Outputing Array from Nested Why Loops

Hi, The purpose of my code is to save for a number n, say 3, the following in an array A:

A(1) = randi(3)
A(2) = randi(2)
A(3) = randi(1)
A(4) = randi(3)
A(5) = randi(2)
....

I have written the following code, but right now it is not doing what I want - how can I change that?

function [ArrayFinal] = RandomizeTrials(numberofSeq, lengthofArray)

ArrayFinal = zeros(1,lengthofArray)

i = 1;
while i < lengthofArray+1

      while i < numberofSeq+1

      Array(i) = randi(numberofSeq);
      numberofSeq = numberofSeq-1;
      i = i + 1;
      end
      i+3;

end

end

[Stephen23]

Note that you should avoid using i and j for loop variable names, as these are both names of the inbuilt imaginary unit.

Answer 1

[the cyclist]

There were lots of little issues with your code. I hope this was not homework, because I tried to pretty much fix them all:

function [Array] = RandomizeTrials(numberofSeq, lengthofArray)
Array = zeros(1,lengthofArray);
i = 1;
while i < lengthofArray+1
      n=1;
      while n < numberofSeq+1
          Array(i) = randi(numberofSeq+1-n);
          n = n + 1;
          i = i + 1;
      end
      i+numberofSeq;
end
end

[MiauMiau]

No, I was doing them for myself. Thank you, I will check what I did wrong! Why exactly is

numberofSeq = numberofSeq-1;

in the while loop wrong?

[Søren Jensen]

look at my answer and you should get it..

Basically you are changing numberofSeq at every itteration, and if you did not save it in a buffer (like me) or avoid changing it (like the cyclist), the second (third, fourth, fifth...) time you enter your internal loop will be very short (since "n<numberofSeq" already returns "false")

[MiauMiau]

oh so I don't have local variables in Matlab? Ie changing the numberofSeq in the while loop will change it outside of the loop too?

[Stephen23]

"local" variables are available everywhere within one workspace. It is quite handy because often the last loop iteration is a special case and the values can be referred to after the loop too.

[MiauMiau]

How comes then when I run:

j = 5;
i = 4;
for i=1:4
    A(i) = i*i;
    j-1;

end
j

j does not seem to be changed - it is still 5 at the end?

[Ilham Hardy]

if you change the code into this:

j = 5;
i = 4;
for i=1:4
    A(i) = i*i;
    j = j-1;
end
j

j will be 1 at the end of the for loop

[MiauMiau]

got it, thx!

[Stephen23]

Because j-1 does not allocate that new values to anything. MATLAB does not have a short-hand for incrementing/decrementing variables, so you need to explicitly assign that value: j = j-1

Answer 2

[Søren Jensen]

A(1) = randi(3)

A(2) = randi(2)

A(3) = randi(1)

A(4) = randi(3)

A(5) = randi(2)

so you have a minimum bound where the argument for randi resets, or is it reset when it hith the floor (1)?

anyways, using i for both loops should overwrite each other and mess up your intention i think. try this:

saved = numberofSeq; % saved for use in the outer itteration i = 1;

while i<(lengthofArray+1)

numberofSeq = saved;

for k=1:(numberofSeq+1)

      Array((i-1)*numberofSeq+k) = randi(numberofSeq);

      numberofSeq = numberofSeq-1;

end i=i+numberofSeq;

end

% you seem to be adapt in other coding languages, so you can probably guess my intention.. unless i missed yours

Answer 3

[Stephen23]

It is possible to vectorize this code a bit by using randi to generate vectors instead of many separate scalar values: this will likely be faster generally, and certainly if out_len/num_seq is a large value.

function out = RandomizeTrials(num_seq, out_len)
X = max(2,ceil(out_len/num_seq));
for k = num_seq:-1:1
    out(k,:) = randi(num_seq-k+1,1,X);
end
out = out(1:out_len);
end