How to add existing values of an array with each other until a certain value is reached

So I am tasked with using a rand function to generate many random numbers between 0-1.

After doing so and storing them in an array, I have to add each element until the sum exceeds 1, then stop there.

Ex: Say x = rand(1, 6) generates (0.12, 0.44, 0.33, 0.32, 0.55, 0.19). I would need sum up just 0.12, 0.44, 0.33, and 0.32.

sum(x) would not work as that just sums of all the generated numbers. I need it to stop immediately after it exceeds 1.

I believe I would need to use the while loop...

[Star Strider]

‘I believe I would need to use the while loop...’

I agree.

[Jos (10584)]

Depending on what "I need it to stop" exactly means, you may also benefit from using cumsum

Answer 1

[Adam Danz]

If you are drawing random numbers between [0, 1] until the sum of your draws reaches 1.0, you'll likely only draw a few numbers (1 to 5, let's say). Its highly unlikely that you'll need much more than that but sometimes you will. So we will draw 5000 just to be safe.

This solution draws 5000 random numbers between [0, 1], calucalutes the cumulative sum, and then detects when the cumulative sum crosses the 1.0 value, eliminating all samples that follow.

randSamp is your list of random variables whose cumulative sum just passes 1.0.

randSamp = rand(1,5000); 
crossIdx = find(cumsum(randSamp) >=1, 1); 
randSamp(crossIdx+1 : end) = []; 

If for some reason you need a while loop,

randSamp = []; 
while sum(randSamp) < 1
    randSamp(end+1) = rand(); 
end

[Daniel Kwak]

Thank you. Just out of curiousity, let's say I wanted to start summing the elements from the 3rd element, then using your code as reference, it would be

crossIdx = find(cumsum(randSamp) >=1, 3)?

[Adam Danz]

The line find(foobar, 3) just finds the first 3 true indices of the logical vector foobar.

If you want to start the cumulative sum at the 3rd element of randSamp,

randSamp = rand(1,5000); 
startAt = 3;                             %start at 3rd element.
csum = cumsum(randSamp(startAt:end));    %cumulative sum from sample 3, onward.   
crossIdx = find(csum >=1, 1);            %find index of the first number > 1.
randSamp(crossIdx + startAt : end) = []; %don't forget to offset by 'startAt'.

[Daniel Kwak]

That makes sense. Thank you for the help.