How to add offset to all coordinates in a two-column numpy array?

Question

The python code below creates data for a numpy array that I use to graph a unit box on a graph:

box = np.array([[x, y] for x in np.arange(0.0, 1.01, 0.01) for y in
               np.arange(0.0, 1.01, 0.01)])

I want to transform box -- by adding a number to the x component and a different number to the y component -- into another numpy array so the new box appears elsewhere on the graph.

I am having some trouble figuring out if I can slice a numpy array to do the addition I need or what the correct loop syntax would be.

My question is: how do I add, say 1 to each x element and 3 to each y element?

So, if some element in the initial numpy array was [0.8, 0.5], that particular element would then be (in the new array): [1.8, 3.5]. All other elements would also have their x and y values updated the same way.

It appears that you have never selected an answer, which means that your questions are all on the unanswered queue. You should select an answer, even for closed questions, if it helped you fix your problem. It benefits the people that put time into helping you, plus you'll get a few rep points for doing it. Select an answer by clicking on the green check box next to it. You can go through your [old questions](https://stackoverflow.com/users/5120843/morkus?tab=questions) and fix them too. — Mad Physicist, Jul 02 '20 at 17:56

Mad Physicist · Answer 1 · 2020-07-02T17:40:47.023

You can use broadcasting. Right now you have an (n, 2) array. You can add a two element array to it directly.

offset = [1., 3.]
box2 = box + offset

This works because dimensions align on the right for broadcasting (and the list offset automatically gets converted to an array). (n, 2) broadcasts just fine with (2,).

To do the operation in-place (using same memory instead of creating a new output array):

box += offset

While you are at it, you may want to take a look at np.meshgrid and this question for examples of how to create the box much more efficiently than python list comprehensions.

Sai Sreenivas · Answer 2 · 2020-07-02T17:39:08.300

1

You can do something like this (just to explain how it works for individual columns):

# data is the array containing x,y values
 
data[:,:1] += 1 # for the first column
data[:,1:] += 3 # for the second column

print(data)

edited Jul 02 '20 at 17:39

answered Jul 02 '20 at 17:19

Sai Sreenivas

1,690
1
7
16

Yeah, I got that far. But, in a loop? I can't get the syntax right. I'll try with the +=. – Morkus Jul 02 '20 at 17:29
@Morkus. What does that mean? – Mad Physicist Jul 02 '20 at 17:32
Not sure how this would work in the code, though. data_new = data (x[:,:1] += 1, x[:,1:] += 3) doesn't work. Does this need to be in a loop? – Morkus Jul 02 '20 at 17:34
1

@Morkus. No, it applies to the entire array. Try it in your console. Don't be afraid to play around – Mad Physicist Jul 02 '20 at 17:35
No loop is required. To be more simple you can use other ansr provided for this question – Sai Sreenivas Jul 02 '20 at 17:35
The question seems to be asking pretty specifically how to draw a new box, not do the operation in-place – Mad Physicist Jul 02 '20 at 17:36
Use answer provided by @MadPhysicist . Its more simple – Sai Sreenivas Jul 02 '20 at 17:36
@Madphysicist - Sai above suggested +=1 and +=3, but did not give me enough context to implement. – Morkus Jul 02 '20 at 17:36
The array is called data. – Morkus Jul 02 '20 at 17:37
@Morkus sorry for that. Actually there is no need of loop, this will add values to all rows – Sai Sreenivas Jul 02 '20 at 17:37
2

@Morkus. There seems to be a disconnect here. Rather than asking about what will happen, run the code line by line and see what it does. That will answer more of your concerns than anything we can tell you. – Mad Physicist Jul 02 '20 at 17:39

How to add offset to all coordinates in a two-column numpy array?

2 Answers2