How can I count the length of char in a char array without using any loop?

so i was practicing my c language and there i come to a need that how can i count the length of character in a character array because when i use sizeof() in that i got the whole length of array..although i can use "for loop" but i can't use that in that question so is there any other way to do this..

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

int main()
{
    int i,N;
    char a[20];
    printf("enter the string");
    scanf("%s",a);
    puts(a);
    N = sizeof(a); 
    printf("%d",N);
    return 0;
}

OUTPUT:-

enter the string                                                                                     
helloguys                                                                                            
helloguys                                                                                            
20     

This can be achieved in 2 ways:

1) Using strlen() function defined in string.h:

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

int main()
{
    int i,N;
    char a[20];
    printf("enter the string");
    scanf("%s",a);
    puts(a);
    N = strlen(a); 
    printf("%d",N);
    return 0;
}

Be aware that, strlen() uses loop internally. According to the definition of strlen() in string.h library, the possible implementation as per cppreference.com is:

std::size_t strlen(const char* start) {
    const char* end = start;
    while(*end++ != 0);
    return end - start - 1;
}

Therefore the second way without using using loops directly or indirectly can be recursion.

2) Using recursion:

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

int LengthofString(char *string) 
{  
    if (*string == '\0')  
        return 0; 
    else
        return 1 + LengthofString(string + 1); 
}

int main()
{
    int i,N;
    char a[20];
    printf("enter the string");
    scanf("%s",a);
    puts(a);
    N = LengthofString(a);
    printf("%d",N);
    return 0;
}

PS: Beware about the predefined library functions and their internal implementations before using them.