How to get string input n times?

Question

I have a C++ program. I want to get a number from the user (t) and force the user to enter line t times but the program's execution terminates after 1 iteration. This is the code:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    string str;
    int t;
    cin >> t;
    for (int i=0; i< t; i++) {
        getline(cin, str);
        cout << str;
    }
    return 0;
}

Can anyone explain me why this happening and how to solve it? Thank you my friends.

Hmmm, it will iterate `t-1` times (`-1` because newline character is not consumed by `cin >> t;`), not 1 times. ([demo](https://wandbox.org/permlink/Aao7gTXm0bWdBCwG)) — MikeCAT, Dec 13 '19 at 16:05

score 3 · Answer 1 · answered Dec 13 '19 at 16:11

The newline character is still in the buffer when you do cin >> t so the next line you read will be blank. When you mix formatted input (>>) and unformatted (std::getline) you often get in situations like this and you need to take measures when switching to unformatted input. Example remedy:

#include <iostream>
#include <limits>
#include <string>

using namespace std;

int main() {
    string str;
    int t;
    cin >> t;
    cin.ignore(numeric_limits<streamsize>::max(), '\n'); // skip the rest of the line

    for(int i = 0; i < t; i++) {
        if(getline(cin, str))    // check that the getline actually succeeded
            cout << str << '\n';
        else
            break;
    }
    return 0;
}

Nick Reed · Accepted Answer · 2019-12-13T16:33:11.427

2

When you enter your first character (the times to repeat), a character is left in the cin buffer - newlines are not consumed by cin >>. As a result, getline(cin, str) reads this character and takes it as the first input, which then empties the buffer out and lets you enter the others.

You can clear the buffer with std::cin.ignore(1); to remove that trailing character - this lets your code run as anticipated. Why not just use cin >> str, though? That solves the problem and avoids a call to getline.

#include <iostream>
#include <string>

using namespace std;

int main()
{
    string str;
    int t;
    cin >> t;

    //clear one character out of buffer
    cin.ignore(1);

    //note that 1 is used for demonstration purposes
    //in development code, INT_MAX, numeric_limits<streamsize>::max(),
    //or some other large number would be best, followed
    //by std::cin.clear()

    for (int i=0; i< t; i++) {
        cout << "input: ";
        //you could use cin >> str; instead of getline(cin, str);
        getline(cin, str);
        cout << "got: " << str << std::endl;
    }
    return 0;
}

Demo

edited Dec 13 '19 at 16:33

answered Dec 13 '19 at 16:10

Nick Reed

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Thanks. it works – Ali Abbasifard Dec 13 '19 at 16:17
@AliAbbasifard `ignore(1)` works when the user is nice and doesn't enter anything more on the line. Try entering `2x`. – Ted Lyngmo Dec 13 '19 at 16:25
@TedLyngmo solid point; I'd used `ignore(1)` for the sake of example, but it'd be better to do `ignore(INT_MAX)` or something similar. Will edit code accordingly. – Nick Reed Dec 13 '19 at 16:30
Make it `numeric_limits::max()` to be sure :-D – Ted Lyngmo Dec 13 '19 at 16:31
1

@TedLyngmo Wasn't even aware that was a thing, I've always been using `INT_MAX`. Learn something new every day! – Nick Reed Dec 13 '19 at 16:35
1

It's the "_unspecified_" part of `typedef /*unspecified*/ streamsize;` that makes it a little safer ... as if a user would ever go that nuts :-) – Ted Lyngmo Dec 13 '19 at 16:37

How to get string input n times?

2 Answers2