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# For Loop runs too many times

Asked by 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..

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Answer by Matt Fig on 9 Dec 2012
Edited by 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...

Tico on 9 Dec 2012

Hi Matt.. Thnx for your answer. The thing is when I run the loop, I get the following output:

C =
1     1     1
1     0     0
C(:,:,1) =
1     1     1
1     0     0
C(:,:,2) =
0     0     1
1     0     1
C(:,:,1) =
1     1     1
1     0     0
C(:,:,2) =
0     0     1
1     0     1
C(:,:,3) =
0     0     1
1     1     0
C(:,:,1) =
1     1     1
1     0     0
C(:,:,2) =
0     0     1
1     0     1
C(:,:,3) =
0     0     1
1     1     0
C(:,:,4) =
1     1     0
1     1     0

And also, I need the loop coz I am trying to simulate a game and I have to populate the 3d array n times. It's a really long script (probably badly implemented) but I don't how to do it without a loop. I need to populate the array n times, then compare the rows of each individual matrix with random numbers..

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!

Answer by Azzi Abdelmalek on 8 Dec 2012
Edited by 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)

Tico on 9 Dec 2012

Thx Azzi but when I ran your code I still get more than 4 matrices. And some of them are just zeros matrices. That said, I have been digging around and I think I have a solution. All I have to do is set the step value to 4.. The code below seems to work.

for j=1:4:4
A=randi([0 1],2,3);
C(:,:,j)=A
end

But when I try it using for i=1:n:n I am getting n-1 zeros matrices and one random matrix.. Any help?

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