C++ DO loop being strange

Question

First of all, here is my code:

string text;
do {
   cout << "Enter text: ";
   ws(cin);
   getline(cin, text);
} while (!text.empty());
// do stuff

What I want my code to do ?
- Check if the user input is empty ;
if the userinput = empty, loop to the beginning and he has to enter new text.
If userinput != empty, get out of the loop and continue the program.

What is my code doing ?
- When I enter a text, the loop start from the beginning and I have to retype a text.
- When I do not enter anything and just type enter, my program is indefinitely waiting and I need to Ctrl+C to exit.

You discard whitespace and never check the state of the stream. — AndyG, Aug 25 '15 at 16:07
@Angew do loop : do the code at least one time then check the while, if the while condition is not fulfilled, then re-do the loop. — Amat Erasu, Aug 25 '15 at 16:11
@Soren you answered my second problem. Now the main one is that when I type "enter" the loop do not restart. — Amat Erasu, Aug 25 '15 at 16:12
your call of the `ws` eats the enter so you never get to the end of the while loop by just typing enter -- you are better off not using the `ws` function and just strip the whitespace of the `text` before testing in the while — Soren, Aug 25 '15 at 16:19
`ws` blocks and waits for input; much like `getline` does. It only returns when non-whitespace is entered. Furthermore, I think many implementations only pass on `cin` input after you pressed enter. In short, don't use `ws` on cin. You need other code to strip the whitespace off the input text. — Stein, Aug 25 '15 at 16:22
Also see this question -- http://stackoverflow.com/questions/216823/whats-the-best-way-to-trim-stdstring — Soren, Aug 25 '15 at 16:22
Oh so the problem is ws and I need another code to do what ws do and then what I want to will works ? — Amat Erasu, Aug 25 '15 at 16:36
I was hinting at the same thing as Soren - that your condition was reversed. — Angew is no longer proud of SO, Aug 25 '15 at 16:36

Shoe · Answer 1 · 2015-08-25T16:15:03.607

3

It's not a repeat-until, but a do-while loop, use:

string text;
do {
   cout << "Enter text: ";
   ws(cin);
   getline(cin, text);
} while (text.empty());
//      ^^

Basically if the condition is true then it loops again, otherwise it breaks the loop. In your case it's looping if the string is not empty, and breaking the loop if the string is empty.

edited Aug 25 '15 at 16:15

answered Aug 25 '15 at 16:12

Shoe

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Thanks for the answer, but that answer my second problem. Now I need the loop to restart when the userinput is empty -- for example user directly type "enter" – Amat Erasu Aug 25 '15 at 16:14
When text is empty and when he just type enter. – Amat Erasu Aug 25 '15 at 16:17
@AmatErasu That's what the above code is doing. It will restart the loop when the user input is empty. How else would the user send an empty text without pressing "enter" while having no input entered? – Shoe Aug 25 '15 at 16:17
I tried this code and when I type just enter on my keyboard, then program is not exiting nor reloading the loop. – Amat Erasu Aug 25 '15 at 16:35
If it doesn't exit, why do you think it's not reloading the loop? – Lightness Races in Orbit Aug 25 '15 at 17:24

C++ DO loop being strange

1 Answers1