Standard Input for Unsigned Character

Question

I am trying to send unsigned characters through a program, and I would like to be able to get the numbers through standard input (ie std::cin). For example when I type 2 I would like it send ☻ (unsigned char 2). when I use the code:

 std::cout << "Enter values: ";
            {
                unsigned char d;
                unsigned char e = 2;
                std::cin >> d;
                WriteFile(file, &d, 1, &written, NULL);
                std::cout << "d= " << d << "\n"; 
                std::cout << "e= " << e;
            }

I get

 Enter values: 2
 d=2
 e=☻

Can anyone tell me why d is being interpreted Incorrectly as unsigned char 50 while e is being interpreted correctly as unsigned char 2?

And of course after your explanation can You explain how to get User input and convert it so that I send 2 rather than '2'.

`e` very correctly has the value `2`, while `d`, indeed also correctly, has the value `'2'`. Those are quite different values. — Bart van Nierop, Aug 01 '14 at 19:45
possible duplicate of [uint8\_t can't be printed with cout](http://stackoverflow.com/questions/19562103/uint8-t-cant-be-printed-with-cout) — πάντα ῥεῖ, Aug 01 '14 at 19:45
The only reason that is a duplicate is because that question, too, displays a complete lack of understanding the difference between an integer and a character literal. — Bart van Nierop, Aug 01 '14 at 19:46
@BartvanNierop I understand the difference I just don't know how to get the integer from the standard input. — jeffpkamp, Aug 01 '14 at 19:50
@jeffpkamp Then you should update your question to reflect that the value of `d` is _incorrect_, and the value of `e` is in fact what you're looking for. — Bart van Nierop, Aug 01 '14 at 19:51
@BartvanNierop I think that's explicitly stated twice in my question. Any ideas on how I can get 2 from the standard input rather than '2'? — jeffpkamp, Aug 01 '14 at 19:56

vsoftco · Accepted Answer · 2014-08-01T19:55:43.667

3

Because std::cin >> d; reads by default a char type, so the input 2 translates into the character 2 (with ASCII code 50) and not the character represented by the ASCII code 2. This is a normal behaviour, otherwise trying to read numbers from cin will end up being a mess.

On the other hand, in unsigned char e = 2; you explicitly assign a value (2) to the variable, so the compiler blindly assigns it to e.

You probably want this:

#include <iostream>
#include <string>
using namespace std;

int main(int argc, char *argv[])
{
    string myString;
    cin >> myString;
    char c = atoi(myString.c_str());
    cout << c << endl;
}

edited Aug 01 '14 at 19:55

answered Aug 01 '14 at 19:43

vsoftco

55,410
12
139
252

okay, I figured as much. So how can I get the ascii value 2 from a console input without using alt+2? – jeffpkamp Aug 01 '14 at 19:48
@jeffpkamp You don't. It's not a printable character. – Bart van Nierop Aug 01 '14 at 19:50
@jeffpkamp I believe the standard says that `std::cin` is text-mode, so I do not think you can read ascii directly. I am not sure though. What you can do is to read it as a number, then convert the character that represents the number into an integer. Can use for example `char c = atoi(myString.c_str())` to convert a previously read string `myString` into an integer. See the code I just posted above. – vsoftco Aug 01 '14 at 19:52
Why go through `std::string`, instead of going through `int`? This seems needlessly complicated. – hyde Aug 02 '22 at 09:55

score 0 · Answer 2 · answered Aug 01 '14 at 19:44

0

When you enter 2 via std::cin, it's correctly interpreted as the character literal '2'.

You should replace

unsigned char e = 2;

with

unsigned char e = '2';

answered Aug 01 '14 at 19:44

maxdefolsch

179
5

Standard Input for Unsigned Character

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