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New to MATLAB?

For Loop runs too many times

Asked by Tico

Tico (view profile)

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..

0 Comments

Tico

Tico (view profile)

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2 Answers

Answer by Matt Fig

Matt Fig (view profile)

on 9 Dec 2012
Edited by Matt Fig

Matt Fig (view profile)

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

Tico

Tico (view profile)

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

Matt Fig (view profile)

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!

Matt Fig

Matt Fig (view profile)

Answer by Azzi Abdelmalek

Azzi Abdelmalek (view profile)

on 8 Dec 2012
Edited by Azzi Abdelmalek

Azzi Abdelmalek (view profile)

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

Tico

Tico (view profile)

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

Azzi Abdelmalek (view profile)

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

Azzi Abdelmalek

Azzi Abdelmalek (view profile)

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