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1. Functions Chapter 3 of the text Within a program, may have to perform the same computation over and over Many programs share the same computation (e.g. sorting) • To break a program into smaller pieces • easier to write • reusable for other problems • It may take some input parameters on which it computes (e.g. printf("7 squared is %i",7*7);) • it may compute and return a value (e.g. sqrt(windspeed);) Motivation: What is a function? • It is a named piece of code (e.g. printf) How is it used? • It is executed by giving its name 142 E -1

2. Examples already seen 1 int main(void) { … return 0; } Function definition of the function main 2 printf(“Temperature is %.2f”,celsius); 3 scanf(“%i”,&age); #include<stdio.h> tells the compiler to use the standard input/output library (where printf and scanf are) a call to the function printf a call to the function scanf Prewritten functions like scanf and printf are packaged in libraries. You can use as many libraries as you want (check appendix B for a list of the libraries available with any C compiler) 142 E -2

3. Useful libraries printf, scanf, ... contains many common functions, e.g. random number generators… also contains #define constants e.g. EXIT_SUCCESS see the list on page AP13 y x is pow(x,y) x is sqrt(x) ... • stdio.h • stdlib.h • math.h You can ALSO define your own functions within a program. 142 E -3

4. Your own functions function header function name function parameter function type (here double) the functionMUST return a double function definition Recall: C is strongly typed functions have a type To write your own function: _ give it a name _ write the code it is to execute Example: write a function to compute the cube of a number. double cube(double var) { /*compute and return the cube of var*/ return (var*var*var); } 142 E -4

5. Calling a function a function can be anywhere an expression (of the same type) can be. A function can be called from within main or any other function (note that main is just a particular function) int main(void) { double x,y,z; ... x=cube(y/2.0) ... z=cube(cube(x)); ... printf(“%f cubed is %f”,y,cube(y)); … return EXIT_SUCCESS; } 142 E-5

6. Declaring your own functions For functions, we need to provide a prototype _ after the preprocessor commands (# lines) _ before the first function (the first piece of code between braces {} ), which is generally main just a rule of style (but follow it!) In C, identifiers (=names of things) MUST be declared BEFORE they are used. e.g. need to write int i, before using the variable i 142E-6

7. An Example prototype for cube call to cube definition of cube #include <stdio.h> double cube(double value); int main(void) { double x,y; … y=cube(x); … return 0; } double cube(double var) { return (var*var*var); } 142E-7