Asked by subha
on 25 Aug 2014

Variable poshidstates have 4 possible values(0,1,2,3). Everytime, poshidstates has to select 1 value out of this four.In this below code, i=100,j=500. Because of below mentioned code part , my program run more than 8 hour instead of just 1 hour(I have to call this code for 600 batches and each batch for 50 times). how can i replace this?

val_vect=[0 1 2 3]; for i=1:numcases; for j=1:numhid; prob=[poshidprobs0(i,j),poshidprobs1(i,j),poshidprobs2(i,j),poshidprobs3(i,j)]; K=find(mnrnd(1,prob)==1); poshidstate(i,j)=val_vect(K); end end

Answer by Iain
on 29 Aug 2014

Accepted Answer

Ok, now I see why it takes so long. You're repeating that calculation 30,000 times.

But seeing what you're doing, this might be faster:

temp = rand(100, 500); poshidstate = (temp > poshidprobs0) + (temp > (poshidprobs0 + poshidprobs1)) + (temp > (poshidprobs0 + poshidprobs1 + poshidprobs2));

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## 16 Comments

## Geoff Hayes (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_233498

Is it always guaranteed that

probhas an element that is equal to one? And that it has at most one element like this?As for speeding it up, have you pre-allocated memory to

poshidstate? Just prior to entering theforloop doThough, to be honest, I don't think that this pre-allocation will reduce the running time by all that much.

## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_233502

yes. I have done this already. With that only , it took 8hrs to run. Yes, when add all those four probs it gaurantees to give 1. The idea behind this code is, i have defined already probability of getting those 4 values and named as poshidprobs 0,1,2,3. I want poshidstates should select atleast one value out of those four. So, i preferred categorical distribution.

## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_233896

@ salaheddin k gets only one value every time. out of the four probs value, only one can be selected everytime

## Geoff Hayes (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_233962

Subha - where in the code are you spending the eight hours? I don't have the

Statistics Toolboxso can't observe how expensivemrndis, but running the above with your dimensions of i==100,j=500, takes around a second.So out of curiosity if you replace

with

how long does it take to run the code?

And as Salaheddin commented, have you verified that the result returned by

is in fact a 1x1 scalar, and not a vector of multiple indices?

## Iain (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_233975

The only two things I can think of that would make this code so slow is poshidprobs0,1 2 & 3 being anonymous functions, or you're running out of RAM.

## Geoff Hayes (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_233983

@Iain - that is a good point. I had assumed that the

poshidprobswere matrices.@Subha - are the

poshidprobsXmatrices or functions?## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234118

My code runs this part of code to 600 batches(600 example or 600 times) and each batch will be called for 50 epochs(50 times). When i remove this part from my code and run with some approximation. It could finish it within less than one hour. But when i include this part, it tooks approximately 8 hrs. poshidprobs is matrix.100*500 matrix.

## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234119

@lain- what you meant by anonymous function

## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234120

@geoff- yes k is 1*1 scalar. Not a vector of mutiple indices

## Geoff Hayes (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234126

Subha - when you say

When i remove this part from my code and run with some approximation, what part of the code have you removed? Please describe the line.And, what are

poshidprobs0,poshidprobs1,poshidprobs2, andposhidprobs4? Are they matrices or function calls?## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234127

what i guess is, prob is taken elementwise. So it tooks so much time as i have too many batches and too many epochs. If somehow it could be calculated in the form of matrix, time could be reduced. Any suggestion in this angle?

## Geoff Hayes (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234128

Subha - I missed from your previous comment that the

poshidprobsXare matrices.But when you say

When i remove this part from my code and run with some approximation, what part of the code have you removed? Please describe the line, and the approximation.Given how you have mentioned

batchesandepochs, I get the feeling that we are not seeing all of your code, only a piece of it...## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234131

@geoff- AS you asked,i have replaced k=1 and run it. it takes approximately 58 min. poshidprobs 0,1,2,3 are matrices with size(100*500)

## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234132

@Geoff. I removed whole lines whatever i posted here. Instead that i used, poshidstate = poshidprobs1+ poshidprobs2+poshidprobs3 . During that time, it took around 40 min only. It was working fine. But it is an approximation.I want to use the one,posted here. numbatches=600.

yes this is part of my program. As my code is around 100 lines long, i couldnot post everything over here. But in short, my program looks likes this.

## Iain (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234155

Ok, now I see why it takes so long. You're repeating that calculation 30,000 times.

But seeing what you're doing, this might be faster:

## subha (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/152204-how-do-i-replace-this-code-to-fast-up#comment_234331

Hi lain, Thanks a lot..I am so happy. You saved lots and lots of time .Problem resolved. I am searching for this for long. Now its just take 40 minutes. Thank u so much.No words to express.So happy. Please post this as answer. So many will be looking for this. Thanks mathworks

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