conditionals, loops, and other statements cs-2301, b-term 20091 conditionals, loops, and other kinds...
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Conditionals, Loops, and Other Statements
CS-2301, B-Term 2009 1
Conditionals, Loops, and Other Kinds of Statements
CS-2301, System Programmingfor Non-Majors
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
Conditionals, Loops, and Other Statements
CS-2301, B-Term 2009 2
Reading Assignment
• Chapter 3 of Kernighan and Ritchie• Helpful to follow along in class
• Note on §3.8 :– we do not allow goto statements in this course
• There is rarely, if any, situation at WPI where a goto in C would be the right thing
• There is rarely a situation in any program where a goto is the right thing!
Conditionals, Loops, and Other Statements
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Review
• Expression – a sequence of operators and operands that return a value
• Assignment – expression with the side effect of changing the value of the left operand
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Definition – Side Effect
• The changing of the value of some data in the course of evaluating or executing something else
• Sometimes unrelated data!
• Examples:–– Explicit assignments
• x = y + 3; i++; --j;– printf()
• Writes to internal buffer; flushes buffer to screen on '\n'
– scanf() – returns values from input• Explicit – assigns data to arguments
• Implicit – keeps track of where it is in internal buffer
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Definition – Statement
• Instruction to “do” something
• Specifies order of execution & evaluation
• §A.9 – a statement in C may be any of:–• labeled-statement
• expression-statement
• compound-statement
• selection-statement
• iteration-statement
• jump-statement
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Expression Statement
• expressionoptional ;
• Note: semicolon is terminator
• Exists primarily for its side effects
• E.g.,• x = y + 3; i++; --j;• printf(“string”, arg1, arg2, arg3);
• The following is perfectly legal in C:–• y + 3;
• Evaluates the expression, then throws the result away!
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Compound Statement
• A sequence of statements surrounded by {}• Example:–
{x = 0;i++;printf("The value of x is %d\n", x);
}
• Reason:– so that we can group together statements in loops, if-else, functions, etc.
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Compound Statement (continued)
• Compound statements may be nested{...;{ x = 0; i++;
} /* no semicolon needed here*/
printf("The value of x is %d\n", x);...;
}
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Compound Statement (continued)
• Compound statements may include declarations inside{double angle;angle = atan2(y, x) * 360/(2*pi);printf("Angle is %f degrees\n", angle);
}
• Declarations inside of {} are not known outside of {}
• Book says declarations must be at beginning of {}• Later versions of C relax this rule
• Declaration must always precede first use of declared identifier
Note: declaration-definition
of angle
allocates memory.
This is relinquished at end
of {}.
atan2(y,x) calculates the
arctangent of yx without
zero-divide problems
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Questions?
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If-else Statements
if (expr)statement1 /*do this if expr is non-zero*/
else
statement2 /*do this if expr is zero*/
• The else clause is optional!
May be any kind of statement –
loop, assignment, if-else, {}, etc.
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If-else Examples
if (j > limit)return j; /* note semicolon*/
elsej += stepValue; /* note semicolon*/
...
if (++k >= 4)k = 0; /* increment k mod 4*/
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If-else Examples (continued)
if (n < maxInput) {
scanf("string", &arg1, &arg2, …);
n++;
printf("string2", arg3, arg4, …);
} else { /* note NO semicolon*/
printf("Summary\n", arg5, …);
return n;
}
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Concatenated If-else Statements
if (expr1)
statement1 /*do this if expr1 is non-zero*/
else if (expr2)
statement2 /*i.e., expr1 == 0, expr2 != 0*/
else if (expr3)
statement3 /expr1 and expr2 are zero, expr3 != 0*/
else if (expr4)
statement4 /expr1, expr2 , expr3 all zero, expr4 != 0*/
else statement5 /*i.e., all expr are zero*/
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Concatenated If-else Statements
• Last else is always attached to last if
• If it should be attached to any other if, use {} to control the flow of execution.
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Switch Statement
• Somewhat like a concatenated if-else• Occasionally easier to read
• Each arm is called a case• Evaluate switch expression, select the
appropriate case (if any)– default case is optional
• Difference from concatenated if-else• Need break statement to exit switch after a case• Otherwise, control falls through to next case
• See §3.4 and p. 59
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Switch Statement Example
int month, daysInMonth, leapYear;…switch (month) {
case 0: printf("January\n");daysInMonth = 31;break;
case 1: printf("February\n");daysInMonth = leapYear ? 29 : 28;break;
case 2: printf("March\n");daysInMonth = 31;break;
case 3: printf("April\n");daysInMonth = 30;break;
…} // switch
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Switch Statement Example
int month, daysInMonth, leapYear;…switch (month) {
case 0: printf("January\n");daysInMonth = 31;break;
case 1: printf("February\n");daysInMonth = leapYear ? 29 : 28;break;
case 2: printf("March\n");daysInMonth = 31;break;
case 3: printf("April\n");daysInMonth = 30;break;
…} // switch
case values m
ust be
constants
break sta
tement needed to
jump over subsequent cases
defaul
t case is optional
(not shown)
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Questions?
