Pengantar C/C++

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Outline

C overview C language elements Variable declarations and data types Executable statements General form of a C program Arithmetic expressions

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C overview C is a high-level programming language Developed in 1972 By Dennis Ritchie at AT&T Bell Laboratories From two previous programming

languages, BCPL and B Used to develop UNIX Used to write modern operating systems Hardware independent (portable) By late 1970s C had evolved to “Traditional

C”

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C overview (cont.)

Standardization Many slight variations of C existed, and

were incompatible Committee formed to create a

“unambiguous, machine-independent” definition

Standard created in 1989, updated in 1999

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C language elements Preprocessor

A system program that modifies a C program prior to its compilation

Preprocessor directive A C program line beginning with # that provides

an instruction to the preprocessor Library

A collection of useful functions and symbols that may be accessed by a program

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C language elements (cont.) Constant macro

A name that is replaced by a particular constant value before the program is sent to the compiler

Comment Text beginning with /* and ending with */

that provides supplementary information but is ignored by the preprocessor and compiler

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C language elements (cont.)/** Converts distance in miles to kilometers*/#include <stdio.h> /* printf, scanf definitions */#define KMS_PER_MILE 1.609 /* conversion constant */

int main(void){

double miles, /* input – distance in miles */ kms; /* output – distance in kilometers */

/* Get the distance in miles */printf(“Enter the distance in miles> ”);scanf(”%lf”, &miles);

/* Convert the distance to kilometers */kms = KMS_PER_MILE * miles;

/* Display the distance in kilometers */printf(“That equals %f kilometers.\n”, kms);

return (0);}

preprocessor directive

standard header file comment

constant

reserved word

standard identifier

comment

special symbol

reserved word punctuation

special symbol

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C language elements (cont.) Function main

int main(void) Declarations

A part of a program that tell the compiler the names of memory cells in a program

Executable statements Program lines that are converted to machine languag

e instructions and executed by the computer Reserved word

A word that has special meaning in C

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C language elements (cont.)

Reserved Word Meaning

int integer; indicates that the main function returns an integer value

void indicates that the main function receives no data from the operating system

double indicates that the memory cells store real numbers

return returns control from the main function to the operating system

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C language elements (cont.)

Standard identifier A word having special meaning but one that

a programmer may redefine (but redefinition is not recommended!)

Example: printf scanf

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C language elements (cont.)

Invalid identifier Reason invalid

1Letter begin with a number

double reserved word

int reserved word

TWO*FOUR character * not allowed

joe’s character ‘ not allowed

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C language elements (cont.) User-defined identifiers

1. An identifier must consist only of letters, digits, and underscores

2. An identifier cannot begin with a digit3. A C reserved word cannot be used as an

identifier4. An identifier defined in a C standard library

should not be redefined Examples:

letter_1, letter_2, inches, cent,CENT_PER_INCH, Hello, variable

Only first 31 characters taking into account

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C language elements (cont.)per_capita_meat_consumption_in_1980

per_capita_meat_consumption_in_1995 The two identifiers would be viewed as identical by a C co

mpiler

C compiler considers uppercase and lowercase usage significantly different

Reserved Words

Standard Identifiers

User-Defined Identifiers

int, void, double, return

printf, scanf

KMS_PER_MILE, main, miles, kms

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C language elements (cont.) Program style

“looks good” is easier to read and understand Programmers spend considerably more time on prog

ram maintenance (updating, modifying) than they do on its original design or coding

Meaningful name for a user-defined identifier easy to understand salary to store a person’s salary THAN s or bag

el If more than 2 or more words, the underscore charac

ter (_) will be better dollars_per_hour THAN dollarsperhour

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C language elements (cont.) Choose identifiers long enough to convey the meanin

g, but not too long lbs_per_sq_in THAN pounds_per_square_inch

Don’t choose names that are similar to each other Avoid selecting 2 names that are different only in the

ir use of uppercase and lowercase letters, such as LARGE and large

Not to user 2 names that differ only in the presence or absence of an underscore, such as xcoord and x_coord

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Variable declarationsand data types Variable

