Functions

Here is the basic structure of a function:

return_type function_name (arg_type arg_name, arg_type arg_name, etc...) {

< statements >

return value;

}

 

• The arg_type's and return_type can be any defined type in C. These include int, char, float, double (see variables), and a special type, void. If a function is declared as type void, there is no return value.
   
• function_name can be any name you make up. However, the same restrictions that apply to variable names apply to function names.
   
• The <statements> and return value; statment are the body of the function. Up to now, your programs have been written entirely within main(). So, all the statements in your programs were part of the main function's body. You can use everything you've learned so far when writing functions. Functions can have variable declarations, if-else conditions, and loops.
   
• Usually, the last statement in a function will be the return statement. The type of the value returned must match the return_type defined in the first line of the function. The value can be in the form of a number, a variable, or an expression. For example, the return statement could be any of these:
     return 10;
     return count;
     return count + 6*n;
   
• The statements in a function are executed only when the function is 'called'. A function call looks like this:

  function_name (value, value, etc...);

  (value, value, etc.) indicates values that are 'passed into' the function. They must match both in number and type the list of 'parameters' in the function declaration. When you call a function, the argument variables defined in the parameter list will be initialized with these values.

• When a function is called, the code in the function will be executed. When the function 'returns', the program will continue where it left off. If the function call occurred, for example, in the 6th line of main(), then when the function returns, the program will resume at that 6th line.
   
• The value returned by a function can be used like any other value of its type. It can be used to set the value of a variable or printed or used in an expression.
   
  
• The page on variable scope has more information about functions.
   
• You will definitely want to read the chapter on functions in your textbook carefully, since functions are an important part of C. Also, be sure to do the exercises on functions. Like loops, the best way to understand functions is just to practice using them.

Questions from Students

1. is it true that you cannot modify the argument of a function? I think that the king books says if you modify and return the variable, it would be the same as the original one. but I tried the following code which does modify its argument, and it works! I am confused.

   int leadingDigit (int n) {
      while(n>=10)
        n/=10;
      return n;
   }
   

   • If you call the leadingDigit() function you describe below like this:

     int a,b;
     a = 567;
     b = 890;
     a = leadingDigit(b);
     printf("a is %d and b is %d\n", a, b);
     

     This code will print "a is 8 and b is 890"
     When you passed b in as the argument to the leadingDigit function, the value 890 was passed in. The value of b was unchanged even though the value of n was changed in leadingDigit().