Getting just the first letter of the user input in C++

Question

I wrote a console program which is getting some spesific character inputs from user to perform operations. Program asks for an input, user types "i" for insert or "r" for remove etc.. The problem is when user types more than one letter, the program behaves weird.

char selection;
bool finish = false;
while (!finish){
    print_menu();
    cin >> selection;
    finish = perform_operation(selection);
}

This is how i get the user input. And i want to get just the first character of the user input and ignore the rest of them, and behave so on. How can i do it?

Note: I'm not allowed to use "string" class, but i can use every str functions!

Thanks for help!

Another case:

What if i would like to get spesific amount of characters? For example: the user inputs a name which is 10 characters but i want to get just the first 5 characters. What should i do?

Do you want to get first "non white" character from input and ignore the following character till the `\n` ? — PiotrNycz, Oct 12 '15 at 11:41

score 6 · Answer 1 · answered Oct 12 '15 at 11:16

6

If you want to read one character, read one character.

Don't have a loop that reads as many characters as possible.

char selection;
print_menu();
if (cin >> selection)
   finish = perform_operation(selection);
else
   throw std::runtime_error("Bad input!");

I've added basic error checking to your I/O operation for good measure.

answered Oct 12 '15 at 11:16

Lightness Races in Orbit

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This is a program which is getting user input as long as a person doesn't exit from the program. So i definitely have to use a while loop. Usin if block is not an option for this case – Tolga Şen Oct 12 '15 at 12:51
@TolgaŞen: Either you want to read just one character, or you want to read multiple characters. Which is it? – Lightness Races in Orbit Oct 12 '15 at 13:28
I'd like to read exact the same amount of characters which i give to the function. It might be just one or multiple also – Tolga Şen Oct 12 '15 at 19:34

Emadpres · Accepted Answer · 2015-10-12T14:17:16.220

1

for getting just one character you can use also this:

char selection;
cin >> selection; // input: abcd
cin.clear();
fflush(stdin);
printf("you entered the character: %c", selection); //selection='a'
// and nothing remained in input stream. you can safely `cin>>` again :]

You can apply this code to your existing one.

In order to get a specific number of charter you can use below sample:

char* TakeNCharacterFromInput(int _count)
{
    char *input = new char[_count+2];
    std::cin.getline(input, _count+1);
    cin.clear();
    fflush(stdin);
    return input;
}

int main(int args, char* argv[]) {
    char *in1 = TakeNCharacterFromInput(5);//input:123456789 => in1:12345
    char *in2 = TakeNCharacterFromInput(2);//input:123456789 => in2:12
    char *in3 = TakeNCharacterFromInput(1);//input:123456789 => in3:1
    delete[] in1;
    delete[] in2;
    delete[] in3;
}

edited Oct 12 '15 at 14:17

answered Oct 12 '15 at 11:18

Emadpres

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2

Um your suggested change doesn't actually change anything. – Lightness Races in Orbit Oct 12 '15 at 11:21
Yeah edited code works fine but i forgot to ask, i really need a function that takes an input with given spesific amount of characters. For example i need user to enter a name but i just want to get first 20 characters of that name and ignore the rest. How can i do that? – Tolga Şen Oct 12 '15 at 13:00
cin.clear() and fflush(stdin) is not working for me, I still can get the others inputs from cin. I'm Using Ubuntu. – Pedro77 Feb 19 '20 at 11:44

molbdnilo · Answer 3 · 2015-10-12T11:49:00.197

1

Ignore everything except the first character until the next line break:

char selection;
bool finish = false;
while (!finish){
    print_menu();
    cin >> selection;
    finish = perform_operation(selection);
    cin.ignore(10000, '\n'); // Totally arbitrary large number.
}

edited Oct 12 '15 at 11:49

answered Oct 12 '15 at 11:45

molbdnilo

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Maybe `cin.ignore(10000, '\n');` ? – PiotrNycz Oct 12 '15 at 11:48
And maybe instead of 10000 - use `std::numeric_limits::max() - 1`? – PiotrNycz Oct 12 '15 at 11:49
@PiotrNycz Not just maybe. – molbdnilo Oct 12 '15 at 11:49
@PiotrNycz I don't think beginners should be invited to decode the most line noise-like parts of C++. (Some days I could swear that C++ *is* Perl 6.) – molbdnilo Oct 12 '15 at 11:51
And I would encapsulate this two lines (is >> c; is.ignore(...)) in one function - `readMeanuChar(std::cin)` ... – PiotrNycz Oct 12 '15 at 11:52
read this from http://en.cppreference.com/w/cpp/io/basic_istream/ignore "count characters were extracted. This test is disabled in the special case when count equals std::numeric_limits::max() " - and answers on SO are for everybody - not only for OP... – PiotrNycz Oct 12 '15 at 11:53
Anyway - +1 - because I forget that `ignore()` exists - helpful knowledge - thanks... – PiotrNycz Oct 12 '15 at 11:54

score 0 · Answer 4 · edited May 23 '17 at 12:02

0

skip always reprinting the menu

print_menu();

then use a function that waits for ENTER:

Several ways to do so, here are some possible one-line approaches:

Use getch() (need #include ).

Use getchar() (expected for Enter, need #include ).

Use cin.get() (expected for Enter, need #include ).

Use system("pause") (need #include ).

see also this thread C++ wait for user input

edited May 23 '17 at 12:02

Community

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answered Oct 12 '15 at 11:24

ralf htp

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Getting just the first letter of the user input in C++

4 Answers4