A name associated with a memory cell whose value can change

Variable declarations Statements that communicate to the compiler

the names of variables in the program and the kind of information stored in each variable

Data types A set of values and operations that can be

performed on those values

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Data type int In mathematics, integers are whole numbers The int data type is used to represent integers

in C ANSI C specifies the range: -32767 to 32767 int can be used in arithmetic operations: add, subtr

act, multiply, and divide; also in comparing two integers

Examples: -10500, 435, +15, 125, 32767

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Data type double It used to represent real numbers Examples: 3.14159, 0.0005, 150.0 double can be used in arithmetic operations: add, subtra

ct, multiply, and divide; also in comparing them Scientific notation can be used for very large or very small

values Example: 1.23E5 = 1.23e5 = 1.23 105

So, in C scientific notation we read the letter e or E as “times 10 to the power”

If the exponent is positive it means “move the decimal point <the value> places to the right”

If the exponent is negative it means “move the decimal point <the value> places to the left”

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Data type double (cont.)

Valid double Constants Invalid double Constants

3.14159

0.005

12345.0

15.0e-04 (value is 0.0015)

2.345e2 (value is 234.5)

1.15e-3 (value is 0.00115)

12e+5 (value is 1200000.0)

150 (no decimal point)

.12345e (missing exponent)

15e-0.3 (0.3 is invalid exponent)

12.5e.3 (.3 is invalid exponent)

34,500.99 (comma is not allowed)

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Data type double (cont.)

Data type double is an abstraction for the real numbers because it does not include them all

Some real numbers are too large or too small, and some real numbers cannot be represented precisely because of the finite size of a memory cell

Valid double Constants Invalid double Constants

3.14159

0.005

12345.0

15.0e-04 (value is 0.0015)

2.345e2 (value is 234.5)

1.15e-3 (value is 0.00115)

12e+5 (value is 1200000.0)

150 (no decimal point)

.12345e (missing exponent)

15e-0.3 (0.3 is invalid exponent)

12.5e.3 (.3 is invalid exponent)

34,500.99 (comma is not allowed)

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Data type char It used to represent an individual

character value - a letter, a digit, or a special symbol

Each type char value is enclosed in apostrophes (single quotes) Examples: ‘A’ ‘z’ ‘2’ ‘9’ ‘*’ ‘:’ ‘”’ ‘ ‘

You can store a character in a type char variable and compare character data.

C allows you to perform arithmetic operations on type char data, but is has to be care

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Executable statements Programs in memory

Before execution After execution of a Program of a Program

10.00

16.09

miles

kms

machine languagemiles-to-kms

conversion program

memory

?

?

miles

kms

machine languagemiles-to-kms

conversion program

memory

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Assignment statements Instruction that stores a value or a comp

utational result in a variable Example: kms = KMS_PER_MILE * miles; In C the symbol = is the assignment oper

ator. Read is as “becomes,” “gets,” or “takes the value of” rather than “equals” because it is not equivalent to the “equal sign” of mathematics.

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Assignment statements (cont.)kms = KMS_PER_MILE * miles;

Beforeassignment

Afterassignment

KMS_PER_MILE

1.609

miles

10.00 ?

kms

*

KMS_PER_MILE

1.609

miles

10.00 16.090

kms

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Assignment statements (cont.)sum = sum + item;

Beforeassignment

Afterassignment

sum

100

item

10

+

sum

110

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Assignment statements (cont.) If x and new_x are type double variables, the st

atementnew_x = x;

copies the value of variable x into variable new_x. The statement

new_x = -x; instructs the computer to get the value of x, nega

te that value, and store the result in new_x

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Input/Outputoperations and functions Input operation

An instruction that copies data from an input device into memory

Output operation An instruction that displays information stored in memory

The most common I/O functions are supplied as part of the C standard I/O library

#include <stdio.h> Function call

A C function that performs an input or output operation

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The printf function

printf(“That equals %f kilometers.\n”, kms);

