How to check if the input is a valid integer without any other chars?

Question

#include <iostream>
#include <limits>

using namespace std;

int main()
{
    int x;
    cout << "5 + 4 = ";
    while(!(cin >> x)){
        cout << "Error, please try again." << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }
    if (x == (5 + 4)){
        cout << "Correct!" << endl;
    }
    else{
        cout << "Wrong!" << endl;
    }
    return 0;
}

How can I check if the user inputs a valid integer? In this program I wrote above, if the user inputs 9, it should be correct, however, if the user inputs 9a for example, it should return an error, but it doesn't for some reason. How can I correct it?

How I did it using cin.peek()

#include <iostream>
#include <limits>
#include <stdio.h>

using namespace std;

int main()
{
    int x;
    bool ok;
    cout << "5 + 4 = ";

    cin >> x;

    while(!ok){
  cin >> x;

  if(!cin.fail() && (cin.peek() == EOF || cin.peek() == '\n')){
  ok = true;
  }
  else{
  cout << "Error, please try again." << endl;
  cin.clear();
  cin.ignore(numeric_limits<streamsize>::max(), '\n');
  }
    }

    if (x == (5 + 4)){
  cout << "Correct!" << endl;
    }
    else{
  cout << "Wrong!" << endl;
    }

    return 0;
}

score 7 · Accepted Answer · answered Nov 29 '13 at 13:57

7

You could read a string, extract an integer from it and then make sure there's nothing left:

std::string line;
std::cin >> line;
std::istringstream s(line);
int x;
if (!(s >> x)) {
  // Error, not a number
}
char c;
if (s >> c) {
  // Error, there was something past the number
}

answered Nov 29 '13 at 13:57

Angew is no longer proud of SO

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1

You could also just `peek()`. – jrok Nov 29 '13 at 14:00
You generally want to skip trailing white space as well, since the user can't see it, and won't understand why there is an error if it occurs. – James Kanze Nov 29 '13 at 14:03
@JamesKanze There shouldn't be any trailing whitespace - I'm using `>>` to extract the string (as the OP did), not `getline()`. – Angew is no longer proud of SO Nov 29 '13 at 14:08
Ah, yes. But then that doesn't correspond to what he seems to be trying to do (since he skips to the end of the line when there is an error). – James Kanze Nov 29 '13 at 14:36

score 3 · Answer 2 · answered Nov 29 '13 at 13:50

bool isIntegerNumber(const std::string& string){
  std::string::const_iterator it = string.begin();
  int minSize = 0;
  if(string.size()>0 && (string[0] == '-' || string[0] == '+')){
    it++;
    minSize++;
  }
  while (it != string.end() && std::isdigit(*it)) ++it;
  return string.size()>minSize && it == string.end();
}

score 3 · Answer 3 · answered Nov 29 '13 at 14:02

You have a line oriented input, so you should probably be using getline. Something like:

bool
getIntFromLine( std::istream& source, int& results )
{
    std::string line;
    std::getline( source, line );
    std::istringstream parse( source ? line : "" );
    return parse >> results >> std::ws && parse.get() == EOF;
}

should do the trick.

Using this, your loop would be:

while ( !getIntFromLine( std::istream, x ) ) {
    std::cout << "Error, please try again." << std::endl;
}

Note that this technique also means that you don't have to worry about clearing the error or resynchronizing the input.

score 3 · Answer 4 · answered Nov 29 '13 at 14:03

For the reason this happens, take a look at this link:

Extracts and parses characters sequentially from the stream to interpret them as the representation of a value of the proper type, which is stored as the value of val. Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to false). Then (if good), it calls num_get::get (using the stream's selected locale) to perform both the extraction and the parsing operations, adjusting the stream's internal state flags accordingly. Finally, it destroys the sentry object before returning.

Then observe the behavior if you attempt something like this:

int x = 0;

cin >> x;
std::cout << x << std::endl;
std::cout << cin.good() << std::endl;

g++-4.8 -std=c++11 -O3 -Wall -pedantic -pthread main.cpp && echo "900a100" | ./a.out
// Output:
// 900
// 1

If you input "a100" instead, it outputs:

0
0

score 2 · Answer 5 · edited May 23 '17 at 10:27

2

try this:

std::string input;
std::cin >> input;

if ( std::all_of(input.begin(), input.end(), std::isdigit) )
{
     //input is integer
}

Refer this :

C++ Fix for checking if input is an integer

edited May 23 '17 at 10:27

Community

1
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answered Nov 29 '13 at 13:47

bgs

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Thus "-10" is not a valid integer ? :) – Johan Nov 29 '13 at 13:50
Error: `all_of` was not declared in this scope. I need code for C++, not C++11 also for both signed/unsigned integers. – user2699298 Nov 29 '13 at 13:54
3

@user2699298 C++11 *is* C++. I guess you mean you need C++03. – Angew is no longer proud of SO Nov 29 '13 at 13:54

score 1 · Answer 6 · answered Nov 29 '13 at 14:37

One I have seen that works for some situations is:

Read the input as string. cin >> str
Decode to number: atoi, or sscanf, or stringstream, etc.
print the number into a string (using sprintf or stringstream)
check if its equal to read string. (using strings ==, not char*)

Quick and simple to do. Uses the Cin>>str word breaking rules, accept negative numbers, rejects overflowing numbers. But it does reject "+10", which in somesituations you are happy wiht, and in some you are not.

If you can use regex, that last part is not a problem. – Zac Howland Nov 29 '13 at 15:24 — Zac Howland, Nov 29 '13 at 15:24

score 1 · Answer 7 · answered Nov 29 '13 at 15:21

If you can use C++11 (and your compiler has full regex support), you can also use the <regex> library:

#include <iostream>
#include <limits>
#include <regex>
#include <string>
#include <utility>

int main()
{
    std::string line;
    std::pair<int, bool> value = std::make_pair(0, false);
    std::cout << "5 + 4 = ";
    while (!value.second)
    {
        while (!std::getline(std::cin, line))
        {
            std::cout << "Error, please try again." << std::endl;
            std::cin.clear();
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        }

        if (!std::regex_match(line, std::regex("(\\+|-)?[[:digit:]]+")))
        {
            std::cout << "Error, please try again." << std::endl;
        }
        else
        {
            value = std::make_pair(std::stol(line), true);
        }
    }

    if (value.first == (5 + 4))
    {
        std::cout << "Correct!" << std::endl;
    }
    else
    {
        std::cout << "Incorrect!" << std::endl;
    }

    return 0;
}

How to check if the input is a valid integer without any other chars?

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