Neverending loop in C++, that compares two char's

Question

I have this:

#include <iostream>
#include <conio.h>

int main ()
{
     char str1[] = "abc123";
     char str2[] = "abc123";
     do
     {
          std::cout << "String: ";
          std::cin >> str2;
     } while (str1 != str2);
     std::cout << "str1 is equal to str2, press any key to quit.\n";
     _getch();
     return 0;
}

The program should end when str1 is equal to str2, and str2 is a value set by the user. The problem is, even if I type the same value as str1, it keeps looping, it doesn't matter if I put the same contents as in str1 or not.

Answer 1

Use std::string instead of raw arrays and pointers, in order to avoid silly mistakes like opening up for buffer overflows, comparing pointers instead of strings, and so on.

---

#include <iostream>
#include <string>

int main ()
{
     std::string str1 = "abc123";
     std::string str2 = "abc123";
     do
     {
          std::cout << "String: ";
          std::cin >> str2;
     } while (str1 != str2);
     std::cout << "str1 is equal to str2.\n";
}

Answer 2

You are not properly comparing the two strings, you are comparing the addresses of the two char arrays.

You have to use the strcmp function.

while(strcmp(str1,str2)!=0)

Or use the std::string class, which allows you to use the overloaded operator == in order to compare strings.

Answer 3

str1 and str2 are char arrays, which really means they are pointers to char arrays. Their values are constant, and will never be equal. Replace the declarations with:

 string str1 = "abc123";
 string str2 = "abc123";

and you will get better results. (but will need header)

Answer 4

You are comparing the pointers.

Use strcmp rather.

Moreover, using cin is unsafe as left may be too small.

Modify as

} while(strcmp(str1, str2))

Answer 5

In the main function stack frame when you are allocating two array:

The two arrays get two different fixed addresses which never changes !

Even if the contents of the array are changed everytime !

char str1[] = "abc123";  // str1 gets an unique fixed address  say 1000
char str2[] = "abc123";  // str2 gets an unique fixed address  say 1007

Thus the statement while (str1 != str2); keeps on comparing the address 1000 and 1007 and

thus the loop condition becomes true all the time .