How to validate combination of data type int and string?

Question

my code is:

int num;
string check = "true";
cin >> num;
if (cin.fail())
        check = "false";

cout << check;
}

I want to make sure the input is an integer only. When the input is like r3 (non-digits followed by integer), the output is false. But when the input is like 3r (digits followed by non-digits), the output is true. How to solve this problem?

@HattedRooster but getline cannot be used by int data type rite? — Nino, Aug 19 '21 at 07:49
Does this answer your question? [Checking cin input stream produces an integer](https://stackoverflow.com/questions/18728754/checking-cin-input-stream-produces-an-integer) — Hassan Ashas, Aug 19 '21 at 07:55

score 1 · Answer 1 · answered Aug 19 '21 at 07:50

You can take input as a string and then check if the string is a number:

#include <iostream>
#include <string>
#include <cstdlib>

int main()
{
    std::string nstr;
    long n;
    char *end_point;
    std::cin >> nstr;
    n = std::strtol(nstr.c_str(), &end_point, 10);
    std::cout << (*end_point == '\0' ? "true" : "false") << '\n';
}

Roman Pavelka · Answer 2 · 2021-08-19T08:25:38.377

0

You have to only check first character if it is a digit:

#include <iostream>
#include <string>
#include <cctype>

int main()
{
    std::string s;
    std::cin >> s;
    std::cout << (isdigit(s[0]) ? "true" : "false") << "\n";
}

$ ./a.out 
123
true
$ ./a.out 
r1
false
$ ./a.out 
1r
true

If you would like to include signed numbers to yours test:

#include <iostream>
#include <string>
#include <cctype>

int main()
{
    std::string s;
    std::cin >> s;

    bool unsigned_int = isdigit(s[0]);
    bool signed_int = (s[0] == '-' || s[0] == '+') && isdigit(s[1]);

    std::cout << (unsigned_int || signed_int ? "true" : "false") << "\n";
}

$ ./a.out 
123
true
$ ./a.out 
+1xyz
true
$ ./a.out 
-98xxx
true
$ ./a.out 
abc
false

edited Aug 19 '21 at 08:25

answered Aug 19 '21 at 07:53

Roman Pavelka

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But doesn't OP want something like `1r` to show as `false`? – mediocrevegetable1 Aug 19 '21 at 07:55
From what I can tell, if OP was fine with `1r` being false then their current method works just fine. I think they want to change it specifically because it doesn't work when there are non-digits after a number. – mediocrevegetable1 Aug 19 '21 at 07:58
1

What about negative integer values ? – Fryz Aug 19 '21 at 08:02

score 0 · Answer 3 · answered Aug 19 '21 at 08:30

#include <iostream>
#include <string>

using namespace std;

int main()
{
    int num;
    string input;
    string check = "true";

    cin >> input;
    if (input.length() != to_string(num = atoi(input.c_str())).length())
    {
        check = "false";
    }   

    cout << check;
    return 0;
}

How to validate combination of data type int and string?

3 Answers3