C++ output of string.size() not correct

Question

I wrote the following code:

#include <iostream>
#include <time.h>
#include <string.h>

int main()
{
    std::string alphabet = "abcdefghijklmnopqrstuvwxyz";
    std::string word;

    unsigned long long int i = 0;
    srand(time(NULL));
    while(true)
    {
        int randomIndex = rand() % 27;
        word += alphabet[randomIndex];

        // If the word might be the desired word, "hi"
        if (word.size() == 2) 
        {
            // The word should only be printed if its length is equal to 2
            std::cout << i << " : " << word << word.size() <<  std::endl;
            if (word == "hi") 
            {
                break;
            }
            else // The word isn't "hi". We reset the variable word and continue looping
            {
                word = "";
            }
            i += 1;
        }

    }
    return 0;
}

It is supposed to put together random letters until word is equal to "hi". Until that, only 2-character words should be printed, but for some reason the program seems to be thinking that 1-character words have a length of 2. Therefore it also prints 1-character words.

Can anyone please help me?

You're generating indices 0 to 26 randomly, but at index 26 there's the terminating 0 char. std::string::size returns the number of chars appended, even in one of them is non-printable such as the terminating 0 char. — fabian
– fabian, Commented Sep 4, 2022 at 12:40
exactly, std::string is for good reason not a C-style string in terms of what its length is defined to be. — Marcus Müller
– Marcus Müller, Commented Sep 4, 2022 at 12:43
int randomIndex = rand() % 27; should be int randomIndex = rand() % 26; by using 27 you are going 1 position past the z stored in the string. Thankfully this value is \0 — drescherjm
– drescherjm, Commented Sep 4, 2022 at 13:55

hun · Accepted Answer · 2022-09-04 14:27:33Z

Let's change the print debug line as follows.

std::cout << randomIndex << '\t' << i << " : " << word << " --> " << word.size() <<  std::endl;

Here, when we check the strings that look like a single character but 2 characters, we come across them in a random number of 26.

The length of the string "abcdefghijklmnopqrstuvwxyz" is 26. Since the indices of the arrays start from 0, they must be in the range of random numbers [0, 25] we produce.

Then let's update the line of code where we generate the random number as follows.

int randomIndex = rand() % 26

In the wrong code, the 26th character corresponds to the memory cell at the end of the string. Let's take a different example to understand.

int main()
{
    char arr[5];
    std::string a = "a";
    a+=arr[1];
    std::cout << a << " " << a.size() << '\n'; // 2
    
    a+=arr[5];
    std::cout << a << " " << a.size() << '\n'; // 3

    a+=arr[6];
    std::cout << a << " " << a.size() << '\n'; // 4
    
    return 0;
}

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