Computer Programming - Functions

A function is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reusing. You have already seen various functions like printf() and main(). These are called built-in functions provided by the language itself, but we can write our own functions as well and this tutorial will teach you how to write and use those functions in C programming language.
Good thing about functions is that they are famous with several names. Different programming languages name them differently, for example, functions, methods, sub-routines, procedures, etc. If you come across any such terminology, then just imagine about the same concept, which we are going to discuss in this tutorial.
Let's start with a program where we will define two arrays of numbers and then from each array, we will find the biggest number. Given below are the steps to find out the maximum number from a given set of numbers −
1. Get a list of numbers L1, L2, L3....LN
2. Assume L1 is the largest, Set max = L1
3. Take next number Li from the list and do the following
4.    If max is less than Li
5.       Set max = Li
6.    If Li is last number from the list then
7.       Print value stored in max and come out
8. Else prepeat same process starting from step 3

Let's translate the above program in C programming language −
#include <stdio.h>

int main() {
   int set1[5] = {10, 20, 30, 40, 50};
   int set2[5] = {101, 201, 301, 401, 501};
   int i, max;
   
   /* Process first set of numbers available in set1[] */
   max = set1[0];
   i = 1;    
   while( i < 5 ) {
      if( max <  set1[i] ) {
         max = set1[i];
      }
      i = i + 1;
   }
   
   printf("Max in first set = %d\n", max );
    
   /* Now process second set of numbers available in set2[] */
   max = set2[0];
   i = 1;    
   while( i < 5 ) {
      if( max <  set2[i] ) {
         max = set2[i];
      }
      i = i + 1;
   }
   printf("Max in second set = %d\n", max );
}
When the above code is compiled and executed, it produces the following result −
Max in first set = 50
Max in second set = 501

If you are clear about the above example, then it will become easy to understand why we need a function. In the above example, there are only two sets of numbers, set1 and set2, but consider a situation where we have 10 or more similar sets of numbers to find out the maximum numbers from each set. In such a situation, we will have to repeat, processing 10 or more times and ultimately, the program will become too large with repeated code. To handle such situation, we write our functions where we try to keep the source code which will be used again and again in our programming.
Now, let's see how to define a function in C programming language and then in the subsequent sections, we will explain how to use them.
The general form of a function definition in C programming language is as follows −
return_type function_name( parameter list ) {
   body of the function
   
   return [expression];
}
A function definition in C programming consists of a function header and a function body. Here are all the parts of a function −
Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
Function Name − This is the actual name of the function. The function name and the parameter list together constitute the function signature.
Function Name − This is the actual name of the function. The function name and the parameter list together constitute the function signature.
Parameter List − A parameter is like a placeholder. When a function is invoked, you pass a value as a parameter. This value is referred to as the actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
Parameter List − A parameter is like a placeholder. When a function is invoked, you pass a value as a parameter. This value is referred to as the actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
Function Body − The function body contains a collection of statements that defines what the function does.
Function Body − The function body contains a collection of statements that defines what the function does.
While creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform a defined task.
Now, let's write the above example with the help of a function −
#include <stdio.h>

int getMax( int set[] ) {
   int i, max;
   
   max = set[0];
   i = 1;    
   while( i < 5 ) {
      if( max <  set[i] ) {
         max = set[i];
      }
      i = i + 1;
   }
   return max;
}
main() {
   int set1[5] = {10, 20, 30, 40, 50};
   int set2[5] = {101, 201, 301, 401, 501};
   int max;

   /* Process first set of numbers available in set1[] */
   max = getMax(set1);
   printf("Max in first set = %d\n", max );
    
   /* Now process second set of numbers available in set2[] */
   max = getMax(set2);
   printf("Max in second set = %d\n", max );
}
When the above code is compiled and executed, it produces the following result −
Max in first set = 50
Max in second set = 501

If you are clear about functions in C programming, then it is easy to understand them in Java as well. Java programming names them as methods, but the rest of the concepts remain more or less same.
Following is the equivalent program written in Java. You can try to execute it to see the output −
public class DemoJava {
   public static void main(String []args) {
      int[] set1 = {10, 20, 30, 40, 50};
      int[] set2 = {101, 201, 301, 401, 501};
      int max;

      /* Process first set of numbers available in set1[] */
      max = getMax(set1);
      System.out.format("Max in first set = %d\n", max );

      /* Now process second set of numbers available in set2[] */
      max = getMax(set2);
      System.out.format("Max in second set = %d\n", max );
   }
   public static int getMax( int set[] ) {
      int i, max;
      max = set[0];
      i = 1;    
      
      while( i < 5 ) {
         if( max <  set[i] ) {
            max = set[i];
         }
         i = i + 1;
      }
      return max;
   }
}
When the above program is executed, it produces the following result −
Max in first set = 50
Max in second set = 501

Once again, if you know the concept of functions in C and Java programming, then Python is not much different. Given below is the basic syntax of defining a function in Python −
def function_name( parameter list ):
   body of the function
   
   return [expression]

Using this syntax of function in Python, the above example can be written as follows −
def getMax( set ):
   max = set[0]
   i = 1   
   
   while( i < 5 ):
      if( max <  set[i] ):
         max = set[i]
      
      i = i + 1
   return max

set1 = [10, 20, 30, 40, 50]
set2 = [101, 201, 301, 401, 501]

# Process first set of numbers available in set1[]
max = getMax(set1)
print "Max in first set = ", max
    
# Now process second set of numbers available in set2[]
max = getMax(set2)
print "Max in second set = ", max
When the above code is executed, it produces the following result −
Max in first set =  50
Max in second set =  501


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