Expression result unused in Vigenere

Question

I am getting this error: expression result unused and I don't know what that means. I use the code it's pointing to (green underline), just like I do in my Caesar cipher which works:

The other posts on here about this error are not helping me, because my code seems to be correct (to me).

Here is a portion of my code:

    string key = argv[1];
    string plain_text = GetString();
    int n = strlen(plain_text);
    int m = strlen(key);

    for (int i = 0, j = 0; i < n, j < m; i++, j++)
    {
        if (islower(plain_text[i]) && isalpha(plain_text[i]))
        {   
             // Change ASCII to alphabet, apply wrap formula, then back to ASCII
             plain_text[i] = (((plain_text[i] - 'a') + (key[j] - 'a')) % 26) + 'a';
        }
        else if (isupper(plain_text[i]) && isalpha(plain_text[i]))
        {
             // Change ASCII to alphabet, apply wrap formula, then back to ASCII
             plain_text[i] = (((plain_text[i] - 'A') + (key[j] - 'A')) % 26) + 'A';
        }

    } // End of for loop

Answer 1

If you want to iterate in accordance with multiple conditions, then you should do something like this

for ( initialization ; test_condition ; re-initialization )
{
      // body of the loop
}

The test_condition should be a logical expression which should either return true or false. For eg.

test_condition = check1 *logical operator* check2 *logical operator* check3...    and so on

However , is not a logical operator. See how to do check multiple conditions simultaneously here.

The thing i < n, j < m is not a valid logical expression, you need something like i < n || j < m (if you want to execute the loop for even any one condition to be true) or i < n && j < m (if you want the loop to execute only and only iff both the conditions are true).