How to find the loaction of an element in an array

Say I have two arrays, one for outside temperature and the other for heat rate. All elements in the array line up and correspond to each other. Is there a way that I could get the location of a certain temperature (from the temperature array) and pull the heat value element (from the heat array) that corresponds to that temperature element location? For example:

T=[0 1 2 3]'; Q=[100 200 300 400]';

I'd want to choose T=2 --> which is in the (3,1) position. How can find out the corresponding Q value? I'm doing this because because the arrays are very large and some of the temperatures are the same values and I want to group the Qs for the same temperature. Thanks

Answer 1

[Matt Fig]

T=[0 1 2 3]'; 
Q=[100 200 300 400]';

Q_T = Q(T==2)  % Or, Q(find(T==2)), but not as efficient...

Be aware that if your actual values are not integers, you may need to use a tolerance instead of ==.

If you are grouping using cell arrays, consider:

T = [0 0 1 1 2 2 3 4 5 5].'; % T with some repeats.
Q = [9 6 7 8 8 9 6 6 7 8].';
[J,J,J] = unique(T)
G = accumarray(J(:),Q(:),[],@(x) {x}); % Group Q by unique T...

[Neev Podder]

Same question I want to ask. I understand your answer but I have floating point numbers in my array. How do I use a tolerance instead of == in my if statement.

[Naz]

It's better if you multiply all the values so you have integers (say be a thousand). Then, after you pull the integer just devide by 1000 to get back your float decimal value.

[Naz]

Also, check out 'nearest'. That should do it.

[Jan]

@Neev: Please post a new problem to a new question.

Checking existence with a tolerance has been discussed repeatedly here.

Do you want a relative or absolute tolerance?