5.2. Communicate from a function

In the previous section, we discussed an example where functions helped in dividing our problem into sub-problems. Functions make it easy to think of problems. The functions we discussed in the previous section did not return any information back to the caller function. In this section, we discuss problems that require returning a value to the caller function.

5.2.1. Return a non-void variable type

For example, we want to write a function that calculates the factorial of a number. The function is to take an input parameter \(n\), calculate the value of \(n \times (n -1) \times (n-2) .... 3 \times 2 \times 1\), and returns it to the calling function. The following figure shows the code along with the order of execution. Download factorial.c if you want to run the program yourself.

Fig. 5.6 Order of execution in a code that gets the factorial.

1. As always, the execution starts from the main function.

2. factorial function takes in 4 as an input.

3. The variable n in factorial function is assigned the value of 4.

4. The function factorial returns the value of 24 to the caller function, which is the main function.

5. factorial(number) now is evaluated as 24.

6. The value of result is set to the evaluated value of factorial(number), which is 24.

Output

Factorial of 4: 24.

5.2.2. Summary of syntax of a program with functions

To summarize the syntax of a C program that has functions, in the following figure you will find the skeleton of a C program that has a non-void and void return function.

Fig. 5.7 Basic skeleton of a program that uses functions.

The order of the parameters passed to a function MUST be the same as the order of the parameters in the header of the function.

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