Matlab: Find minimum value from a set of values for each column in an array

Question

I have a 2-D array A=zeros(1000,1024). I want to iteratively compute the difference between each of the value of the i-th row (with i=1-999) and the values of 1000th row.

Right now I think of looping over the 1-999 rows, compute the differences of the current row and the 100th row and store it in a seperaate data structure (B=zeros(999,1024)). After, I compute the minimum of each column using another for-loop, which iterates over the columns of B.

Do you know of a more efficient, faster approach?

Luis Mendo · Accepted Answer · 2014-09-10 16:03:06Z

3

If you only want the minimum of each column, you can save many operations by doing the subtraction at the end:

min(A(1:end-1,:),[],1) - A(end,:)

edited Sep 10, 2014 at 16:03

answered Sep 10, 2014 at 13:59

Luis Mendo

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Comments

Dan · Accepted Answer · 2014-09-10 13:58:10Z

1

This is a classic use case for bsxfun:

M = rand(1000,1024);
V = M(end,:);

MminusV = bsxfun(@minus, M(1:end-1,:), V);

min(MminusV)

answered Sep 10, 2014 at 13:58

Dan

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2 Comments

Divakar Over a year ago

My solution was wrong actually as I had included the last row into bsxfun. Now its correct but identical to yours :)

Dan Over a year ago

@Divakar but Luis' beats them both ;)

Divakar · Accepted Answer · 2014-09-10 14:42:27Z

1

Try this -

min(bsxfun(@minus,A(1:999,:),A(1000,:)),[],1)

It seems you want to subtract from the last row, so you can make it general -

min(bsxfun(@minus,A(1:end-1,:),A(end,:)),[],1)

edited Sep 10, 2014 at 14:42

answered Sep 10, 2014 at 13:57

Divakar

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5 Comments

Luis Mendo Over a year ago

+1 but I think min(bsxfun(@minus,A(1:end-1,:),A(end,:)),[],1) suits the question better

Dan Over a year ago

Do you need the ,[],1?

Luis Mendo Over a year ago

@Dan That's to prevent min from working along a row if there's just one row

Divakar Over a year ago

@Dan Sort of got into the habit to specify the dimension to avoid confusion.

Divakar Over a year ago

@LuisMendo Dan's suggestion works too, but just a safer practice I am trying these days :)

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