How to return a string when I have a function that gives an array

Question

We were asked to make function that that converts Decimal numbers into Binary and takes an integer as an input but must give a string as an output. How do I store the array into one string?

string DecToBin(int num)
{

    string res;

    for (int n = 15; n >= 0; n--)
    {
        if ((num - pow(2, n)) >= 0)
        {
            res[n] = 1;
            num -= pow(2, n);
        }
        else
        {
            res[n] = 0;
        }

    }

    for (int n = 15; n >= 0; n--)
    {
        res[n];
    }

}

When you declare the string it's of size zero, which means doing e.g. `res[n]` will be indexing out of bounds for *every* `n`, leading to [*undefined behavior*](http://en.wikipedia.org/wiki/Undefined_behavior). However, if you read e.g. [this `std::string` reference](http://en.cppreference.com/w/cpp/string/basic_string) I'm sure you can find something which *appends* characters to a string. — Some programmer dude, Dec 13 '14 at 11:40
And also, `res[n] = 1` is wrong you need to assign to a character like this `res[n] = '1'` — Iharob Al Asimi, Dec 13 '14 at 11:42
Also, while I'm sure the `pow` function have a special case when the first argument is `2`, the usual way is to use the shift operator, e.g. `1 << n`. Also, do you have a check that the number you get as argument is no more than 16 bits? On most platforms today, `int` is 32 bits. — Some programmer dude, Dec 13 '14 at 11:47
@JoachimPileborg I'm new to the shift operator, and no I currently don't have any function to check if the argument is more than 16 bits, but I will work on that later — Renz, Dec 13 '14 at 11:56

score 1 · Answer 1 · answered Dec 13 '14 at 11:43

1

You just return the string

return res;

that's it.

answered Dec 13 '14 at 11:43

Iharob Al Asimi

52,653
6
59
97

1

But take into consideration the comments in your original post. – Iharob Al Asimi Dec 13 '14 at 11:45
oh thank you! it worked!! but now the answer is in reverse order :O – Renz Dec 13 '14 at 11:54
that could be a different question, i have seen many algorithms to convert an `int` to a binary string representation but not yours. I suggest you learn about [Bitwise Operators](http://en.wikipedia.org/wiki/Bitwise_operation) – Iharob Al Asimi Dec 13 '14 at 11:56
Yes I actually tried using assign function, it also worked but it returns some thrash values with it – Renz Dec 13 '14 at 11:57
using bitwise operators still gives me a reverse answer when I try to output it – Renz Dec 13 '14 at 12:16
Ask a different question then. And also, are you aware of endiannes? – Iharob Al Asimi Dec 13 '14 at 12:19
Ok, I'll do that, and nope I don't know about endiannes. – Renz Dec 13 '14 at 12:22

score 1 · Answer 2 · answered Dec 13 '14 at 12:49

You could do something like this,

#include <iostream>
#include <string>

std::string foo(int n)
{
    std::string s;
    while(n > 0){
        s += n%2 == 0 ? '0' : '1';
        n = (n >> 1);
    }
    return std::string(s.rbegin(), s.rend());
}

int main()
{
    int n = 13;
    std::cout << foo(n) << std::endl;
    return 0;
}

Prints

score 1 · Answer 3 · answered Dec 13 '14 at 14:00

As you have begotten answer for how to return the result I add a supplement on the generation of the string.

As an alternative you can include <climits> which gives you CHAR_BIT and then calculate number of bits by e.g.:

#include <climits>

int bits = CHAR_BIT * sizeof(int);

or you can use <limits> and say something like:

#include <limits>

int bits = std::numeric_limits<int>::digits;

Then it is only to start the loop from the end:

string int2bitstr(int n) 
{
    int bits = numeric_limits<int>::digits;
    string s;
    int i;

    for (i = bits - 1; i >= 0; --i)

And say something like:

        s += n & (1 << i) ? '1' : '0';

or:

        s += (n >> i) & 1 ? '1' : '0';

or the bit (no pun intended) quicker:

        s += ((n >> i) & 1) + '0';

Reason the last one work is because one add character value of the glyph 0 to the number 0 or 1 – in effect giving '0' or '1'.

score 0 · Answer 4 · answered Dec 13 '14 at 13:19

0

Just add

return res;

to the end of your function.

answered Dec 13 '14 at 13:19

Mohammad Alshaar

523
4
21

How to return a string when I have a function that gives an array

4 Answers4