Issue with random function

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Matt 2015-9-21

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编辑： Stephen23 2015-9-21

采纳的回答： Matt

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Hello guys,

I have an issue with the function RANDOM that is not producing random numbers as it should. Here is the code:

for j=1:25
random(j, 1)=rand*(time(j, 1)/2);
random(j, 2)=rand*(time(j, 1)/2);
random(j, 3)=rand*(time(j, 1)/2);
end

If I input this in the command window and assign a value to "time", it works, but in my script, I am always getting the same number on every column! I tried to debug it with breakpoints, but I cannot see why from one line to the other, the "rand" function cannot produce random numbers.

Also inside this for loop, I then have different assignments to the time matrix, eg: time(j, 2) or time(j, 3) so it is not ok to just generate random numbers. I have to connect them with specific entries of the time matrix.

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Answer 1

Matt 2015-9-21

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I found the issue to the problem. I named a variable "rand" earlier in my code. Matlab didn't give me any information this would in fact replace the function RAND by the value of the variable rand I declared, so effectively the RAND function was not producing random numbers anymore!

I didn't know using a variable called rand would overwrite Matlab's RAND function. Good to know, problem solved! :-)

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Matt 2015-9-21

Yes. I remember reading something similar to "this is a variable allowed to matlab, it may cause issues to use it", but I cannot remember in what condition.

I just assumed it would not overwrite the function, as I implicitly never used variable names such as sum or mean.

Stephen23 2015-9-21

编辑：Stephen23 2015-9-21

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Rather than doing this in a loop, it would be faster and more efficient to generate the random numbers all at once. This avoids having to expand the output array in a loop, which is very inefficient.

>> X(:,1) = 0:24;
>> Y = bsxfun(@times,X/2,rand(25,3));

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Answer 2

James Tursa 2015-9-21

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What is the class of the variable random? Is it a double at the command line but something else in your script?

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Matt 2015-9-21

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I have declared it as being zeros right before the for loop:

random=zeros(25, 3)

Matt 2015-9-21

I must add that when I put the command RAND right after the start of the for loop, and then just RAND again before the end of the loop, I get the same number! Why is not not generating random numbers all the time? Strange.

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