can c++ using cin to read one digit like scanf

Question

does c++ have similar measure like scanf("%1d) read only one digit in cin >>

or only #include <cstdin> and use scanf

I try to use setw() but it seem to use in string

Answer 1

Here, take a look at this method

#include <iostream>

int main(){

    std::cout << "Enter a number : ";
    std::string input;
    std::getline(std::cin, input);

    int num = input[0] - 48; // converting char to int by ASCII values
    // int num = input[0] - '0';  can also be used
    cout << num;
    
    return 0;
}

Even if many digits are entered as input, the program only accepts the first digit.

Answer 2

A digit is nothing else than a single character.

In order to read a single character from std::cin, you can use operator>>, or you can use std::istream::get.

Using operator>>:

char digit;
std::cin >> digit;

Using std::istream::get:

char digit;
std::cin.get( &digit );

In order to convert char digit to an integer, assuming that digit is in the range '0' to '9', you can write the following:

int number = digit - '0';

You can do this because the ISO C++ standard guarantees that the characters '0' to '9' are stored consecutively in the character set. For example, in ASCII, the characters '0' to '9' have the character codes 48 to 57. Therefore, if you simply substract 48 (which is the character code for '0') from the character code, you will get a result between 0 and 9, which is the value of the digit.

Here is an example program:

#include <iostream>
#include <cstdlib>

int main()
{
    int number;
    char digit;

    //prompt user for input
    std::cout << "Please enter a digit: ";

    //attempt to read a single character
    std::cin >> digit;

    //verify that no input error occurred
    if ( !std::cin )
    {
        printf( "Input error!\n" );
        std::exit( EXIT_FAILURE );
    }

    //verify that character is in the range '0' to '9'
    if ( ! ( '0' <= digit && digit <= '9' ) )
    {
        printf( "Character is not a digit!\n" );
        std::exit( EXIT_FAILURE );
    }

    //convert digit to integer
    number = digit - '0';

    //print result
    std::cout <<
        "The digit was successfully converted to " << number << ".\n";
}

This program has the following behavior:

Please enter a digit: 7
The digit was successfully converted to 7.

It only accepts digits:

Please enter a digit: a
Character is not a digit!

It will only read the first digit, not a whole number:

Please enter a digit: 38
The digit was successfully converted to 3.