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For Loop runs too many times

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Tico
Tico on 8 Dec 2012
Hi everyone.. I'm new to matlab and am trying to populate a 3D array by the results of a loop:
for j=1:4
A=randi([0 1],2,3);
C(:,:,j)=A
end
And I am getting different 2x3 C matrices where j=1, 2, then j=1,2,3 then j=1,2,3,4. Is there a way I can run the loop just for j=1, then j=2, thenj=3 and j=4? I thought I should be getting a 3D array containing just 4 "pages" (i-e, 4 matrices??
Thnx..

Answers (2)

Matt Fig
Matt Fig on 9 Dec 2012
Edited: Matt Fig on 9 Dec 2012
I am confused about your question. You say you want to have C be a 2-by-3-by-4 array at the end of the loop, correct? That is exactly what you have at the end of your loop!
>> clear all
for jj = 1:4
A=randi([0 1],2,3);
C(:,:,jj)=A; % Semicolon!!
end
whos C % This tells us what we have in C.
Name Size Bytes Class Attributes
C 2x3x4 192 double
Notice that C is the exact size you want. I wonder if you got confused by leaving off the semicolon so that while the loop was running and so you saw C being built-up for every iteration? This is just one reason to not forget to put a semicolon on your lines of code! There is no practical reason to watch all that screen-dump!
Also, why not just do:
C = randi([0 1],2,3,4); % No loop needed...
  2 Comments
Matt Fig
Matt Fig on 9 Dec 2012
Exactly! Did you read what I wrote??? Those outputs are you seeing the array being built up during the loop, not the final array. Put a semicolon on there (or copy and paste the code I gave you) and you will not see that awful mess any more....
Slow down and read what was written!

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Azzi Abdelmalek
Azzi Abdelmalek on 8 Dec 2012
Edited: Azzi Abdelmalek on 9 Dec 2012
C=zeros(2,3,4) % preallocate makes your code faster
for jj=1:4
A=randi([0 1],2,3);
C(:,:,jj)=A
end
The result is one matrix 2x3x4
% avoid using i or j (used to represent imaginary numbers)
  2 Comments
Azzi Abdelmalek
Azzi Abdelmalek on 9 Dec 2012
do you want
clear
k=0;
for j=1:4:4
k=k+1
A=randi([0 1],2,3);
C(:,:,k)=A
end
but for k=1:4:4 gives you just k=1

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