how to find length for string in c

1#include <stdio.h>
2#include <string.h>
3int main()
4{
5    char a[20]="Program";
6    char b[20]={'P','r','o','g','r','a','m','\0'};
7
8    // using the %zu format specifier to print size_t
9    printf("Length of string a = %zu \n",strlen(a));
10    printf("Length of string b = %zu \n",strlen(b));
11
12    return 0;
13}

1// include all the libraries used in the program.
2#include <stdio.h>
3#include <string.h>
4
5// Calculate Length of String Using strlen() Function
6int main() 
7{ 
8    char a[100]; int length;
9	printf("Enter a string to calculate its length\n"); gets(a);
10	length = strlen(a);
11	printf("Length of the string = %d\n", length);
12	return 0; 
13}
14
15// Calculate Length of String without Using strlen() Function
16int main() 
17{
18    char s[] = "Programming is fun";
19    int i;
20
21    for (i = 0; s[i] != '\0'; ++i);
22    
23    printf("Length of the string: %d", i);
24    return 0;
25}