convert string to uppercase in c

1For those of you who want to uppercase a string and store it in a variable (that was what I was looking for when I read these answers).
2
3#include <stdio.h>  //<-- You need this to use printf.
4#include <string.h>  //<-- You need this to use string and strlen() function.
5#include <ctype.h>  //<-- You need this to use toupper() function.
6
7int main(void)
8{
9    string s = "I want to cast this";  //<-- Or you can ask to the user for a string.
10
11    unsigned long int s_len = strlen(s); //<-- getting the length of 's'.  
12
13    //Defining an array of the same length as 's' to, temporarily, store the case change.
14    char s_up[s_len]; 
15
16    // Iterate over the source string (i.e. s) and cast the case changing.
17    for (int a = 0; a < s_len; a++)
18    {
19        // Storing the change: Use the temp array while casting to uppercase.  
20        s_up[a] = toupper(s[a]); 
21    }
22
23    // Assign the new array to your first variable name if you want to use the same as at the beginning
24    s = s_up;
25
26    printf("%s \n", s_up);  //<-- If you want to see the change made.
27}
28//If you want to lowercase a string instead, change toupper(s[a]) to tolower(s[a]).

1#include <ctype.h> // For function below