Sum Over Negative Observations of a Column

[Charlie Clarke]

I am calling mata in a stata do file. The following code sums over the columns in matrix A. Is there a way to only sum over negative entries?

mata : st_matrix("B", colsum(st_matrix("A")))

So:

A = [1 2; -2 -2; -3 -1]
B = [-5 -3]

[Joseph Coveney]

You can do something like the following. I've spelled it out so that you can see the intermediate results, but you can condense the code in obvious ways. There might even be a function or two already built into Mata to do some of the wrangling more compactly; I'm often amazed to discover on the List what's available there.

.ÿsetÿlinesizeÿ72

.ÿmata:
-------------------------------------------------ÿmataÿ(typeÿendÿtoÿexit
>ÿ)ÿ--------------------------------------------------------------------
:ÿAÿ=ÿ1,ÿ2ÿ\ÿ-2,ÿ-2ÿ\ÿ-3ÿ,ÿ-1

:ÿSelectÿ=ÿA:<0

:ÿSelect
ÿÿÿÿÿÿÿ1ÿÿÿ2
ÿÿÿÿ+---------+
ÿÿ1ÿ|ÿÿ0ÿÿÿ0ÿÿ|
ÿÿ2ÿ|ÿÿ1ÿÿÿ1ÿÿ|
ÿÿ3ÿ|ÿÿ1ÿÿÿ1ÿÿ|
ÿÿÿÿ+---------+

:ÿCÿ=ÿAÿ:*ÿSelect

:ÿC
ÿÿÿÿÿÿÿÿ1ÿÿÿÿ2
ÿÿÿÿ+-----------+
ÿÿ1ÿ|ÿÿÿ0ÿÿÿÿ0ÿÿ|
ÿÿ2ÿ|ÿÿ-2ÿÿÿ-2ÿÿ|
ÿÿ3ÿ|ÿÿ-3ÿÿÿ-1ÿÿ|
ÿÿÿÿ+-----------+

:ÿBÿ=ÿcolsum(C)

:ÿB
ÿÿÿÿÿÿÿÿ1ÿÿÿÿ2
ÿÿÿÿ+-----------+
ÿÿ1ÿ|ÿÿ-5ÿÿÿ-3ÿÿ|
ÿÿÿÿ+-----------+

:ÿend
------------------------------------------------------------------------

.

[Nick Cox]

What do you want to happen if a row contains a mix of positive and negative values?

Joseph's code can be collapsed to

: A = (1, 2 \ -2, -2 \ -3, -1)

: colsum((A :< 0) :* A)
        1    2
    +-----------+
  1 |  -5   -3  |
    +-----------+

[Frank Erickson]

Assuming you only want rows that are entirely negative, you can use the select function:

colsum( select(A, rowsum(A :< 0)) )

If you want at least one column to be negative then use rowsum(A :< 0) >= 1 in the second argument to select.

If you want each column treated independently, then the matrix multiplication approach works.

[Nick Cox]

Frank: That condition selects rows with any values negative, and not exclusively those with all values negative. The rowsum is positive with just one negative value.

[Frank Erickson]

Doh, you're right, thanks Nick Cox

[Charlie Clarke]

Thank you, Joseph and Nick. That works beautifully.