Referencing value of an array based on the values of another - matlab

Question

I have an array d = [0,1000,2000]. Based on this array, I compute the value of another array, let's say J = [0,5000,8000], where J = 3*d+2000.
Now, during iteration using a for loop, if d=0, I want to extract value of J(1); if d = 1000, then I want to extract values of J(1) and J(2); if d = 2000, I want to extract values of J(1),J(2) and J(3). What would be a generic way to cross-reference values of two arrays?

Answer 1

Assuming your values in d are unique and the length of d and J are identical, use MATLAB's find function, especially using this version with n = 1, and direction = 'first', like so:
idx = find(d == ii, 1, 'first')
Here, ii is the desired value, e.g. 0, 1000, and so on. You'll get the index of the (first) occurrence of the desired value in d. Then, just extract all values from J from 1 to idx:
J(1:idx)
Here's a small test script:
d = [0, 1000, 2000]
J = 3 * d + 2000
% If your counter variable only contains valid values from d
for ii = d
idx = find(d == ii, 1, 'first');
printf('d = %d\n', ii);
printf('Extracted values are: ');
printf('%d ', J(1:idx));
printf('\n \n');
end
% If your counter variable may contain any value
for ii = 0:2000
idx = find(d == ii, 1, 'first');
if (~isempty(idx))
printf('d = %d\n', ii);
printf('Extracted values are: ');
printf('%d ', J(1:idx));
printf('\n \n');
end
end
And, the output looks like this:
d =
0 1000 2000
J =
2000 5000 8000
d = 0
Extracted values are: 2000
d = 1000
Extracted values are: 2000 5000
d = 2000
Extracted values are: 2000 5000 8000
d = 0
Extracted values are: 2000
d = 1000
Extracted values are: 2000 5000
d = 2000
Extracted values are: 2000 5000 8000
Hope that helps!
P.S. I haven't thought about vectorization here, since the question states, that a for loop is used in any case.