convert string from upper cas to lower case in C code example

Example 1: convert string to uppercase in c

For 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).

#include <stdio.h>  //<-- You need this to use printf.
#include <string.h>  //<-- You need this to use string and strlen() function.
#include <ctype.h>  //<-- You need this to use toupper() function.

int main(void)
{
    string s = "I want to cast this";  //<-- Or you can ask to the user for a string.

    unsigned long int s_len = strlen(s); //<-- getting the length of 's'.  

    //Defining an array of the same length as 's' to, temporarily, store the case change.
    char s_up[s_len]; 

    // Iterate over the source string (i.e. s) and cast the case changing.
    for (int a = 0; a < s_len; a++)
    {
        // Storing the change: Use the temp array while casting to uppercase.  
        s_up[a] = toupper(s[a]); 
    }

    // Assign the new array to your first variable name if you want to use the same as at the beginning
    s = s_up;

    printf("%s \n", s_up);  //<-- If you want to see the change made.
}
//If you want to lowercase a string instead, change toupper(s[a]) to tolower(s[a]).

Example 2: convert string to lowercase in c

// include all the libraries  used in the program.
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

// Base Function
int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
    }
    else if (strlen(argv[1]) != 26)
    {
        printf("Key must contain 26 characters.\n");
    }
    
    string key = argv[1].toupper();
    string key = argv[1].tolower();
}