Finding letters in a string while ignoring everything else

Question

For my homework, part of what I need to do is take a phrase from the user, and from there take only the letters in the phrase, ignoring numbers, spaces, and special characters. Once I find letters in the string, I need to store them into a separate variable. However, I can't get that variable to store anything outside of the if statement that looks for letters.

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string line, temp;
    cout << "Enter phrase to check:  ";
    getline(cin, line);
    
    for(int i = 0; i < line.size(); i++)
    {
        if((line[i] > 64 && line[i] < 91) || (line[i] > 96 && line[i] < 123))
        {
            temp[i] = line[i];
        }
    }

    cout << temp;
    return 0;
}

When I run the program, temp outputs nothing. But I know the if statement is correctly finding letters, from making it print line[i] inside the if statement.

How should I increment the index? At the end of the if statement? or after the if statement in the for loop? — Patrick Trinidad, Feb 28 '21 at 22:39
@PatrickTrinidad At the end of the if statement. But does string's bracket operator works for setting? You might want to do `temp += line[i]` or `temp.push_back(line[i])`. — Moige, Feb 28 '21 at 22:46

score 3 · Accepted Answer · answered Feb 28 '21 at 22:41

3

your temp variable is an empty string. temp[x] is telling the compiler to change the x-th character of that string(which doesn't make any sense, as the string doesn't have any characters!). You're lucky(or unlucky) that you aren't getting any Segmentation faults(crashes).

Just use the += operator:

temp += line[i];

answered Feb 28 '21 at 22:41

Nikita Demodov

553
5
17

Ah, I didn't know that having an empty string did that. Thank you very much, doing += worked. I appreciate you taking time to help me. – Patrick Trinidad Feb 28 '21 at 22:43
@PatrickTrinidad If an answer helped you and answered the question, you can mark it as `accepted` by clicking the checkmark on the left :). – Nikita Demodov Feb 28 '21 at 22:44

score 2 · Answer 2 · edited Mar 01 '21 at 02:14

2

Try

temp.push_back(line[i]);

It will work.

edited Mar 01 '21 at 02:14

cigien

57,834
11
73
112

answered Feb 28 '21 at 22:40

maria maria

66
5

TheChubbyPanda · Answer 3 · 2021-02-28T23:02:47.597

The way you're currently doing it (temp[i] = line[i];) means that each non special character in line will be placed at the same index in temp. This should usually fail since temp (a string) does not resize on indexing.

For changing the size of a string, there exists a function called string::push_back as detailed here.

Instead of indexing using temp[i], you would instead use temp.push_back(line[i]);

This function allows the string to resize itself to accommodate the new char if need be and won't throw a segmentation fault.

NB: std::string::push_back is designed to append a single char to a string. There exist multiple other ways of doing this, including Nikita Demodov's answer which shows the use of the += operator which is much more lenient and will allow appending of other strings etc. push_back is most common to the std::vector where it is used to append single items to the list.

Finding letters in a string while ignoring everything else

3 Answers3