C++ store cin.getline value to double variable and to char variable

Question

In my program, I want to limit the amount of numbers a user can input using cin.getline(variable, N) . My code looks like this (this is not the whole code):

#include <iostream>

int main()
{   
    input:
    long double num1;
    long double num2;
    long double result;
    char response;
    cout << "Enter first number and then press enter" << endl;
    cin >> num1;
    cout << "Enter + to add, - to substract, * to multiply, / to divide, v to find the sqare root and ^ to find the power" << endl;
    cin.getline(response, 2); //Here is the problem!
}

When I run this, I get the following error:

How can I store the value returned by cin into a double and a char variable?

Please let me know if you want extra information.

UPDATE: I found a different solution for my project. The solution is specific to my code, and it won't work in other circumstances, so it's pointless to upload it. Thank you for your time.

`cin >> response;` like you already did it with the `double`, but that will read the `` left by the `cin >> num1;`, so you have to add a `cin.ignore();` in between: https://godbolt.org/z/9Yo5bxx6a — mch, Dec 28 '21 at 12:07
You're only limiting the number of characters that you will read, NOT the number of characters that can be entered. If you tried testing it by typing more than one character, your C-string is malformed due to missing a null character. — sweenish, Dec 28 '21 at 12:53

digito_evo · Answer 1 · 2021-12-29T15:38:41.250

-2

You can't get a number using cin.getline. Instead get a buffer of chars and then convert it to double.

#include <iostream>

int main()
{   
    input:
    long double num1;
    long double num2;
    long double result;
    char response[3] { }; // resonse should be able to take in 3 chars (2 + 1 for '\0')

    cout << "Enter first number and then press enter" << endl;
    cin >> num1;
    cout << "Enter + to add, - to substract, * to multiply, / to divide, v to find the sqare root and ^ to find the power" << endl;
    cin.getline(response, 2);
}

Then you can use std::strtold to convert any string buffer (char*) to a long double.

edited Dec 29 '21 at 15:38

answered Dec 28 '21 at 12:09

digito_evo

3,216
2
14
42

@xDev120 What is `input:` up there? – digito_evo Dec 28 '21 at 12:21
It's a label for a goto later in the code – xDev120 Dec 28 '21 at 12:22
This is terrible. why a 10 char buffer when you only use 3? Why use a char array at all??? And then construct a string from it and convert to long double?? Look what the printed line before it says: it expects a '+', '-' or such character, not a number. – JHBonarius Dec 28 '21 at 12:46
Why are you trying to convert the operator character to a double? And why are you converting the C-string to `std::string` at all? `std::atof()` is what you'd want. – sweenish Dec 28 '21 at 12:50
@JHBonarius Please check the updated answer. – digito_evo Dec 29 '21 at 15:33
@sweenish The answer is updated. I appreciate your help. – digito_evo Dec 29 '21 at 15:35

C++ store cin.getline value to double variable and to char variable

1 Answers1