1. Functions Chapter 3 of the text Motivation:
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
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
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);)
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2. Examples already seen int main(void) { … Function definition of
return 0; } 1
Function definition of
the function main 2
printf(“Temperature is %.2f”,celsius);
a call to the function printf 3
scanf(“%i”,&age);
a call to the function scanf
Prewritten functions like scanf and printf
are packaged in libraries.
#include<stdio.h>
tells the compiler to use the standard
input/output library (where printf and
scanf are)
You can use as many libraries as you want
(check appendix B for a list of the libraries
available with any C compiler)
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3. Useful libraries stdio.h printf, scanf, ... stdlib.h
contains many common
functions, e.g.
random number generators…
also contains #define
constants e.g. EXIT_SUCCESS
math.h x
see the list on page AP13 y
is pow(x,y)
is sqrt(x)
...
You can ALSO define your own functions
within a program.
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4. Your own functions 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.
function header
function name
function
parameter
function
definition
double cube(double var) {
/*compute and return the cube of var*/
return (var*var*var); }
function
type (here
double)
the function MUST return
a double
Recall: C is strongly typed
functions have a type
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5. Calling a function 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; }
a function can be anywhere an
expression (of the same type) can be.
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6. Declaring your own functions
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
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!)
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7. An Example prototype for 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);
call to cube
definition
of cube
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