Function argument Enclosed in parentheses following the functi

on name; provides information needed by the functions

Format string In a call to printf, a string of characters enc

losed in quotes (“), which specifies the form of the output line

function name

function arguments

format string

print list

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The printf function (cont.) Print list

In a call to printf, the variables or expressions whose values are displayed

Placeholder Symbol beginning with % in format string that indicates w

here to display the output value

Placeholder Variable Type Function Use

%c

%d

%f

%lf

char

int

double

double

printf/scanf

printf/scanf

printf

scanf

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The printf function (cont.) Newline escape sequence

The character sequence \n, which is used in a format string to terminate an output line

Multiple placeholders If the print list of a printf call has several variables,

the format string should contain the same number of placeholders.

C matches variables with placeholders in left-to-right order

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The printf function (cont.) If letter_1, letter_2, and letter_3 are typ

e char variables and age is type int, the printf call

printf(“Hi %c%c%c – your age is %d\n”, letter_1, letter_2, letter_3, age);

displays a line such as

Hi EBK – your age is 35

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More about \n Cursor

A moving place marker that indicates the next position on the screen where information will be displayed

Exampleprintf(“Here is the first line\n”);printf(“\nand this is the second.\n”);

It produces 2 lines of text with a blank line in between:

Here is the first line

and this is the second.1st new line

2nd new line

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More about \n (cont.)printf(“This sentence appears \non two lines. \n”);

The character after the \n appear on a new output line:

This sentence appears

on two lines.

new line

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Displaying Prompts

Prompt (Prompting message) A message displayed to indicate what data t

o enter and in what formprintf(“Enter the distance in miles> ”);

scanf(“%lf”, &miles);

It displays a prompt for square meters (a numeric value).

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The scanf functionscanf(“%lf”, &miles);

It calls function scanf to copy data into the variable miles

It copies the data from the standard input device, usually is the keyboard the computer will attempt to store in miles whatever data the program user types at the keyboard

The format string “%lf” consists of a single placeholder for number, as an input

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The scanf function (cont.)

Notice that in a call to scanf, the name of each variable that is to be given a value is preceded by the & character

the & is the C address-of operator tells the scanf function where to find each variable into which it is to store a new value

If the & were omitted scanf only know a variable’s current value, not its location in memory, so scanf would unable to store a new value in the variable

Example: Scanning data line BOB

miles

30.5

number entered 30.5

letter_1

B

letters entered BOB

letter_2

O

letter_3

B

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The return statementreturn(0);

It transfers control from the program to the operating system

The value 0 is considered the result of function main’s execution, and it indicates that your program executed without error

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General form of a C program

preprocessor directivesmain function heading{

declarationsexecutable statements

}

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Program style Spaces in program

The consistent and careful use of blank spaces can improve the style of a program

A blank space is required between consecutive words in a program line

The compiler ignores extra blanks between words and symbols insert space to improve the readability and style of a program

Leave a blank space after a comma and before and after operators, e.g., *, /, +, -, =

Indent the body of the main function and insert blank lines between sections of the program

But it’s wrong / * and * /, also MAX ITEMS

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Comments in programs Programmers make a program easier to understand by usi

ng comments to describe the purpose of the program, the use of identifiers, and the purpose of each program step part of program documentation

Program documentation Information (comments) that enhances the readability of

a program

double miles, /* input – distance in miles */ kms; /* output – distance in kilometers */

We document most variables in this way

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Program style –using comment Header section

The programmer’s name The date of the current version A brief description of what the program does

/* * Programmer: Ucup Date completed: Sep 12, 2005 * Instructor: Irfan Class: CI1301 * * Calculates and displays the area and circumference * of a circle */

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Program style –using comment (cont.) Before you implement each step in the initial algorithm, you shou

ld write a comment that summarizes the purpose of the algorithm step

This comment should describe what the step does rather than simply restate the step in English

/* Convert the distance to kilometers. */kms = KMS_PER_MILE * miles;

is more descriptive and hence preferable to

/* Multiply KMS_PER_MILE by miles and store result in kms. */kms = KMS_PER_MILE * miles;

