C++ Is it possible to accept any type of input?

Question

I am very new to c++ and am trying to program a simple calculator. I would like to add an input option were if you type 'ans' it will replace the first number with the answer to the previous calculation. Unfortunately I already defined that input as a float which causes it not to read any string input. If I defined that input as a string, the calculator would crash because you cant multiply two strings. This is the piece of code I am stuck on.

#include <iostream>

int main()
{
    float a;
    std::cin >> a;
    if (a == 'ans') {
        std::cout << "this is a string input";
    }
    else {
        std::cout << a * 2;
    }

}

I believe that when ever I enter 'ans' it goes straight to the else part of the if and tries to multiply a * 'ans' because it returns 0.

If anybody has any idea... Thanks

[std::stof](https://en.cppreference.com/w/cpp/string/basic_string/stof) — Mooing Duck, Feb 27 '20 at 00:12
`'ans'` you likely used the wrong quotation marks. I don't think you wanted a multicharacter literal: [https://stackoverflow.com/questions/3960954/c-multicharacter-literal](https://stackoverflow.com/questions/3960954/c-multicharacter-literal) — drescherjm, Feb 27 '20 at 00:12
@Tas Since `'ans'` is an `int` it's _technically_ possible to match, but it's certainly not what the OP was attempting to do. — N. Shead, Feb 27 '20 at 00:13
Read it as a string, figure out if it's `ans` or a number, and then convert it to a number if needed. — eesiraed, Feb 27 '20 at 00:13
There are a number of things going on here and I can’t post a real answer now, but in Sunday: a) C++ is statically-typed; before you ever run the program, `a` is and always will be a `float`. b) You should get at least two warnings on the `if` part because it does not do what you think; try enabling more warnings. c) When it fails to parse `ans` as a float, it fails, and uses the pre-existing value of `ans`; it could be anything or nothing, not just 0. d) What you need to do is read it in as a string, and use the library to parse that into a float yourself. — Daniel H, Feb 27 '20 at 00:14

score 2 · Accepted Answer · edited Feb 27 '20 at 00:47

2

My suggestion would be to take a std::string as input, check whether it is "ans" or any other string input, and then, if it is not, convert it to a float using std::stof.

Also, keep in mind that 'ans' is not a string, but a multicharacter literal, as others have noted. This is not what you want here. In C++, use single quotes for character literals and double quotes for string literals.

edited Feb 27 '20 at 00:47

Ted Lyngmo

93,841
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answered Feb 27 '20 at 00:23

Jeremy Nuttall

36
2

would std::stos convert it back to a string? – Feb 27 '20 at 00:52
@Whityj `std:stos` doesn't seem to exist (I had to check because I had not heard of it). Use `std::to_string()` to convert a numeric (variable) into a `std::string`. – Ted Lyngmo Feb 27 '20 at 01:01

asmmo · Answer 2 · 2020-02-27T00:29:33.953

1

There are many ways to overcome this problem. One of them is to use std::stof() which takes a std::string and size_t *, then returns the number contained in the std::string and in the same time the value of the var pointed to by the size_t * changes to the number of the processed chars of the std::string if its leading chars are numbers, as follows

#include <iostream>
#include<string>
int main()
{
    std::string a{};

    std::cout<<"Enter a thing: \n";
    std::cin >> a;
    size_t processedCharsNo{};

    float num{};
    if(!isalpha(a[0]))num = std::stof(a, &processedCharsNo);
    if (processedCharsNo == 0) {
        std::cout << "this is a string input";
    }
    else {
        std::cout << num * 2;
    }

}

edited Feb 27 '20 at 00:29

answered Feb 27 '20 at 00:20

asmmo

6,922
1
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1

Nitpick: You do not need to initialize a variable that's never going to be read before it's written to (`processedCharsNo`), and this `float num {std::stof(a, &processedCharsNo)};` is generally better received as `float num = std::stof(a, &processedCharsNo);` – Ted Lyngmo Feb 27 '20 at 00:24
Just a note: When doing this kind of extraction, it's generally a good idea to check that it actually worked: `if(std::cin >> a) { /* ok proceed */ } else { /* nah, extraction failed completely */ }`. – Ted Lyngmo Feb 27 '20 at 01:07

C++ Is it possible to accept any type of input?

2 Answers2