Program:

/*C program to count uppercase and lowercase 
characters in a string.*/

#include <stdio.h>

int main()
{
    char str[100];
    int countL, countU;
    int counter;

    //assign all counters to zero
    countL = countU = 0;

    printf("Enter a string: ");
    gets(str);

    for (counter = 0; str[counter] != NULL; counter++) {

        if (str[counter] >= 'A' && str[counter] <= 'Z')
            countU++;
        else if (str[counter] >= 'a' && str[counter] <= 'z')
            countL++;
    }

    printf("Total Upper case characters: %d, Lower Case characters: %d", countU, countL);

    return 0;
}

Output

    Enter a string: This is a Test String.
    Total Upper case characters: 3, Lower Case characters: 14