C program to show the number of times different letters appear in a string

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

int main()
{
    char a[100];
    int count=0;
    int i,j;
    printf("Enter any sentence:");

    gets(a);

    printf("Frequencies of letters in the input string:\n");

    for(i=0; i<strlen(a); i++)
    {

        if(a[i]>='A' && a[i] <='Z'|| a[i]>='a' && a[i]<= 'z')
        {
            count=1;

        for(j=i+1; j<strlen(a); j++)
            {
                if(toupper(a[i])==toupper(a[j]))
                {

                    a[j]=' ';
                    count++;
                }
            }

            printf("%c %c: %d\n ", toupper(a[i]),tolower(a[i]), count);
        }


    }



    return 0;
}