Why does this code for reversing a string not work in C++?

Question

string binary = "12345";
string binary2 = "";
size = binary.size() - 1;
for (i = size; i >= 0; i--)
{
    binary2[size - i] = binary[i];
}
cout << binary2;

I keep getting a subscript error during run time. But, I don't know what the problem is. I would appreciate an explanation if possible.

Because `binary2` is an empty string? Anyway, `string binary2(binary.rbegin(), binary.rend());`. — juanchopanza, Feb 26 '14 at 12:59
binary2.resize(binary.size()); http://www.cplusplus.com/reference/string/string/resize/. With that said, please use @juanchopanza s method. — dutt, Feb 26 '14 at 13:01
if `i` is unsigned as `std::size_t`, you got a infinite loop with UB inside. — Jarod42, Feb 26 '14 at 13:14

Lightness Races in Orbit · Accepted Answer · 2014-02-26T13:09:13.397

8

binary2 is empty. It has no characters in it. Accessing characters with [] does not magically create character that do not already exist: it only changes the value of existing characters. You are trying to change the value of characters that do not exist.

You can either initialise binary2 to " ", like this:

string binary = "12345";
string binary2(binary.size(), ' ');
// now your loop

or you can do string reversing properly:

string binary = "12345";
string binary2(binary.rbegin(), binary.rend());

edited Feb 26 '14 at 13:09

answered Feb 26 '14 at 13:02

Lightness Races in Orbit

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A more rigid option for #1 in case `binary` changes would be `string binary2(binary1.size(), '\0');` (or another character of your choosing). – chris Feb 26 '14 at 13:05
I haven't learned that yet. Thanks for the addition. Out of curiosity, is their a way to initialize a string to a certain size that is given in a number and can change. Because if I used spaces and binary changed to 123456. It wouldn't work, – user29568 Feb 26 '14 at 13:06
@user29568: Why are you not using any documentation? There is a string constructor to do this. http://en.cppreference.com/w/cpp/string/basic_string/basic_string – Lightness Races in Orbit Feb 26 '14 at 13:08
@LightnessRacesinOrbit To be honest, our teacher just throws things on the board and doesn't really explain things. So, I haven't learned that either. I use Programming and Principles using C++ by Stoustrup to study. But, I haven't reached that far in yet. – user29568 Feb 26 '14 at 13:11
Thank You for offering a full explanation and answer. – user29568 Feb 26 '14 at 13:15

score 2 · Answer 2 · edited May 23 '17 at 12:10

2

First of all your binary2 string is empty. This could be fixed by appending to it instead. However a little bit of searching led me to the following answer by Greg Rogers found here: https://stackoverflow.com/a/198210/2299061

#include <algorithm>
std::reverse(str.begin(), str.end());

edited May 23 '17 at 12:10

Community

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answered Feb 26 '14 at 13:13

This

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That is for reversing a string in place, but OP wants a second string to be a reversed version of a first one. In this case, the reverse operator initialization seems more efficient. But +1, `std::reverse` is often overlooked. – juanchopanza Feb 26 '14 at 13:15

score 0 · Answer 3 · edited Feb 26 '14 at 13:13

0

string binary2 = "" should be changed to string binary2 = " ";

edited Feb 26 '14 at 13:13

juanchopanza

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answered Feb 26 '14 at 13:12

gtv

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Yes, we've covered that. – Lightness Races in Orbit Feb 26 '14 at 13:13

Why does this code for reversing a string not work in C++?

3 Answers3