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Arithmetic expressions

Arithmetic

Operator

Meaning Examples

+

-

*

/

%

addition

subtraction

multiplication

division

remainder

5 + 2 is 7

5.0 + 2.0 is 7.0

5 – 2 is 3

5.0 – 2.0 is 3.0

5 * 2 is 10

5.0 * 2.0 is 10.0

5.0 / 2.0 is 2.5

5 / 2 is 2

5 % 2 is 1

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Arithmetic expressions (cont.) Result of integer division

3 / 15 = 0 18 / 3 = 615 / 3 = 5 16 / - 3 varies16 / 3 = 5 0 / 4 = 017 / 3 = 5 4 / 0 is undefined

7 / 2

3

72

6

1 7 % 2

299 / 100

2

299100

200

99 299 % 100

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Arithmetic expressions (cont.) Result of % operation

3 % 5 = 3 5 % 3 = 24 % 5 = 4 5 % 4 = 15 % 5 = 0 15 % 5 = 06 % 5 = 1 15 % 6 = 37 % 5 = 2 15 % -7 varies 8 % 5 = 3 15 % 0 is undefined

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Arithmetic expressions (cont.) The formula

m equals (m / n) * n + (m % n) Examples:

7 equals (7 / 2) * 2 + (7 % 2) equals 3 * 2 + 1

299 equals (299 / 100) * 100 + (299 % 100) equals 2 * 100 + 99

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Data type of an expression

ace + bandage is type int if both ace and bandage are type in

t; otherwise, it’s type double It’s as an example of the general form

ace arithmetic_operator bandage Mixed-type expression

An expression with operands of different types The data type of such a mixed-type expression will be

double

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Mixed-type assignment statement Mixed-type assignment

The expression being evaluated and the variable to which it is assigned have different data types

Example:m = 3;n = 2;p = 2.0;x = m / p;y = m / n;

x = 9 0.5;n = 9 0.5;

3 2 2.0 1.5 1.0

m n p x y

4.5 4

x n

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Expressions withmultiple operators Unary operator

An operator with one operand Examples:

negation () and plus () operators x = y; p = x y;

Binary operator An operator with two operands

When and are used to represent addition and subtraction, they are binary operators

x = y z; z = y x;

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Rules forevaluating expressions Parentheses rule

All expressions in parentheses must be evaluated separately. Nested parenthesized expressions must be evaluated from the inside out, with the innermost expression evaluated first.

Operator precedence rule Operators in the same expression are evaluated in the following ord

er Unary , first , , % next Binary , last

Association rule Unary operators in the same subexpression and at the same precede

nce level (such as and ) are evaluated right to left (right associativity). Binary operators in the same subexpression and at the same precedence level (such as and ) are evaluated left to right (left associativity).

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Rules forevaluating expressions (cont.)

x y z a b c dcan be written in a more readable form using parentheses:

(x y z) (a b) (c d) The formula for the area of a circle

a = r2

can be written in C asarea = PI radius radius;

where the constant macro PI = 3.14159

area = PI * radius * radius

*1 c

*

area

2area = PI * radius * radius

3.14159 2.0 2.0

6.28318

12.56636

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Rules forevaluating expressions (cont.)

z (a b 2) w -ycontaining type int only.

z - (a + b / 2) + w * -y

/1 a,b

+2 a

-3 a

*4 b

-5 c

+6

z

8

z

3

a

9

b

2

w

-5

y

z - (a + b / 2) + w * -y

8 3 9 2 -5

4 5

7 10

1

11

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Writing mathematicalformulas in C

Always specify multiplication explicitly by using the operator * where needed (formulas 1 and 4) Use parentheses when required to control the order of operator evaluation (formulas 3 and 4) Two arithmetic operators can be written in succession if the second is a unary operator (formula 5)

Mathematical Formula

C Expression

1. b2 – 4ac

2. a + b – c

3.

4.

5. a –(b + c)

b * b – 4 * a * c

a + b – c

(a + b) / (c + d)

1 / (1 + x * x)

a * –(b + c)

dc

ba

21

1

x

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Case study: evaluating a collection of coins It demonstrates the manipulation of type int data (using / and

%) and type char data Problem

Your local bank branch has many customers who save their change and periodically bring it in for deposit. Write a program to interact with their customers and determine the value of a collection of coins.

