# [Tutor] Subtract a 1D Array from a 2D Array

Steven D'Aprano steve at pearwood.info
Sat May 13 11:33:36 EDT 2017

```On Sat, May 13, 2017 at 09:33:19AM -0400, Stephen P. Molnar wrote:

> I have two arrays:
>
>         X           Y          Z
>     	            a
>         0	        0	       0
>      2.059801	    0	       0
>     -1.203126	 3.402953	   0
>     -1.108639	-1.567853	 2.715601
>     -0.938564	-1.32733	-2.299003
>     	           a_c
>     0.4283375	 0.91755	 0.208299

I'm having trouble interpreting what that means. What are X, Y, Z? Are
they part of the array? Which array? Why is there a lone "a" in the
middle of row 2, and a lone "a_c" in the middle of row 8?

Can you give some Python code that creates the two arrays? And then show
us what result you expect to get?

Since the actual values in the array are not important, please use
SIMPLE values. We shouldn't have to do detailed mental arithmetic
with seven-significant figures to understand what you are trying to
do. You should use nice simple single-digit (or at most two-digit)

My *guess* is that you have an array called "a" with 3 columns and 5
rows, and a second array called "a_m" with 3 columns and 1 row, and you
want to subtract a_m from each row of a.

a = [[ 9  8  7 ]
[ 5  5  5 ]
[ 4  4  4 ]
[ 3  3  3 ]
[ 2  2  2 ]]

a_m = [[ 1  2  3 ]]

And you want the result of a - a_m to be:

[[ 8  6  4 ]
[ 4  3  2 ]
[ 3  2  1 ]
[ 2  1  0 ]
[ 1  0 -1 ]]

Am I close? If this is not what you want, please explain what you
actually do want.

--
Steve
```