Cin in a while loop condition in C++

Question

The following code is suppose to grab the first word of a sentence (if there's one) and print it letter by letter (The printing part works). The problem that I've encountered is that whenever I input a sentence in the prompt, it grabs every word in the sentence and not the first word (cin is suppose to stop at the first space,enter, etc... so I think the part that is wrong here is the while loop). How can I fix the issue? I think there's something I'm not understanding.

#include <iostream>
using namespace std;

int main(void){
    int i = 0;
    int length= 25;
    string word[length];
 
    cout << "Type another word: (Press Ctrl + D to quit):";
    while( i < length && cin >> word[i]){
    i++;
    cout << "Type another word: (Press Ctrl + D to quit):";
    }

    for(int j = 0; j < i; j++){
     cout<< endl << word[j]<< endl;
     for(int k = 0; k < word[j].length(); k++)
     cout << word[j].at(k) << endl;
    }
}

As we can see this grabs the entire sentence and because of this it prints another prompt not needed.

EXAMPLE OF OUTPUT:

Type another word: (Press Ctrl + D to quit):testing this
Type another word: (Press Ctrl + D to quit):Type another word: (Press Ctrl + D to quit):december
Type another word: (Press Ctrl + D to quit):
testing
t
e
s
t
i
n
g

this
t
h
i
s

december
d
e
c
e
m
b
e
r

I will attach an 'ideal' output:

Type another word: (Press Ctrl + D to quit):testing this
Type another word: (Press Ctrl + D to quit):december
Type another word: (Press Ctrl + D to quit):
testing
t
e
s
t
i
n
g

december
d
e
c
e
m
b
e
r

it reads one word at a time, but it still reads all the words you enter. It will read the 2nd word the next time through the loop — Garr Godfrey, Mar 31 '21 at 23:40
In `string word[length];`, `length` is not a compile-time constant, making `word` a Variable-Length Array, which is [not standard C++](https://stackoverflow.com/questions/1887097/). Make `length` be `const`, or use `std::vector` instead. — Remy Lebeau, Apr 01 '21 at 01:26

user4581301 · Answer 1 · 2021-04-01T00:00:29.547

When you wrote cin is suppose to stop at the first space,enter, etc... so I think the part that is wrong here is the while loop you almost nailed it. cin >> words[i] reads one word into the array. The loop body increments i and then goes around to read again. Word by word. Not the next sentence. The next word. Operator >> doesn't understand sentences. Just words.

Instead use std::getline to read the whole line in, then get the first word from that line. Easiest way to do that is probably to turn the line into a std::istringstream and use operator >> to extract a single word.

For example:

#include <iostream>
#include <string> // was missing.
#include <sstream> // defines std::istringstream
using namespace std;

int main(){ // no need for void. Compiler's smart enough to figure 
            // that out from empty braces.
    int i = 0;
    const int length= 25; // needs to be constant to be used to size 
                          // an array
    string word[length];

    cout << "Type another word: (Press Ctrl + D to quit):";
    std::string line; // storage for the line
    while( i < length && std::getline(cin, line)){ //read whole line
        std::istringstream strm(line); // make a stream out of the line
        if (strm >> word[i]) // read first word from line
        {
            i++;
        }
        else  // tell user they made a mistake or do something fancy. 
        {
            std::cout << "No word on that line.\n";
        }
        cout << "Type another word: (Press Ctrl + D to quit):";
    }

    for(int j = 0; j < i; j++){
        cout<< endl << word[j]<< endl;
        for(int k = 0; k < word[j].length(); k++)
            cout << word[j].at(k) << endl;
    }
}

Documentation for std::getline

Documentation for std::istringstream

Cin in a while loop condition in C++

1 Answers1