Stop input loop when input is done | std::cin

Lario Sedic 0
Viewed 78

I want to write a function that gets a set of integers and saves them to a vector.

To get the integers I'm using a while loop.

Enter the vector elements: 1 2 3 4 5

I want the loop to stop looking for input after the last element is inputted or until a non-numberis inputted, however I'm having trouble with it.

It is an assignment, so it needs to read input from std::cin.

This is my code:

#include <iostream>
#include <vector>

bool NumInVect(int num, std::vector<int> vect)
{
  bool numInVect = false;
  for (int i = 0; i < vect.size(); i++)
    {
      if (vect[i] == num)
      {
        numInVect = true;
      }
    }
  return numInVect;
}

int main() {
  std::vector<int> tempVect;
  std::vector<int> finalVector;
  
  std::cout << "Enter the vector elements: ";
  
  int currVal;
  
  
  while (std::cin >> currVal)
  {
    std::cout << "-\n";
    
    if (!NumInVect(currVal, tempVect))
    {
      tempVect.push_back(currVal);
    }
    std::cout << currVal << std::endl;

  }

//the code never reaches this loop, since its stuck in the while loop
std::cout << "knak\n";
  for (int i = 0; i < tempVect.size(); i++)
    {
      std::cout << tempVect[i];
    }
  
}

I've tried doing multiple things like using std::cin.eof()/.fail(), std::cin >> currVal in the while loop, a do-while loop, but I can't seem to figure out how to get this to work. Does anyone have any tips on what to look into or how to approach this?

cin c++
3 Answers

You should add a condition in your while loop so that it breaks when you want like:

if (temptVect.size() >= 5)
    break;
César Debeunne0

If your intention is to get all the input from a given line, and add individual numbers to the vector, then you want to read a line of input into a string, then use an istringstream to process that line of input.

A simplified example:

#include <iostream>
#include <sstream>
#include <string>
#include <vector>

int main() {
    std::vector<int> tempVect;
    std::string line;
    std::getline( std::cin, line );

    std::istringstream iss( line );
    int curVal;
    while ( iss >> curVal ) {
        tempVect.push_back( curVal );
    }
}
ChrisMM0

I want the loop to stop looking for input after the last element is inputted or until a non-numberis inputted,

This loop should do exactly that.

  while (std::cin >> currVal)
  {
    std::cout << "-\n";
    
    if (!NumInVect(currVal, tempVect))
    {
      tempVect.push_back(currVal);
    }
    std::cout << currVal << std::endl;

  }

This test while (std::cin >> currVal) will fail if read does not work and exit the loop. Since currVal is an integer, it will fail as soon as it tries to read something that is not an integer (note space and new lines are completely ignored).

What may be the issue is that std::cin is buffered. You can type a long time before filling up the buffer and causing the operator>> to do anything. To force this operator to read you have to force a flush this is usually done by hitting <enter> on the keyboard.

After the buffer is flushed it will read all the numbers on the input and exit the loop if a read fails. So you need to explicitly type not a number before hitting the <enter>.

eg:

1 2 3 4 5 X<enter>

or:

1<enter>
2<enter>
3<enter>
4<enter>
5<enter>
X<enter>

You need that non number to force the read to fail and the loop to exit.

Alternatively, you could say I want to read all the numbers on one line then exit the loop.

In that case: You should read a line and the processes the line. This is only a slight change from your above code.

std::string line;                     // Hold a line of text
std::getline(std::cin, line);         // Read a line of text.

std::stringstream lineStream(line);   // Convert string into stream
while (lineStream >> currValue)       // Read each integer from the stream.
{                                     // Exits when the read fails to read
                                      // a number becuase it is empty.
    // The other stuff like before.
}
Martin York0
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