More efficient way than scanf and printf for scaning and printing integers

Question

I just want to know that how can we read integers(from stdin) and write integers(on stdout) without using scanf/printf/cin/cout because they are too slow. Can anyone tell me how fread and fwrite can be used for this purpose? I know only a little about buffering etc. I want the substitute for following two codes->

[1]

    long i , a[1000000];
    for(i=0;i<=999999;i++)
           scanf("%ld",&a[i]);

and

[2]

    for(i=0;i<=999999;i++)
           printf("%ld\n",a[i]);

Any other efficient method is appreciated. Thanks in advance!!

Answer 1

I suggest you're looking in the wrong place for speed improvements. Let's look at just printf( ):

1. printf( )is limited by the huge time it takes to physically (electronically?) put characters on the terminal. You could speed this up a lot by using sprintf( ) to first write the output chars into an array; and then using printf( ) to send the array to the tty. printf( ) buffers lines anyway, but using a large, multi-line output array can overcome the setup delay that happens for every line.

2. printf( ) formatting is a tiny part of its overhead. You can be sure that the folks who wrote this library function did their best to make it as fast as it could be. And, over the forty years that printf( ) has been around, many others have worked it over and rewritten it a few zillion times to speed it up. No matter how hard you work to do the formatting that printf( ) takes care of, it's unlikely you can improve very much on their efforts.

scanf( ) delays and overhead are analogous.

Yours is a noble effort, put I don't believe it can pay off.

Answer 2

printf is an operation on a FILE * and it buffers, puts operates on a FD and does not. Build output in buffer and then use puts. printf also has to parse the format string and takes variable type args; if you know the value is an integer, you can avoid all that by doing some math and add value of each digit to '0' and formatting the number yourself.

Answer 3

boost::karma (part of boost::spirit) promises quite good generator performance. You might try it.

Answer 4

If it is crucial that the numbers are read fast, and written fast, but not necessarily important that the output is presented to the user fast, or read from the file fast. You might consider making a buffer between the input/output streams and the processing.

Read the file into the buffer, it can be done in a separate thread, and then extract the number from this buffer instead. And for generating the output write to a memory buffer first, and then write the buffer to a file afterwards, again this can be done in a separate thread.

Most IO routines are relatively slow, since accessing information on the disk is slow (slower than the memory or cache). Of course this only makes sense if it is not about optimising the entire output/input phase. In which case there is no point in going through a separate (own implement) buffer.

By separating the parsing of the numbers and the IO part, you will increase the perceived speed of the parsing tremendously.

If you are looking for alternatives faster than scanf / printf you might consider implementing your own method that is not depending on a format string. Specialised implementations are often faster than the generalised ones. However do consider it twice before you start reinventing the wheel.