MATLAB Answers

Why does exceeding array index in this program not show an error?

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mahaju on 26 Jun 2020
Commented: per isakson on 26 Jun 2020
This is most basic code to reproduce my problem:
clear all; clc; close;
R = 64;
array_b = zeros(1, 1000);
for i = 1:10000
for kk=15:-1:2, % if ,WOLA_R=64
Running this program does not show any errors. If I put a breakpiunt at the line
and try to view the value of array_b(1,(kk+1)*R:-1:kk*R+1), I get
??? Index exceeds matrix dimensions.
which is what I expect. Size of array_b is (1, 1000) and for kk = 15 (first iteration of the loop), (kk+1)R:-1:kkR+1 = 1024 > 1000, so I should get an error.
But if I run the program normally I don't get an error. Why is that? This is supposed to copy the contents of array_b from lower indices to higher indices, 64 elements at a time. During the first iteration of the loop, if it does not make an error, where do the elements that are supposed to go from 1001 to 1024 go to?


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Answers (1)

per isakson
per isakson on 26 Jun 2020
Edited: per isakson on 26 Jun 2020
Because there are values of the second index, which exceeds 1000 (as you have noticed)
K>> (kk+1)*R:-1:kk*R+1
ans =
Columns 1 through 6
1024 1023 1022 1021 1020 1019
Columns 7 through 12
1018 1017 1016 1015 1014 1013
the assignment then increases (silently) the size of array_b
K>> whos array_b
Name Size Bytes Class Attributes
array_b 1x1024 8192 double
Matlab works that way
K>> a=1
a =
K>> a(3)=3;
K>> a
a =
1 0 3


mahaju on 26 Jun 2020
Thank you very much. So if I initially make array_b 1024 elements long, does the program behave exactly the same way?
per isakson
per isakson on 26 Jun 2020
"1024 elements long" pre-allocating to the exact size makes the program a bit faster. I cannot think of any other difference. Since you start by assigning to the highest index, I guess you could save a tine bit of time by skipping the pre-allocation. That was once true.

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