C :: Program Where User Inputs A String And Prints Out Length Of String

I would like to understand a function on strings. Below is a code that I took from my teacher where the user inputs a string and prints out the length of the string.

Code:
#include <stdio.h>
#include <string.h>
int main() {
char str[100];
int i = 0;

[Code] ....

Now I understand that it returns the count in "int" so my question is:

Let's say i declared

Code: int count = 0;
at the beginning of the code and then made
Code: count = strlen(str);
why wouldn't i have the same result? Is there a way to do it also?