compare string element with numeric

Question

#include <bits/stdc++.h>

using namespace std;

int main() 
{
    string s("092282");
    cout << s[0];
    if (s[0] < (char)9)
    {
        cout << "yesss";
    }
}

In this text I am not able to understand how to compare string element which is numeric constant and a numeric.

Can you please elaborate more on what you don't understand? I don't understand what you don't understand about the above code. — NathanOliver, Dec 17 '18 at 15:36
What do you expect '(char)9' to do? Using the cast operator doesn't do what you expect — JVApen, Dec 17 '18 at 15:37
`s[0]` is a char, if you want to compare it with numeric digit, do it like this: `if (s[0] - '0' < 9)` — kiner_shah, Dec 17 '18 at 15:37
Side note: be careful with your [`#include `](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h). — TrebledJ, Dec 17 '18 at 15:41
@sparshgoil You need to understand the difference between an integer like `9` and a character like `'9'`. Even through your string is all numbers it's made up of characters not integers. So the correct comparison is `if (s[0] < '9')` as JVApen said. — john, Dec 17 '18 at 15:43
@sparshgoil, characters '0', '1', ..., '9', have ASCII values 48, 49,..., 57. You can read about ASCII [here](https://en.wikipedia.org/wiki/ASCII) — kiner_shah, Dec 17 '18 at 15:44
@JVApen That returns true for characters like `!`, among others. You should use `std::isdigit` instead. — Lightness Races in Orbit, Dec 17 '18 at 15:46
@LightnessRacesinOrbit: Agreed, however, it ain't undefined behavior like the `s[0] - '0'`at some platforms. — JVApen, Dec 17 '18 at 15:56
@JVApen No but it will give the wrong answer. Why's `s[0] - '0'` UB? — Lightness Races in Orbit, Dec 17 '18 at 16:51
Because char, depending on the platform is signed or unsigned. Signed overflow is undefined — JVApen, Dec 17 '18 at 16:55

Kuba · Accepted Answer · 2018-12-17T16:33:34.777

0

To can compare characters with intergers but char c = '9' is encoded as integer using ASCII table standard. In this table character '9' is equal to 57. You can simply check this.

if('9' == (char)57)
    cout << "yes";

output:

yes

Change your code as below:

int main() 
{
    string s("092282");
    cout << s[0] << endl;
    if (s[0] < '9')
    {
        cout << "yesss";
    }
}

edited Dec 17 '18 at 16:33

answered Dec 17 '18 at 16:18

Kuba

123
6

You **can** compare characters with integers. You have to be careful that the values you're dealing with are appropriate. And for the comparison `if ('9' == (char)57)` the cast is superfluous; `if ('9' == 57)` does the same thing. Both values will be converted to `int` for the comparison. – Pete Becker Dec 17 '18 at 16:23
Note that using ASCII is quite common, but not required. – Pete Becker Dec 17 '18 at 16:24
1

Actually just noticed that's the OP's bug not yours so no downvote – Lightness Races in Orbit Dec 17 '18 at 16:52

compare string element with numeric

1 Answers1