length of string c

#include <stdio.h>
#include <string.h>
int main()
{
    char a[20]="Program";
    char b[20]={'P','r','o','g','r','a','m','\0'};

    // using the %zu format specifier to print size_t
    printf("Length of string a = %zu \n",strlen(a));
    printf("Length of string b = %zu \n",strlen(b));

    return 0;
}

int main() { char a[100]; int length;
printf("Enter a string to calculate its length\n"); gets(a);
length = strlen(a);
printf("Length of the string = %d\n", length);
return 0; }