Analysis To solve this problem, you need to get the count of each type of coin

(quarters, dimes, nickels, pennies) from a customer. From those counts, you can determine the total value of the coins in cents. Once you have that figure, you can do an integer division using 100 as the divisor to get the dollar value; the remainder of this division will be the leftover change. In the data requirements, list the total value in cents (total_cents) as a program variable, because it is needed as part of the computation process but is not a required problem output. To personalize the interaction, get each customer’s initials before getting the coin counts.

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Case study: evaluating a collection of coins (cont.) Data requirements

Problem inputschar first, middle, last /* a customer’s initials */int quarters /* the count of quarters */ int dimes /* the count of dimes */ int nickels /* the count of nickels */ int pennies /* the count of pennies */

Problem outputsint dollars /* value in dollars */ int change /* leftover change */

Additional program variablesint total_cents /* total value in cents */

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Case study: evaluating a collection of coins (cont.) Design

Initial algorithm1. Get and display the customer;s initials2. Get the count of each kind of coin3. Compute the total value in cents4. Find the value in dollars and change5. Display the value in dollars and change

Step 3 and 4 may need refinement. Their refinement are:Step 3 Refinement

3.1 Find the equivalent value of each kind of coin in pennies and add these valuesStep 4 Refinement

4.1 dollars in the integer quotient of total_cents and 1004.2 change is the integer remainder of total_cents and 100

Implementation The program is shown in the following

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Case study: evaluating a collection of coins (cont.)/* * Determines the value of a collection of coins */#include <stdio.h>

int main(void) {char first, middle, last; /* input – 3 initials */int pennies, nickels; /* input – count of each coin type */int dimes, quarters; /* input – count of each coin type */int change; /* output – change amount */int dollars; /* output – dollar amount */int total_cents; /* total cents */

/* Get and display the customer’s initials */printf(“Type in 3 initials and press return> ”);scanf(“%c%c%c”, &first, &middle, &last);printf(“Hello %c%c%c, let’s see what your coins are worth.\n”,

first, middle, last);

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Case study: evaluating a collection of coins (cont.)

/* Get the count of each kind of coin */printf(“Number of quarters> ”);scanf(“%d”, &quarters);printf(“Number of dimes > ”);scanf(“%d”, &dimes);printf(“Number of nickels > ”);scanf(“%d”, &nickles);printf(“Number of pennies > ”);scanf(“%d”, &pennies);

/* Compute the total value in cents */total_cents = 25 * quarters + 10 * dimes + 5 nickles + pennies;

/* Find the value in dollars and change */dollars = total_cents / 100;change = total_cents % 100;

/* Display the value in dollars and change */printf(“\nYour coins are worth %d dollars and %d cents.\n”,

dollars, change);

return(0);}

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Case study: evaluating a collection of coins (cont.)Type in 3 initials and press return> BMCHello BMC, let’s see what your coins are worth.Number of quarters> 8Number of dimes > 20Number of nickels > 30Number of pennies > 77

Your coins are worth 6 dollars and 27 cents.

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Formatting value of type int Simply add a number between the % and the d of the %d pla

ceholder in the printf format string Field width

The number of columns used to display a value

Value Format Displayed Output

Value Format Displayed Output

234

234

234

234

%4d

%5d

%6d

%1d

234234234234

-234

-234

-234

-234

%4d

%5d

%6d

%1d

-234

-234-234-234

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Formatting value of type double It must indicate both the total field width needed and the n

umber of decimal places desired The form of the format string placeholder is %n.mf where n

is a number representing the total field width, and m is the desired number of decimal places

Displaying x using format string placeholder %6.2f

Value of x

Displayed Output

Value of x Displayed Output

-99.42

.123

-9.536

-99.42

0.12-9.54

-25.554

99.999

999.4

-25.55

100.0

999.40

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Formatting value of type double (cont.)