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Iterative Statement
• while loop• for loop• do-while loop
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while loops
while (expression)statement
• Evaluate expression• If true, execute statement and then repeat• Repeat until expression becomes false
• statement may be executed zero or more times!
Often a compound statement
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int sum = 0;int count = 0;int input;while (scanf("%d", &input) != EOF){
sum += input;count++;printf("Input value of %f recorded.\n", input);...
}if (count > 0)
printf("Average is %f\n", (double)sum/count);else
printf("No inputs recorded\n");
while loop exampleNote initialization of sum
and count
What is this?
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while loop examples (continued)
• Study example on p. 59 (§3.4)
• A program to count digits, white space characters, and other characters.
• Includes a while loop with a switch statement inside
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do-while loop
• do statement
while (expression);
• Similar to while loop, but guaranteed to execute statement at least once
• See §3.6
Note: semicolon is
required here
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Breaking out of a Loop
• When it becomes necessary to terminate a loop prematurely– break; /*exits smallest containing
switch or loop*/
• When it becomes necessary to terminate the current iteration and start the next one– continue; /*terminates this iteration only*/
• See p. 65, §3.7
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Questions?
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for loop
• A counting loopfor (expr1; expr2; expr3)
statement
• Evaluate expr1 to initialize• Evaluate expr2 to test
• If true, execute statement• If not true, exit for loop
• Evaluate expr3 to prepare for next iteration• Repeat expr2 to test
Remember: zero is false,
non-zero is true
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for loops and while loops
• The for loopfor (expr1; expr2; expr3)
statement
• is exactly equivalent to the followingexpr1; while (expr2) {
statementexpr3;
}
• See p. 60
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The most common kind of for-loop
int i;
for (i = 0; i < limit; i++) {
...;
/* do something with ith entity */
...;
}
Loop “body” is typically
a compound statement
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The most common kind of for-loop
int i;
for (i = 0; i < limit; i++) {
...;
/* do something with ith entity */
...;
}
It is traditional in C that for-
loops start counting at zero
and test that the counter is less
than the upper limit
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The most common kind of for-loop
int i;
for (i = 0; i < limit; i++) {
...;
/* do something with ith entity */
...;
} • This loop iterates limit times.
– Iterations are numbered i = 0, 1, 2, …, limit-1
• Reason:– arrays are indexed this way!
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Nested for-loops
int i, j;for (i = 0; i < limit; i++) {...; /*prepare subgroup i*/for (j=0; j < limit2; j++) {
...;/* do something with item j of
subgroup i*/...;
}...;
}
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An Extension in Modern C Compilers
• The following construct (not in book) is legal in C99:–for (int i = 0; i < limit; i++){expression involving i;...;printf(“Iteration %d completed.\n”, i);...;
}
• The loop counter i is declared in for loop• Not visible outside of loop!
• Common practice• Good programming style
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Notes on Loop Style
for (int i = 0; i < limit; i++) {
...; /*prepare for subgroup i*/
for (int j=0; j < limit2; j++) {
...;
/* do something with item j of
subgroup i*/
...;
} /* end of loop j */
...;
} /* end of loop i */
Declare loop variables in
for-statements – C99
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Notes on Loop Style
for (int i = 0; i < limit; i++) {
...; /*prepare for subgroup i*/
for (int j=0; j < limit2; j++) {
...;
/* do something with item j of
subgroup i*/
...;
} /* end of loop j */
...;
} /* end of loop i */
Include a comment at end of
each loop to show which loop
it is
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Questions?
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Simple Example
• Read a series of numbers
• Add up the numbers and also the squares of those numbers “on-the-fly”
• After all numbers are input, print out:–– The mean – The standard deviation
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Simple Example (continued)
#include <stdio.h>#include <math.h>
int main(int argc, char*argv[]) {
unsigned int n; double mean; double sum = 0; double sumOfSquares = 0;
printf("Enter sequence“" of numbers:- ");
for (n = 0; ; n++) {int rc;double input;
rc = scanf("%lf", &input);
if (rc == EOF) break;else if (rc == 0) continue;
sum += input;sumOfSquares += input *
input;
} //for (n = 0 ...
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Simple Example (concluded)
printf("The total number of input entries“" was %u.\n", n);
if (n > 0 ) {
printf("The mean is %g.\n", mean = sum/n);
printf("The standard deviation is %g.\n",
sqrt(sumOfSquares/n - pow(mean, 2)));
} // if (i > 0)
return 0;
} //main
Example can be found here
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Questions?