First iteration in for loop is slower
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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.
采纳的回答
Ken Atwell
2014-6-27
0 个投票
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.
2 个评论
I Zendejas
2014-6-28
Image Analyst
2014-6-28
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.
更多回答（1 个）
ALVARO
2023-2-2
0 个投票
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.
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