Added Even Odd in C Article

projects/even-odd/_posts/2020-10-06-c.md

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+---
+title: Even Odd in C  
+layout: default  
+date: 2020-10-05
+featured-image:  
+tags: [c, even-odd]  
+authors:
+- shubhragupta-code
+---
+
+In this article, we will check whether an integer is even or odd using C.
+
+## How to Implement the Solution
+Let's take a look on code for this program in C.
+
+```c
+#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
+```c
+#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.&nbsp;
+
+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:
+```console
+./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
+To run a C program, we need to install GCC compiler and run the following command in terminal.
+```console
+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]
+
+## Further Reading
+- [Hello World in C][4] on The Renegade Coder
+
+[2]: https://www.tutorialspoint.com/c_standard_library/c_function_atoi.htm
+[3]: https://www.tutorialspoint.com/c_standard_library/c_function_strcmp.htm
+[4]: https://www.tutorialspoint.com/c_standard_library/c_function_printf.htm
+[5]: https://www.onlinegdb.com/online_c_compiler
+[6]: https://repl.it/languages/C
+[7]: https://therenegadecoder.com/code/hello-world-in-c/
+[8]: https://en.wikipedia.org/wiki/Include_directive