How to turn an int into a string?

Question

I've been trying to find how to do it online. I'm restricted from not using ready-made functions, like to_string or boost::lexical_cast, not even the <sstream> library. How can I do it with these limitations?

Does this answer your question? Easiest way to convert int to string in C++ — JHBonarius
– JHBonarius, Commented Sep 10, 2020 at 19:42
please learn to use the search function of this site or Google. — JHBonarius
– JHBonarius, Commented Sep 10, 2020 at 19:43
One digit at a time? I would guess whoever gave you the assignment with the restrictions also gave some guidance on how to start. — Retired Ninja
– Retired Ninja, Commented Sep 10, 2020 at 19:43
you have a stupid teacher ( I hate teachers that act they are teaching you C++, but don't allow you to actual use C++.). you can try char and doing '0'+i per digit... i.e. using the char for 0 and adding an offset. — JHBonarius
– JHBonarius, Commented Sep 10, 2020 at 19:53
To be fair, I had to implement the equivalent manually once for a real-life program. Around 2005 or so. For underpowered cell phones. — Karl Knechtel
– Karl Knechtel, Commented Sep 10, 2020 at 20:02

catnip · Accepted Answer · 2020-09-10 20:12:42Z

2

Here's one way to do it:

std::string int_to_string (int i)
{
    bool negative = i < 0;
    if (negative)
        i = -i;
    std::string s1;

    do
    {
        s1.push_back (i % 10 + '0');
        i /= 10;
    }
    while (i);
    
    std::string s2;
    if (negative)
        s2.push_back ('-');
    for (auto it = s1.rbegin (); it != s1.rend (); ++it)
        s2.push_back (*it);
    return s2;
}

I avoided using std::reverse, on the assumption that it would be off-limits.

Live demo

answered Sep 10, 2020 at 20:12

catnip

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2 Comments

Mike Vine Over a year ago

This has UB when i == INT_MIN.

JHBonarius Over a year ago

With teachers like his, iterators could as well be off limits, lol. Or std::string, and he'd want you to dynamically allocate a char array, hahaha.

JHBonarius · Accepted Answer · 2020-09-10 20:06:45Z

1

You can use '0' + i to get the char value of 0 and offset it. I.e.

#include <cmath>
#include <string>
#include <iostream>

int main() {
    int number = 12345678;

    int nrDigits = std::log10(number)+1;

    std::string output(nrDigits, '0'); // just some initialization

    for (int i = nrDigits - 1; i >= 0; i--) {
        output[i] += number % 10;
        number /= 10;
    }

    std::cout << "number is " << output << '\n';
}

answered Sep 10, 2020 at 20:06

JHBonarius

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2 Comments

catnip Over a year ago

Problems if number <= 0. I would count the digits a different way.

JHBonarius Over a year ago

@PaulSanders exercise for the reader ;) of course this is not the best solution. But it's not my assignment and i would just use to_string.

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