First iteration in for loop is slower

I'm decoding some data which randomly can have sign, all I know is that there is a character before the point character and there are two characters after the point. I have noticed that the first iteration in the for loop is slower than the next iterations. I'm wondering whether you have any idea about it. Is that because a kind of initialization?

Here you go a part of my code:

close all; clear all; clc;

data='-0.01-0.020.03';
inc = [0 0 0];
inc_t = 0;
xyz_accel = [0 0 0];

for j=1:3
    tic
    if data(1+inc_t)=='-'
        xyz_accel(1,j) = str2double( data(inc_t+1:inc_t+5) );
        inc(1,j) = 5;
    else
        xyz_accel(1,j) = str2double( data(inc_t+1:inc_t+4) );
        inc(1,j) = 4;
     end
     inc_t = sum(inc);
     toc
end

xyz_accel

Elapsed time is 0.017351 seconds.

Elapsed time is 0.000255 seconds.

Elapsed time is 0.000327 seconds.

xyz_accel =

   -0.0100   -0.0200    0.0300

Thanks in advanced.

Answer 1

[Ken Atwell]

MATLAB works a bit harder up front to accelerate performance later. You're most likely seeing this effect.

The performance difference will tighten up if you remove the 'clear all'. The gap will not completely close, but the first iteration speeds up by 10-fold on my Mac.

[I Zendejas]

OK Mr.Ken Atwell, it is good to know that. It is just that I searched similar questions and they have exactly the opposite case, they get iterations slower than the previous.

Thank you very much.

[Image Analyst]

There can be reasons for that too, such as you're plotting or displaying an increasing number of things, or you're not preallocating memory and so you're dynamically reallocating larger and larger array sizes.

Answer 2

[ALVARO]

I guess this comment only applies after 2015 version, but I believe what matlab is doing is jitting the for loop (Just in Time Compilation). It would be nice is someone could confirm this.