FUNCTION

Dong-Chul KimBioMeCISCSE @ UTA

04/21/23 1

• A function is a self-contained unit of program code designed to accomplish a particular task.

• Some functions can cause an action (printf, scanf), and others can return a value for your program to use (strlen, strcmp).

• Why use functions?• Makes reuse of code easier• Reduces number of places to update• Hide implementation, letting programmer focus on functionality –

more modular

Functions

• You want to write a program that does the following:• Read in a list of numbers• Sort the numbers• Find their average• Print out their average

#include <stdio.h>int main(void){ int list[10]; readlist(list, 10); sortlist(list, 10); average(list, 10); printavg(list, 10); return 0;}

Example

• What do you need to know about functions?• How to define functions properly?• How to call them up for use?• How to set up communication between functions?

Outline

• Function prototype• Tells the compiler what sort of function your_function() is

• A function call• Causes the function to be executed

• A function definition • Specifies exactly what the function does

Three terms

#include <stdio.h>void printstars(int num); /*Function prototype*/int main(void) {

int numstar;do{printf("Please input an integer (>0):");scanf("%d", &numstar);}while(numstar <= 0);printstars(numstar);/*A function call*/return 0;

}

void printstars(int num) /*A function definition */{

int i;for (i=0; i< num; i++){

printf("*");}printf("\n");

}

Example

• We must let the compiler know about the function prior to using it by either:• Placing the function definition before main• Placing a function declaration (or prototype)

before main and define the function later

Function Declarations

• When defining our own functions, the general form isreturn_type function_name(input_type variable_name)

{

/* something happens here */

}

• Example:int addnumbers(int number1, int number2)

{

int sum = number1 + number2;

return sum;

}

Function Definitions

• The variable names in the function definition do not need to match the names in the function call, but the quantity should match.

• To return a value, we use the return keyword.

• We can declare variables in our function just as we did in main.

• We can call other functions from within our function.

#include <stdio.h>

int f1(int y){

y = y + 1;printf("In f1, y = %d\n", y);return f2(y);

}int f2(int y){

y = y + 1;printf("In f2, y = %d\n", y);return y;

}int main(void){

int x = 15;f1(x);return 0;

}

• The types of variables that we can pass or receive from a function can be any of the types that we declare variables to be–int, float, array (actually, we pass the address of the array), etc.

• What type do we use if we are not passing or not returning anything? Void

• Example:void print2numbers(int number1, int number2)

{

printf("%d + %d is %d\n", number1, number2,

number1 + number2);

}

Return and Input Types

• We need to know the following when using variables in functions:

• The process used in this lecture for providing variable values to our function is called pass by value. When doing so, a copy of the variable is provided.

• Variables declared outside the function are unknown to the function unless we pass them.

• Variables declared within a function block are known only to that function.

Variable Scope