logical statement accelerates or decelerates speed?

i have the following code. i intended to check whether logical statement will affect the speed significantly.

it turns out to be quite strange. the first loop is twice faster than the second.

can anyone explain it?

my matlab is 2011b. my laptop is back to 2007 and my system is Ubuntu.

==========================

f=1;

tic

for s=1:100

    if f==1

    a=rand(1000,1000);

    a=a*a;

    end
end
toc

tic

for s=1:100

    a=rand(1000,1000);

    a=a*a;
end

toc

Answer 1

[Azzi Abdelmalek]

It's because, when the second for loop wa executed, your computer was more busy. I tried your code, the second for loop was faster 43 sec against 49 sec

[Azzi Abdelmalek]

If I have to test the effect of If ,I will choose a simple code

f=1;
t1=0;t2=0;
for s=1:1000000 
  tic;
  if f==1
  a=1;
  end
  t1=t1+toc;
  tic;
  a=1;
  t2=t2+toc;
end
t1
t2

Answer 2

[Muruganandham Subramanian]

Because, when you are increasing the no. of logical expression,leads to increase the memory allocation (i.e both temporary and data lenghts) and it depends on data types also..

Answer 3

[Fangjun Jiang]

Use profile to find out how much time each statement takes.

I ran your code. It is strange that first loop is faster than the second.

Elapsed time is 6.062211 seconds.
Elapsed time is 6.134868 seconds.

However, if you change the loop to be 1000 but the size of a to be 100, the result is different. I think the impact of the f==1 statement is so minuscule that it is hard to evaluate in this context.

Elapsed time is 0.307561 seconds.
Elapsed time is 0.272184 seconds.