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Sep 4th, 2008

getch() problem

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I just knew about getch() and found it better than cin (a lot) but there's one problem bugging me, can you makea n if statement for getch()?
For example they have:

C++ Syntax (Toggle Plain Text)
int a;
cin >> a;
if (a == 1)
  //blah
else if (a ==2)
 //blah
else
 //blah
int a;
cin >> a;
if (a == 1)
  //blah
else if (a ==2)
 //blah
else
 //blah

But can you do something similar with getch()? I thought putting a in those brackets would work but it kept on $saying too many arguments to function "int getch()"$ . Can anyone help me with this?
Thanks.

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Dannyo329

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Sep 4th, 2008

Re: getch() problem

Something like this should compile:

C++ Syntax (Toggle Plain Text)
int a = getch();
if (a == '\n') cout << "enter";
int a = getch();
if (a == '\n') cout << "enter";

But why do you think getch() is better then cin? Getch() is outdated and non-standard, so if you use it on a modern compiler, you'll get a compiler error.

Besides, comparing getch() with cin is like comparing apples with pears. They're both fruit, but the taste is quite diffrent...

Last edited by Nick Evan; Sep 4th, 2008 at 6:23 am.

Nick Evan

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Sep 4th, 2008

Re: getch() problem

Well its partly of my friends, when I show them the program I've made, then something tells them to press 1, so they press 1 but not enter....

And no, that

Quote ...

int a = getch();
if (a == '\n') cout << "enter";

doesn't work. Now it somehow skips straight to 'else', missing out the 'if' and the 'else if'......
I'm gonna have to try hard on this one.

Last edited by Dannyo329; Sep 4th, 2008 at 6:34 am. Reason: Forgot to add something

Dannyo329

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Sep 4th, 2008

Re: getch() problem

>Now it somehow skips straight to 'else', missing out the 'if' and the 'else if'......
That's because getch doesn't perform text conversions. It's a straight detection of scan codes from the console. This means that on systems (such as Windows) where a newline is two values before being converted to '\n' by C++'s stream buffer, calling getch once will give you the first of those values.

Assuming you're on Windows, here's what is happening. When you press Return, a Windows newline is signaled and two values are sent to the console: 0xD and 0xA, in that order. This is the CR-LF pair you may have heard about. Let's say that '\n' in C++ represents 0xA. A standard C++ text stream will take 0xD-0xA and compact it into 0xA, but getch won't do anything of the sort and will simply return 0xD.

So the result is that you're comparing 0xD with 0xA, which is clearly not true, so the else statement is performed. You can catch the carriage return using the '\r' escape character:

C++ Syntax (Toggle Plain Text)
#include <iostream>
#include <conio.h>
 
const int ESC = 0x1B;
 
int main()
{
  int a;
 
  while ( ( a = getch() ) != ESC ) {
    if ( a == '\r' )
      std::cout<< a <<"\tNewline detected\n";
    else
      std::cout<< a <<"\tNot a newline\n";
  }
}
#include <iostream>
#include <conio.h>

const int ESC = 0x1B;

int main()
{
  int a;

  while ( ( a = getch() ) != ESC ) {
    if ( a == '\r' )
      std::cout<< a <<"\tNewline detected\n";
    else
      std::cout<< a <<"\tNot a newline\n";
  }
}

Narue

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Sep 8th, 2008

Re: getch() problem

There's a compiler error saying something like this:
ISO C++ forbids comparison between pointer and integer

Dannyo329

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Sep 8th, 2008

Re: getch() problem

Post the code that produces this error

Nick Evan

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Sep 10th, 2008

Re: getch() problem

Oh, wait. I put these "" instead of these '' when I tried out the code you've posted. Thanks it works fine now, but is there any reason that I should put this in?

C++ Syntax (Toggle Plain Text)

const int ESC = 0x1B;

And this

C++ Syntax (Toggle Plain Text)

!= ESC

in the while loop ?

C++ Syntax (Toggle Plain Text)
while ( ( a = getch() ) != ESC )
while ( ( a = getch() ) != ESC )

Thanks

Dannyo329

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Sep 10th, 2008

Re: getch() problem

Click to Expand / Collapse Quote originally posted by Dannyo329 ...

but is there any reason that I should put this in?
C++ Syntax (Toggle Plain Text)
const int ESC = 0x1B;
const int ESC = 0x1B;
And this
C++ Syntax (Toggle Plain Text)
!= ESC
!= ESC
in the while loop ?
C++ Syntax (Toggle Plain Text)
while ( ( a = getch() ) != ESC )
while ( ( a = getch() ) != ESC )

Yes.

The first line ( const int ESC = 0x1B; ) is just to improve readability. 0x1B is the ASCII code for an ESC.

Narue put the " != ESC " in the while loop, to avoid that your program enters an infinite loop. When you press ESC, getch() will pick that up and break out of the while loop.

You could replace those 2 lines by this while ( ( a = getch() ) != 0x1B) , but as you can see, it's a lot less readable that way.

Nick Evan

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Sep 10th, 2008

Re: getch() problem

Thanks

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