Value Format

Displayed Output

Value Format Displayed Output

3.14159

3.14159

3.14159

.1234

-.006

-.006

%5.2f

%3.2f

%5.3f

%4.2f

%8.3f

%.3f

3.143.14

3.142

0.12

-0.006-0.006

3.14159

3.14159

3.14159

-.006

-.006

-3.14159

%4.2f

%5.1f

%8.5f

%4.2f

%8.5f

%.4f

3.14

3.13.14159-0.01

-0.00600

-3.1416

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Interactive mode, batch mode, and data files Interactive mode

A mode of program execution in which the user responds to prompts by entering (typing in) data

E.g. our previous program Batch mode

A mode of program execution in which the program scans its data from a previously prepared data file

Input redirection E.g. In the UNIX and MS-DOS OS, program take its input from

file mydata instead of from the keyboard by placing the symbols <mydata at command prompt metric <mydata

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Batch version program/** Converts distance in miles to kilometers*/#include <stdio.h> /* printf, scanf definitions */#define KMS_PER_MILE 1.609 /* conversion constant */

int main(void){

double miles, /* input – distance in miles */kms; /* output – distance in kilometers */

/* Get and echo the distance in miles */scanf(”%lf”, &miles);printf(“The distance in miles is %.2f.\n”, miles);

/* Convert the distance to kilometers */kms = KMS_PER_MILE * miles;

/* Display the distance in kilometers */printf(“That equals %.2f kilometers.\n”, kms);

return (0);}

The distance in miles is 112.00.That equals 180.21 kilometers.

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Echo prints vs Promptsscanf(“%lf”, &miles);

It gets a value for miles from the first (and only) line of the data file. Because the program input comes from a data file, there’s no need to precede with a prompting message. Instead we useprintf(“The distance in miles is %.2f.\n”, miles);

This statement echo prints or displays the value just stored in miles and provides a record of the data manipulated by the program

Output redirection Redirect program output to a disk file instead of to the screen E.g. In the UNIX and MS-DOS OS, program put its output to file myou

tput by placing the symbols >myoutput at command prompt metric >myoutput

For input and output file, it will be better to use the command line metric <mydata >myoutput

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Program with Named Files/** Converts distance in miles to kilometers*/#include <stdio.h> /* printf, scanf, fprint, fscanf, fopen, fclose definitions */#define KMS_PER_MILE 1.609 /* conversion constant */

int main(void){

double miles, /* input – distance in miles */ kms; /* output – distance in kilometers */FILE *inp, /* pointer to input file */ *outp; /* pointer to output file */

/* Open the input and output files */inp = fopen(”c:distance.dat”, “r”);outp = fopen(“c:distance.out”, “w”);

/* Get and echo the distance in miles */fscanf(inp, ”%lf”, &miles);fprintf(outp, “The distance in miles is %.2f.\n”, miles);

/* Convert the distance to kilometers */kms = KMS_PER_MILE * miles;

/* Display the distance in kilometers */fprintf(outp, “That equals %.2f kilometers.\n”, kms);

/* Close files */fclose(inp);fclose(outp);

return (0);}

Contents of input file distance.dat 112.00

Contents of output file distance.out The distance in miles is 112.00.That equals 180.21 kilometers.

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Common programming errors Murphy’s Law: “If something can go

wrong, it will” Debugging – correcting error from a

program According to computer folklore,

computer pioneer Dr. Grace Murray Hopper diagnosed the first hardware error caused by a large insect found inside a computer component)

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Syntax errors The violation of the C grammar rules,

detected during program translation (compilation)

Examples: Missing semicolon at the end of the variable

declaration (in line 271) Undeclared variable miles (detected in lines 275

& 278) Last comment is not closed because of blank in *

/ close-comment sequence (in line 280)

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Syntax errors (cont.)221 /* Converts distance in miles to kilometers */222223 #include <stdio.h> /* printf, scanf definitions */266 #define KMS_PER_MILE 1.609 /* conversion constant */267268 int269 main(void)270 {271 double kms272273 /* Get the distance in miles */274 printf(“Enter the distance in miles> ”);***** Semicolon added at the end of the previous source line

