lecture 5: modular programming (functions – part 1 bj furman 27feb2012
TRANSCRIPT
Learning Objectives Explain the concept of modular program design Explain the concept of a function in C Explain why functions are important in programming Explain the structure of a function
Return data type Parameters
Apply the concept of a function to a practical problem Explain how larger C programs should be structured
using .h and .c files
Modular Programming
Break a large problem into smaller pieces Smaller pieces sometimes called ‘modules’ or
‘subroutines’ or ‘procedures’ or functions Why?
Helps manage complexity Smaller blocks of code Easier to read
Encourages re-use of code Within a particular program or across different programs
Allows independent development of code Provides a layer of ‘abstraction’ a = sqrt(9.0);
Functions
The ‘building blocks’ of a C program You’ve used predefined functions already:
main() printf(), scanf(), pow()
User-defined functions Your own code In combination with predefined functions
Functions - Mathematical View
32)( 2
xxxf
11 is )2(
113443)2(2)2()2( 2
f
f
f(2)? isWhat
)(xf2 11X Function
Returnedvalue
Functions - Definition Structure Function 'header'
Return data type (if any)
Name Descriptive
Arguments (or parameter list)
Notice: data type and name
Statements Variable declaration Operations Return value (if any)
type function_name (type arg1, type arg2 )
{statements;
}
double product(double x, double y)
{ double result;
result = x * y; return result; }
A function that calculates the product of two numbers
Functions - Example Function prototype
Like a variable declaration Tells compiler that the function will
be defined later Helps detect program errors Note semicolon!!
Function definition See previous slide Note, NO semicolon
Function return return statement terminates
execution of the current function Control returns to the calling
function if return expression;
then value of expression is returned as the value of the function call
Only one value can be returned this way
Function call main() is the 'calling function' product() is the 'called function' Control transferred to the function
code Code in function definition is
executed
#include <stdio.h>
/* function prototype */
double product(double x, double y);
int main(){ double var1 = 3.0, var2 = 5.0; double ans;
ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans);}
/* function definition */ double product(double x, double y)
{ double result;
result = x * y; return result; }
Function - Practice 1
Write a function named 'sum' sums two
integers returns the sum
2 min. on your own
Share with neighbor
Steps1. Function header
• return data type• function name• argument list with data types
2. Statements in function definition• variable declaration• operations• return value
Functions that do not return a value
Use the return type of void void my_fun( arg_list,…)
Practice Write two functions, the first prints out first
name, and the second prints out last name
Function - Practice 2 Program to print out two
happy :) :) or sad faces :( :(
Continuously prompts for user input:
) for happy face ( for sad face Quits if 'q' or 'Q' entered
calls two functions happy_face() sad_face()
Work in pairs Pseudocode first!! Divide tasks of writing
the two functions
Steps1. Pseudocode for program logic2. Function header
• return data type (if any)• function name• argument list with data types (if any)
3. Statements in function definition• variable declaration (if any)• operations• return value
Program - Faces logic Pseudocode
1. Declare and initialize variables
2. WHILE user input not equal to q AND not equal to Q1) Switch on user input to
2) Case ')‘:
call happy_face();
break;
5) Case '(‘:
call sad_face();
break;
6) Case ‘q’:
7) Case ‘Q’:break;
8) Case ‘0’:
9) Default:
re-prompt for user input
Structuring C Programs Modularization
Breaking a program up into smaller pieces: Instead of:
one_big_program.c break into groupings of header files (.h) and source code (.c) files:
module_1.h module_1.c etc.
Rationale separates the user-interface description (.h) from the nitty-
gritty details of implementation (.c) The Application Programming Interface (API), the .h file, is
distinct from the implementation, the .c file (which may already be compiled and not readily viewed)
Example: math.h from Ch can construct and test modules independently promotes re-use of code
Example: math.h used in Ch
See C:/ Ch / include / math.h Declaration of constants
#define M_PI 3.14159265358979323846 Declaration of macro subsitutions
#define isgreater(x, y) ((x)>(y)) Declaration of global variables (caution!) Function prototypes
extern double sin(double x); Pertinent comments
Structured Programming
All programs can be written using these control structures: Sequence Decision (three structures)
IF IF-ELSE SWITCH
Repetition (three structures) WHILE DO-WHILE FOR
C Code for D&D 3.15cProgrammer’s block
Pre-processor directive
Declare and initialize variables
While loop(repetition structure)
Main function (statements go between { } )
return statement
Arithmetic with Integers and Mixed Data Types
Arithmetic with integers Result is an integer
1+1 --> 2 4/2 --> 2 2/4 --> ? BE CAREFUL!!!
Arithmetic with mixed data types Automatic conversion of operand so that data types match
Conversion is ‘upward’ in the sizeof() sense Example in Ch
char a = 7;sizeof(a);double b=3;sizeof(b);printf("a+b == %lf and needs %d bytes\n ", a+b, sizeof(a+b));