Even Odd in C

Welcome to the Even Odd in C page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{    
    if (argc == 1 || argv[1][0] == '\0' || (atoi(argv[1]) == 0 && strcmp(argv[1], "0") != 0)) {
        printf("Usage: please input a number\n");
    } else {
        int input = atoi(argv[1]);
        if (input % 2 == 0) {
            printf("Even\n");
        }
        else {
            printf("Odd\n");
        }
    }

    return 0;
}

Even Odd in C was written by:

• AceKnight
• Jeremy Grifski

If you see anything you'd like to change or update, please consider contributing.

Note: The solution shown above is the current solution in the Sample Programs repository as of Mar 30 2019 16:30:59. The solution was first committed on Oct 21 2018 19:46:02. As a result, documentation below may be outdated.

How to Implement the Solution

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{    
    if (argc == 1 || argv[1][0] == '\0' || (atoi(argv[1]) == 0 && strcmp(argv[1], "0") != 0)) {
        printf("Usage: please input a number\n");
    } else {
        int input = atoi(argv[1]);
        if (input % 2 == 0) {
            printf("Even\n");
        }
        else {
            printf("Odd\n");
        }
    }

    return 0;
}

Let's understand this code block by block.

Main Function

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) {

In the first three lines, we are including header files using [include directive][8] to utilise some functions defined in header files later in the program. Here, Standard Input/Output header file(<stdio.h>) is called to use printf() function, C Standard Library(<stdlib.h>) to use atoi() function and <string.h> to use strcmp() function. 

In C, we declare a function using general form:

return_type function_name(parameter){
  ...
}

So, we are declaring main function with return_type integer and argc and argv as parameters to access command line arguments. argc and argv are variables which main function will get when run in command-line. argc stores argument count while argv stores array of strings that are arguments. This should be kept in mind that all command-line arguments are stored as strings.

argv[0] represents first argument which always is equal to name of our program. If we type the following command in terminal:

./even-odd 2

Here, ./even-odd represents argv[0] and 2 represents argv[1].

Control Flow

Before we move onto the control flow, let's look on the functions which we called from header files. [atoi()][2] converts argument string into an integer. [strcmp()][3] compares two strings and [printf()][4] prints formatted string as output.
For the if-else statements, first if condition checks if the argument count is 1 or if the argument provided is only a null string or if the argument is equal to 0(integer). For that it prints correct usage pattern.
Otherwise the value of argv[1] is converted into integer and stored in variable input. % represents modulo operator which gives remainder for integer division. So, in case of input % 2, 0 as remainder shows that input is divisible by 2 and non zero remainder shows indivisibility. Thus, prints Even or Odd depending on input for the next if statement.
Return value is called exit code. So, 0 as exit code represents zero error.

How to Run the Solution

gcc -o even-odd even-odd.c
./even-odd

Another handy option is to compile and run using online C Compiler such as [OnlineGDB][5], [Repl][6]