275 scanf(“%lf”, &miles);***** Identifier “miles” is not declared within this scope***** Invalid operand of address-of operator276277 /* Convert the distance to kilometers */278 kms = KMS_PER_MILE * miles;***** Identifier “miles” is not declared within this scope

279280 /* Display the distance in kilometers * /281 printf(“That equals %f kilometers.\n”, kms);282283 return (0);284 }***** Unexpected end-of-file encountered in a comment***** “}” inserted before end-of-file

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Run-Time errors An attempt to perform an invalid operation,

detected during program execution Occurs when the program directs the

computer to perform an illegal operation, such as dividing a number by zero

If it occur the computer will stop executing your program and will display a diagnostic message that indicates the line where there error was detected

71

Run-Time errors (cont.)111 #include <stdio.h>262263 int264 main(void)265 {266 int first, second; 267 double temp, ans;268269 printf(“Enter two integers> ”);270 scanf(“%d%d”, &first, &second);271 temp = second / first;272 ans = first / temp; 273 printf(“The result is %.3f\n”, ans);274275 return (0);276 }

Enter two integers? 14 3Arithmetic fault, divide by zero at line 272 of routine main

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Undetected errors It leads to incorrect result It has to predict the results your program should produce a

nd verify that the actual output is correct A very common source of incorrect results in C programs is

the input of a mixture of character and numeric data scanf’s different of the %c placeholder on the one hand,

and of the %d and %lf to the other scanf first skips any blanks and carriage returns in the inp

ut when a numeric value is scanned, but, scanf skips nothing when it scans a character unless the %c placeholder preceded by a blank

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Undetected errors (cont.) Revised start of main function for Coin Evaluation

int main(void) {char first, middle, last; /* input – 3 initials */int pennies, nickels; /* input – count of each coin type */int dimes, quarters; /* input – count of each coin type */int change; /* output – change amount */int dollars; /* output – dollar amount */int total_cents; /* total cents */int year; /* current year */

/* Get the current year */printf(“Enter the current year and press return> ”);scanf(“%d”, &year);

/* Get the program user’s initials */printf(“Type in 3 initials and press return> ”);scanf(“%c%c%c”, &first, &middle, &last);printf(“Hello %c%c%c, let’s check your coins’ value in %d.\n”,

first, middle, last, year);

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Undetected errors (cont.)

If the user types in 2005 and then the letters BMC, we would expect the second call to printf to display:Hello BMC, let’s check your coins’ value in 2005.

Instead, it displays the message:Hello

BM, let’s check your coins’ value in 2005.

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Undetected errors (cont.) Why? Let’s see the status of memory

The value of year is correct, but the three characters stored are not ‘B’, ‘M’, “C’, but ‘\n’, ‘B’, and ‘M’. The ‘\n’ in first is the character that results from the user pressing the <return> key after entering the number 2005

The scan of 2005 stopped at this character, so it was the first character processed by the statementscanf(“%c%c%c”, &first, &middle, &last);

Because the letter C was not yet scanned, it will be scanned during the next scanf call. This will lead to further problems.

By the following code, scanf will skip spaces (including carriage returns) before scanning a characterscanf(“ %c%c%c”, &first, &middle, &last);

2005

year

\n

first

B

second

M

third

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Logic errors An error caused by following an incorrect algorithm Usually logic error do not cause run-time errors and

do not display error messages difficult to detect! The only sign of a logic error may be incorrect

program output You can detect it by testing the program thoroughly,

comparing its output to calculated results You can prevent it by carefully desk checking the

algorithm and the program before you type it in Debugging can be time consuming plan your

program solutions carefully and desk check them to eliminate bugs early

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Logic errors on a program#include <stdio.h>

int main(void) {int first, second, sum;

printf(“Enter two integers> ”);scanf(“%d%d”, first, second); /* ERROR! Should be &first, &second */sum = first + second;printf(“%d + %d = %d\n”, first, second, sum);

return (0);}

Enter two integers> 14 35971289 + 5971297 = 11942586