Run single equation many times

Here I have code that finds W over again 1000 times

nsample1 = 255;
nsample2 = 160;
X = 1.2 + 0.15*randn(1,nsample1);
Y = 1.1 + (1.25-1.1)*rand(1,nsample2);
B = [X Y]; 
c = 1e3;
for i = 1:c;
  W(i+1,:) = 10*sum(B);
end

When I run the code it goes 1000 times, all outputs are the same. I want 1000 outputs that are nearly the same to eachother. X and Y using rand and randn to generate random numbers to get a different B each time, do I have something wrong in my for loop? Supposed to be like a monte carlo simulation.

Answer 1

[Geoff Hayes]

Luke - on each iteration of the for loop, you are summing the same B that was initialized outside of the for loop

 W(i+1,:) = 10*sum(B);

You will need to re-initialize B on each iteration of the loop in order to get new values. For example,

nsample1 = 255;
nsample2 = 160;
c = 1e3;
W = zeros(c,1);
for i = 1:c;
    X = 1.2 + 0.15*randn(1,nsample1);
    Y = 1.1 + (1.25-1.1)*rand(1,nsample2);
    B = [X Y]; 
    W(i+1,:) = 10*sum(B);
end

Note also how W is pre-sized outside of the loop. Try the above and see what happens!

[Luke Radcliff]

I see now, you have to put x y and b into the for loop so it re-randomizes the values every time, thank you.

Answer 2

[Walter Roberson]

Your B is a vector. sum(B) is going to be a scalar. And you do the same sum(B) in each repetition of the loop. So all of the W elements are going to be the same, except W(1,:) which you do not store into.

I might have guessed that you want X+Y instead of sum([X,Y]) but your X and Y are different lengths, so I do not know what you are trying to calculate.

[Luke Radcliff]

okay so X and Y represent two different kinds of parts/pieces. The X piece is 1.2 lbs with a standard dev of 0.15 lb and there are 255 random pieces within that spec. Y piece is 1.1 lb to 1.25 lbs, there are 160 random pieces within that spec. W is the the total pallet weight, there are 10 boxes on the pallet. 1 X(255 pieces) and 1 Y(160 pieces) are in 1 box. I combined both vectors into 1 vector and took the sum and multiplied it by 10 to find the total weight of the pallet. Since the parts sizes are different everytime I run the code I want a different W for 1000 times, like a simulation.