how to assign uninitialized string value from another string in C++

Question

I am very new to programming. My question may be silly but it will be helpful if someone can guide me.

Please see my code below:

#include <iostream>
#include <string>
using namespace std;

    int main()
    {
        cout << "you have choose to Reverse the text" << endl;

        string inputstring;
        string outputstring;

        cout << "Enter the string you want to reverse" << endl;
        getline(cin, inputstring);
        int n = inputstring.length();

        for (int i = 0; i < n; i++)
        {
            outputstring[i] = inputstring[n - 1 - i]; // problem in this line
        }
}

Till here it works fine inputstring[n - 1 - i] but when I try to assign its value to outputstring. I am getting error.

juanchopanza · Accepted Answer · 2015-10-10 06:31:24Z

3

outputstring is empty, so you're accessing it out of bounds here:

outputstring[i] = inputstring[n - 1 - i];

You have to ensure outputstring has length of at least n by the time you enter the loop. There are different ways of achieving that.

One solution is to create it with size n after reading in to inputstring. Here, we create it filled with *:

std::string outputstring(n, '*');

You can also resize the string after creation:

outputstring.resize(n);

Now you can access outputstring[N] for N in the range [0, n). This makes your loop valid. See this working example.

Alternatively, you could consider reversing inputstring in-place. Note that in real code, this can be easily done with std::reverse:

std::reverse(inputstring.begin(), inputstring.end());

edited Oct 10, 2015 at 6:31

answered Oct 10, 2015 at 5:45

juanchopanza

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Basile Starynkevitch · Accepted Answer · 2015-10-10 06:23:47Z

1

You just use the assign member function of std::string

 outputstring.assign(inputstring);

or you could even simpler use its operator = like

outputstring = inputstring;

Or, if you just want to assign parts of the string, you can insert, replace, append and use resize to resize the string

As a rule of thumb, always read the documentation related to any C++ feature you are using, so if you use std::string you have to read its documentation before starting coding (to be able to choose the right functions for the job)

Reading the first few chapters of a good book on C++ programming before even typing any C++ code will be very helpful.

OF course learn C++11 or C++14, not some older version of the standard.

edited Oct 10, 2015 at 6:23

answered Oct 10, 2015 at 6:09

Basile Starynkevitch

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1 Comment

juanchopanza Over a year ago

OP doesn't want to assign the whole string. They want to assign elements. They are attempting to reverse the string.

fnc12 · Accepted Answer · 2015-10-10 06:39:44Z

0

outputstring is empty. So during accessing to its i'th element you program causes undefined behavior (link, Exception safety section) . Just change last lines

        for (int i = 0; i < n; i++)
        {
            outputstring[i] = inputstring[n - 1 - i]; // problem in this line
        }

with

        outputstring.reserve(n);  // reserve is optional. Use it just for memory allocating optimization..
        for (int i = 0; i < n; i++)
        {
            outputstring += inputstring[n - 1 - i]; // problem in this line
        }

Operator += appends char or string to initial string.

answered Oct 10, 2015 at 6:39

fnc12

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