Why does this call to isspace() never return true?

Question

Probably this is asked 100 times before, but i can't find or produce a solution. Everything (as far as i have tested) is recognized, only pressing space is not working. I'm typing a space + hit enter, the cursor just moves to the next line and nothing happens. How can i make this work?

#include <cstdlib>
#include <iostream>
#include <cctype>

using namespace std;


int main(int argc, char** argv) {
    cout << "Press a key please:  " << endl;
    char key;
    cin >> key;

    if (isalpha(key))
        cout << key << " is a letter." << endl;
    else if (isdigit(key))
        cout << key << " is a number." << endl;
    else if (isspace(key))
        cout << key << " is a space." << endl;
    else if (ispunct(key))
        cout << key << " is a punctuation" << endl;
    return 0;
}

Answer 1

When you use the formatted input functions (i.e., operator>>()) the input will skip leading whitespace by default. If you want to read a char which may be a space, you'll need to use the std::noskipws manipulator to disable this skipping:

if (std::cin >> std::noskipws >> c) {
    // ...
}

Alternatively, you can use unformatted input functions, e.g., get() to read individual characters without changing the stream setup: the unformatted input functions don't skip leading whitespaces (which is often causing grief when changing between formatted input, e.g., for an int, and unformatted input, e.g., reading a line using std::getline()):

if (std::cin.get(c)) {
    // ...
}

Also, note that you can only pass positive values (and EOF) to any of the is....() functions but char is signed on many platforms. You need to turn your char into a suitable value first, using, e.g., one of

isspace(static_cast<unsigned char>(c))
isspace(std::char_traits<char>::to_int_type(c))

Answer 2

The problem is, that cin uses whitespace as a separator (also tab, newline). Therefore, you have to read your input ignoring the separator:

cin >> noskipws >> x;

Alternatively use get for single characters:

cin.get(x);

Also have a look at How to cin Space in c++?

Answer 3

cin skips all whitespace (spaces, tabs, new lines, etc.) by default. You can either change its behavior, or use a slightly different mechanism. To change its behavior, use the manipulator noskipws, as follows:

char key;
cin >> noskipws >> key;

You can read more about this here 'How to cin Space in c++?'.

Answer 4

How to read a space character.

Replace this:

cin >> key;

which skips whitespace and leaves text in the input buffer, with this:

auto const max_streamsize = numeric_limits<streamsize>::max();

key = char( cin.get() );
cin.ignore( max_streamsize, '\n' );

which doesn't skip whitespace, and consumes all of the input line.

The problem with not consuming all of an input line, such as the terminating '\n' character, is that this remaining text will then be read by the next input operation, without first waiting for physical user input.

---

How to use the C character classifier functions.

The C functions such as isalpha require a non-negative character code, or else the special value EOF, as argument.

With most C++ compilers the char is, however, signed by default. In general that means formal Undefined Behavior if it's passed directly to e.g. isalpha. For example, checking whether a char value 'ø' (Norwegian lowercase Ø) is alphabetical, by passing it straight to isalpha, is Undefined Behavior with most C++ compilers, and may crash in debug mode with Visual C++ and some other compilers.

A simple solution is to cast it to unsigned char (note: cannot pass EOF this way):

typedef unsigned char UChar;

... isalpha( UChar( ch ) ) ...

---

Complete example, like the original code:

#include <iostream>
#include <limits>       // std::numeric_limits
#include <ctype.h>      // isalpha etc.
using namespace std;

auto main()
    -> int
{
    typedef unsigned char UChar;
    auto const max_streamsize = numeric_limits<streamsize>::max();

    for( ;; )
    {
        cout << "Type a character please (x to exit):  ";
        char const key = char( cin.get() );
        cin.ignore( max_streamsize, '\n' );

        if( key == 'x' ) { break; }

        if( isalpha( UChar( key ) ) )
            cout << key << " is a letter." << endl;
        else if( isdigit( UChar( key ) ) )
            cout << key << " is a number." << endl;
        else if( isspace( UChar( key ) ) )
            cout << key << " is a space." << endl;
        else if( ispunct( UChar( key ) ) )
            cout << key << " is a punctuation" << endl;
        else
            cout << key << " is some unknown kind of character" << endl;
    }
}