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

Question

mahaju 2020-6-26

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/555088-why-does-exceeding-array-index-in-this-program-not-show-an-error

评论： per isakson 2020-6-26

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
     array_b(1,(kk+1)*R:-1:kk*R+1)=array_b(1,kk*R:-1:(kk-1)*R+1);
   end
end  

Running this program does not show any errors. If I put a breakpiunt at the line

array_b(1,(kk+1)*R:-1:kk*R+1)=array_b(1,kk*R:-1:(kk-1)*R+1);

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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Answer 1

per isakson 2020-6-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/555088-why-does-exceeding-array-index-in-this-program-not-show-an-error#answer_457354

编辑：per isakson 2020-6-26

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 =
     1
K>> a(3)=3;
K>> a
a =
     1     0     3

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mahaju 2020-6-26

Thank you very much. So if I initially make array_b 1024 elements long, does the program behave exactly the same way?

per isakson 2020-6-26

"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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