what exactly does this C++ code do ?have you ever seen in before ?

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1.00/5 (1 vote)

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in a c++ program I found this

#ifdef _WIN32
inline int getchar_unlocked() { return getchar(); }
#endif
inline int ReadInt()
{
 int c=getchar_unlocked();
 int out=0;
 while(c=='\n' || c==' ')
 c=getchar_unlocked();
 while(c!='\n' && c!=' ' && c!=-1)
 {
 out*=10;
 out+=(c-'0');
 c=getchar_unlocked();
 }
 return out;
}

in conjuction with :

{ freopen("filename.ext","rt",stdin);

every time ReadInt() is called an integer from the filename.txt is returned
eg if filename is
141
5159 55 5818
4
then 5 ReadInt()'s will give the numbers 141,5159,55,5818 and 4
my question is what the hell is this guy doing ?
is he using the standard input stream as a file stream ? and then again...getchar() returns a char from the console...
and where does this

#ifdef _WIN32

piece of code help ?( not the characted_unlocked() line..the 'if' steatement) ...this program is compiled and run on linux so why would someone do this ?

also : how can I read big (8-20 mb) files -most files contain integers only-as fast as possible without using external libraries such as the memory mapped file method ?
fscanf is way faster than ifstream but the code above is faster even than fscanf

thanks for your help :)

Posted 2-Mar-14 7:12am

EbolaHost

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Richard MacCutchan · Accepted Answer · 2014-03-02T07:20:00

Solution 1

It is a rather old fashioned (C-style) way of reading integers from the standard input stream. The call to freopen redirects that file to stdin so it can be read with the calls to getchar. A much better way of doing it would be to use fscanf or one of the STL ifstream classes. The #ifdef _WIN32 at the top is not closed by a corresponding #endif so it's anyone's guess why it is there.

Posted 2-Mar-14 7:20am

Richard MacCutchan

Comments

Vedat Ozan Oner 2-Mar-14 13:48pm

it (#ifdef _WIN32) is closed. just below getchar_unlocked definition.

Richard MacCutchan 3-Mar-14 4:21am

True, How did I manage to miss that?

Vedat Ozan Oner 3-Mar-14 4:34am

it happens (mostly to me :p)

EbolaHost 2-Mar-14 14:44pm

But this method is faster than fscanf...
And the #if is closed after two lines

Richard MacCutchan 3-Mar-14 4:22am

Faster in terms of what? You are talking of microseconds at most, and probably a lot less, nothing that a user could ever see.

CPallini 2-Mar-14 15:39pm

5.

Richard MacCutchan 3-Mar-14 4:24am

Thanks, same for you.

Andreas Gieriet 2-Mar-14 15:48pm

My 5 too!
Cheers
Andi

Richard MacCutchan 3-Mar-14 4:24am

Thanks.

CPallini · Accepted Answer · 2014-03-02T10:06:00

The guy is using input stream redirection in order to use getchar to perform fgetc[^] task, that is reading a character at time from a file.
In the ReadInt function, blanks and newlines are skipped, then digits are then read and used to build the resulting integer, until a blank, a newline or the end of file is found.

The key statements for building the integer from read digits are:

C++

int out=0;
 //..
while(/*...*/)
{
  out *= 10;
  out+=(c-'0');
  //..

That is out is initalised with zero, then at each step it is first multiplied by 10 and then added with the read digit value (e.g. '5' - '0' == 5).
Suppose you are reading the 55 number from the file.

out=0 before reading any digit
out is still 0 after performing the multiplication by 10
out=5, after performing out+=c - '0', the first time
out=50, after perorming out*=10 at next step
out=55, after performing out+=c - '0' again
finally a blank is detected and the function return 55

Please note, the function is not robust: it relies on the fact that every character different from a blank a newline (or the end of file) is a digit (that is not true, generally speaking, so this must be a prerequisite on your input files.
There is no overflow checking on integers (as well negative values are not correctly parsed).
Moreover it may return a spurious zero at the end of the file.