C++ Is there any way to make a program run faster?

Question

For example.. if I had.

#include <iostream>
using namespace std;

int main()
{
   int counter = 0;
   while (true)
   {
      cout << counter << endl;
      counter++
   }
}

And say I was on a race to counting to 1 billion against other computers, is the rate at which this loop runs purely dependent on the computer processor speed? Or is there a limit on how fast my program can run, which could be changeable?

It's limit is mostly from the fact that outputting each line is slow. It could be made a bit faster by not flushing the stream every line, which is what `endl` does. You'd have to replace it with a newline character. — MicroVirus, Nov 06 '15 at 00:00
Pipe the output of the program to /dev/null so the speed of your terminal (or other output window) isn't a factor. — MrEricSir, Nov 06 '15 at 00:04
C++ is faster because templates. Example: `qsort` vs `std::sort` — Guillaume Racicot, Nov 06 '15 at 07:09

score 14 · Accepted Answer · answered Nov 06 '15 at 00:00

14

Get rid of the endl and use "\n" instead. Plan on at least a 4x speed up from that alone.

Write the output to a file instead of the screen. That should be good for another 10x speed improvement (or so--more if you use an SSD).

answered Nov 06 '15 at 00:00

Jerry Coffin

476,176
80
629
1,111

score 3 · Answer 2 · answered Nov 06 '15 at 01:29

3

Use printf from <cstdio>, it's a good bit faster than cout.

printf("%d\n", counter);

answered Nov 06 '15 at 01:29

Andy M

596
3
7

1

Care to elaborate why? – Marc Dirven Apr 13 '21 at 09:51

score 3 · Answer 3 · answered Sep 01 '18 at 14:32

3

If you are using cout but NOT using anything from the library <cstdio>, you can write this in the beginning of the int main() function:

    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

answered Sep 01 '18 at 14:32

user10303922

31
1

score 2 · Answer 4 · edited May 06 '23 at 01:05

You can easily make this code 10x faster by following a few steps:

Don't use #include <iostream> library, instead use #include <cstdio>. It has a lot of complex things which will make your code slower.
Always add void parameter inside your main function as many compilers automatically assume it int main(int argc, char *argv[]) so it your program is gonna assume and be ready for taking arguments from the user making your program slow.
While compiling making sure to add the -O3 flag in gcc or clang. Example: gcc main.c -o main.o -O3.

score 1 · Answer 5 · answered Nov 06 '15 at 00:15

The slowness of your posted program comes from formatting internal representation to a human readable form (textual representation) and outputting the textual representation.

One optimization not mentioned is to buffer your formatted output, then ouput it. For example, write the formatted text to a buffer, then every 100 or so counts, print out the buffer using a block write. The objective is to reduce the number of output transactions and to make each transaction have a larger amount of data. Basically, one output of 1024 characters will be faster than 1024 outputs of 1 character.

The output depends on the OS and other factors that are beyond your program's control. Your program send the data, for output, to the OS and waits for the OS to complete the request. The completion time depends on task priorities and resource availability (at least). So if your program can count in milliseconds but the I/O takes seconds, your out of luck as no program optimization will help.

You do realize that an iostream uses a streambuf, which normally does buffering, about like you've described? (Oh, but at least most compilers I've used recently use a larger buffer than you've described, so your method would probably slow it down). — Jerry Coffin, Nov 06 '15 at 00:20
Yes, I'm aware of the stream buffer, but I like the old fashioned control method. Also, there is the overhead of each call to the output and reducing the quantity of calls reduces the overhead, saving time. — Thomas Matthews, Nov 06 '15 at 00:44

C++ Is there any way to make a program run faster?

5 Answers5