Is there a numpy function that allows you to specify start, step, and number?

Question

We’re all familiar with np.linspace, which creates an array given a start, stop, and num of elements:

In [1]: import numpy as np

In [2]: np.linspace(0, 10, 9)
Out[2]: array([  0.  ,   1.25,   2.5 ,   3.75,   5.  ,   6.25,   7.5 ,   8.75,  10.  ])

Likewise, who could ever forget np.arange, which creates an array given a start, stop, and step:

In [4]: np.arange(0, 10, 1.25)
Out[4]: array([ 0.  ,  1.25,  2.5 ,  3.75,  5.  ,  6.25,  7.5 ,  8.75])

But is there a function that allows you to specify a start, step, and num of elements, while omitting the stop? There should be.

Asked By: dbliss

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Source

Answer 1

def by_num_ele(start,step,n_elements):
    return numpy.arange(start,start+step*n_elements,step)

maybe?

Answered By: Joran Beasley

Answer 2

Here’s one that should always work with floats.

>>> import numpy as np
>>> import itertools
>>> def my_range(start, step, num):
...     return np.fromiter(itertools.count(start, step), np.float, num)
... 
>>> my_range(0, 0.1, 3)
array([ 0. ,  0.1,  0.2])

You could make the np.float an arg (or kwarg) if you want to use it with something other than floats:

>>> import numpy as np
>>> import itertools
>>> def my_range(start, step, num, dtype=np.float):
...     return np.fromiter(itertools.count(start, step), dtype, num)
... 
>>> my_range(0, 5, 3)
array([  0.,   5.,  10.])
>>> my_range(0, 5, 3, dtype=np.int)
array([ 0,  5, 10])

Answered By: Jake Griffin

Answer 3

A deleted answer pointed out that linspace takes an endpoint parameter.

With that, 2 examples given in other answers can be written as:

In [955]: np.linspace(0, 0+(0.1*3),3,endpoint=False)
Out[955]: array([ 0. ,  0.1,  0.2])

In [956]: np.linspace(0, 0+(5*3),3,endpoint=False)
Out[956]: array([  0.,   5.,  10.])

In [957]: np.linspace(0, 0+(1.25*9),9,endpoint=False)
Out[957]: array([  0.  ,   1.25,   2.5 ,   3.75,   5.  ,   6.25,   7.5 ,   8.75,  10.  ])

Look at the functions defined in numpy.lib.index_tricks for other ideas on how to generate ranges and/or grids. For example, np.ogrid[0:10:9j] behaves like linspace.

def altspace(start, step, count, endpoint=False, **kwargs):
   stop = start+(step*count)
   return np.linspace(start, stop, count, endpoint=endpoint, **kwargs)

Answered By: hpaulj

Answer 4

Thanks for that question. I had the same issue.
The (from my perspective) shortest and most elegant way is:

import numpy as np
start=0
step=1.25
num=9

result=np.arange(0,num)*step+start

print(result)

returns

[  0.     1.25   2.5    3.75   5.     6.25   7.5    8.75  10.  ]

Answered By: Markus Dutschke

Answer 5

Some of the other solutions didn’t work for me, so since I was already comfortable using np.linspace I decided to throw together a function that replaces linspace‘s num with a step argument.

def linspace(start, stop, step=1.):
  """
    Like np.linspace but uses step instead of num
    This is inclusive to stop, so if start=1, stop=3, step=0.5
    Output is: array([1., 1.5, 2., 2.5, 3.])
  """
  return np.linspace(start, stop, int((stop - start) / step + 1))

An example output:

linspace(9.5, 11.5, step=.5)
array([ 9.5, 10. , 10.5, 11. , 11.5])

Edit: I misread the question, the original question wanted a function that omitted the stop argument. I’ll still leave this here, as I think it could be useful to some who stumble upon this question as it’s the only one I’ve found that’s similar to my original question of finding a function with start, stop, and step, rather than num

Answered By: Shane Smiskol

Is there a numpy function that allows you to specify start, step, and number?

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