Learn C - C Function Examples

Declaring function in C can be written as follows

  void foo(){ 
      printf("foo() was called\n"); 
  }

We put this function on above main() function. Then, we can call this function, for instance foo().


  #include <stdio.h> 
//w  w w. jav a  2 s  . com
  void foo(){ 
      printf("foo() was called\n"); 
  } 

  int main(int argc, const char* argv[]) { 

    foo(); 
    return 0; 
  }

The code above generates the following result.

Example

We also can declare a function on below of main() function but we must declare our function name.


  #include <stdio.h> 
/*from   www. ja v  a2  s . c om*/
  // implicit declaration for functions 
  void boo(); 

  int main(int argc, const char* argv[]) { 

    boo(); 
    return 0; 
  } 

  void boo(){ 
      printf("boo() was called\n"); 
  }

The code above generates the following result.

Function with Parameters and Returning Value

You may want to create a function that has parameters and a return value.

It is easy because you just call return into your function.


  #include <stdio.h> 
/*from   w  w  w . j a v  a2s  .c o  m*/
  // implicit declaration for functions 
  int add(int a, int b); 

  int main(int argc, const char* argv[]) { 

    int result = add(10,5); 
    printf("result: %d\n",result); 

    return 0; 
  } 

  int add(int a, int b){ 
      return a + b; 
  }

The code above generates the following result.

Function with Array Parameters

We also can declare a function with array as parameters.

To know how array size, our function should declare array size.

Write this code into your program for illustration.


  #include <stdio.h> 
/*  ww w.j  ava 2s  .com*/
  // implicit declaration for functions 
  double mean(int numbers[],int size); 

  int main(int argc, const char* argv[]) { 

    int numbers[8] = {8,4,5,1,4,6,9,6}; 
    double ret_mean = mean(numbers,8); 
    printf("mean: % .2f\n",ret_mean); 

    return 0; 
  } 

  double mean(int numbers[],int size){ 
      int i, total = 0; 
      double temp; 

      for (i = 0; i < size; ++i){ 
          total += numbers[i]; 
      } 

      temp = (double)total / (double)size; 
      return temp; 
  }

The code above generates the following result.

Function Parameter Pointer

We can pass pointer as parameters in our function.

For illustration, we can create swap() to swap our values.


  #include <stdio.h> 
/*w  w  w . ja va  2 s .c om*/
  // implicit declaration for functions 
  void swap(int *px, int *py); 

  int main(int argc, const char* argv[]) { 

    int *x, *y; 
    int a, b; 

    a = 10; 
    b = 5; 

    // set value 
    x = &a; 
    y = &b; 

    printf("value pointer x: %d \n",*x); 
    printf("value pointer y : %d \n",*y); 

    swap(x,y); 
    printf("swap()\n"); 
    printf("value pointer x: %d \n",*x); 
    printf("value pointer y : %d \n",*y); 
    return 0; 
  } 

  void swap(int *px, int *py){ 
      int temp; 

      // store pointer px value to temp 
      temp = *px; 
      // set pointer px by py value 
      *px = *py; 
      // set pointer py by temp value 
      *py = temp; 
  }

The code above generates the following result.