for loop variable optimization in C/C++

Question

Which is faster in following two code snippets? and Why?

Declared loop index variable outside the for statement:

size_t i = 0;
for (i = 0; i < 10; i++) 
{

}

and

Declared loop index variable within the for statement:

for (size_t i = 0; i < 10; i++) 
{

}

In the first example you're initialising variable 'i' twice, you don't need to do that. — Simon Bosley
– Simon Bosley, Commented Apr 6, 2017 at 11:35
You can compare the resulting assembler code on for example godbolt.org and check yourself if they are equal and look up differences when they occur. — nwp
– nwp, Commented Apr 6, 2017 at 11:38
if you call function from single thread, then declaring loop counter as local static variable for function will save you 2 operations with stack (push, pop) per function call. Only you can measure if it will have any impact (number of function calls) — Andrew Kashpur
– Andrew Kashpur, Commented Apr 6, 2017 at 12:23
Look at the assembly code and/or benchmark. It's the only way to be sure. (Though in this case I see no reason there could be any difference.) — user1196549
– user1196549, Commented Apr 6, 2017 at 13:31

Lundin · Accepted Answer · 2017-04-06 11:34:53Z

6

Neither, they are equivalent and will yield the same machine code.

(The compiler will remove the redundant initialization of i twice from the first example.)

Where a variable is declared has very little to do with performance and memory use.

for (size_t i = 0; i < 10; i++) is usually considered the most readable.

for (i = 0; i < 10; i++) has the advantage that you can use the i variable after the loop is done - which makes most sense when the number of iterations is variable.

answered Apr 6, 2017 at 11:34

Lundin

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3 Comments

Peter Over a year ago

Being able to "use the i variable after the loop is done" is not necessarily an advantage.

Jabberwocky Over a year ago

@Peter it is not an advantage, but it can be one.

Lundin Over a year ago

@Peter I was thinking of cases such as for(size_t i=0; i<length; i++) { str[i] = whatever; if(bad) break; } str[i] = '\0';

Luis Cabrero · Accepted Answer · 2017-04-06 11:54:50Z

0

The only difference is that on the fist one, i is a global variable, and the second one is a local variable only for the loop.

edited Apr 6, 2017 at 11:54

answered Apr 6, 2017 at 11:37

Luis Cabrero

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3 Comments

Jabberwocky Over a year ago

There is no such thing as global-use variables. There are global variables, but here i is not global either in the first case. Enhance your question, you are on the right track.

Lundin Over a year ago

@MichaelWalz If you want to be picky, there is no such thing as "global variables" either. The C language has scope and linkage, the formal term would be "file scope variable with external linkage".

Luis Cabrero Over a year ago

Well, try to do this: for(int i=0; i<4;i++){ //do some stuff } printf("%d",i); Thats a local variable for the loop. If you declare it outside, you will be able to use it on another part of the code..

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