How do I create a custom 4x4 array using NumPy?

Question

I am new to Python and I am having a bit of trouble with the array functions. I want to make a 4 by 4 array which contains the numbers from 1 to 16.

I know that using np.zeros((4,4)) outputs a 4x4 array with all zeros. Using np.array(range(17)) I can get an array of the required numbers BUT not in the correct shape (4x4).

It must be fairly simple, surely? All comments are much appreciated.

bogatron · Accepted Answer · 2013-09-23 18:42:39Z

9

The problem is that you are creating an array of 17 values (zero through 16), which can't be reshaped to 4x4. Instead:

>>> a = np.arange(1, 17).reshape(4,4)
>>> a
array([[ 1,  2,  3,  4],
       [ 5,  6,  7,  8],
       [ 9, 10, 11, 12],
       [13, 14, 15, 16]])

answered Sep 23, 2013 at 18:42

bogatron

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6 Comments

turnip Over a year ago

Oh I see, I should have noticed that. So is this the standard way of producing custom arrays?

askewchan Over a year ago

It depends on how "custom" you want them to be, but it's a very common starting point. arange only gives sequential values (but can start at any number and take steps other than one). reshape works on any array as long as the number of values stack nicely.

bogatron Over a year ago

It depends on what you mean by custom. There are numerous ways to create and/or initialize an array. If you just want the shape, you can use np.empty, np.empty_like, np.zeros, etc. If you also want to initialize the values, you can call np.fromfunction or others.

turnip Over a year ago

I am assuming that "initialising the values" means to make my own values? What would be the process if I want a n-by-m array with a list of numbers (that fit nicely)? I tried the following and it works nicely: 'a=(1,2,4,66,88,99,5,10)' 'b=np.array(a).reshape(4,2)' 'print b' This returns a 4-by-2 array of the desired numbers. Efficient? Or is there a better way. PS: I can't seem to get the hang of this mini-markdown formatting for the code lines. I apologise for that.

bogatron Over a year ago

By initialize, I mean "set its initial values". What you did is fine.

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