What's the difference between these two given examples

Question

I'm a beginner in C++ and programming itself actually. I just want to ask, What's the difference between these 2 examples. What is the difference between "len = strlen(str1)-1" and "i = strlen(str1)-1"

Top part of the code will be like this:

#include <iostream>
#include <string.h>
using namespace std;

int main()
{
    char str1[20],str2[20];
    int c, i ,j, len;
    
    cout<<"Enter a word: ";
    cin.getline(str1, 20);

Example 1:

//reverse
for (i = strlen(str1)-1, j = 0; i >= 0; i--, j++){
    str2[j] = str1[i];      
}
//compare string
c = strcmp(str1, str2);

/*This does not work because the value of 'c' will be -1 if the input
is "lol" which is palindrome*/

and Example 2:

//reverse
len = strlen(str1)-1;
for (i = len, j = 0; i >= 0; i--, j++){
    str2[j] = str1[i];      
}

//compare string
c = strcmp(str1, str2);

/*This does work in other hand, because of the variable "len"*/

the rest of the code will be like this

if(c == 0){
    cout<<"It is a Palindrome";
}
//if the value of C is !=0
else{
    cout<<"It is not a Palindrome";
}

}

Why is that? Thanks in advance for those who will answer. :)

Answer 1

Both examples are same except first uses an extra variable len.

This code is actually reversing the string. If str1 contains "123" then str2 will contain "321".

Function strlen(str1) returns the length of str1 but in C++ index of Arrays start from 0 that is why the last element index will be one less than length, hence strlen(str1) - 1.

UPDATE

Even with updated information the answer to first question remains same that both examples are same in nature. Difference in results is a mare co-incident due to a reason explained below.

char str1[20],str2[20];

This code creates two array of 20 char but not initialized. This means the initial values can be random.

Now when you call cin.getline(str1, 20); it not only writes the string you entered but adds a terminating '\0' character at the end of it. Our reversing logic only reverse the string but does not insert terminating '\0' at the end of str2 which means str2 is much longer (until it finds a '\0') than str1. Due to this they never compare correctly.

A simple solution to this issue can be zero-filling the arrays before using them and in C++ there is a simple way to do that:

char str1[20] = { 0 }, str2[20] = { 0 };

It is always a good practice to zero-fill your arrays if you are going to use then as strings.