I'm trying to create a numpy array of intergers ascending intergers (1,2,3,...), such that the n is repeated n times. For example for maximum number 4 I would like
my_arr = [1,2,2,3,3,3,4,4,4,4]
Now this is easy using a for loop
my_arr = numpy.array([])
max = 4
for i in range(1,max + 1):
my_arr = numpy.append(my_arr,np.ones(i)*i)
but this gets horribly slow for large numbers max.
Any suggestions?
Use np.repeat:
import numpy as np
def pattern(n):
x = np.arange(1, n + 1)
return np.repeat(x, x)
# >>> pattern(4)
# array([1, 2, 2, 3, 3, 3, 4, 4, 4, 4])
There is a builtin way to do this.
import numpy as np
def builtinway(maxval=10):
arr = list(range(1, maxval+1))
return np.repeat(arr, arr)
One way to do this pretty quickly and directly would be as follows. In practice, space would become a problem before the number of operations grows too quickly. (Though in practice, you probably wouldn't do this).
def make_arr(maxval=10):
arrlen = maxval * (maxval + 1) // 2
arr = np.ones(arrlen)
for i in range(2, maxval+1):
addend = np.ones(arrlen)
addend[:i*(i-1) // 2] = 0
arr += addend
return arr
np.repeat([1, 2, 3, 4], [1, 2, 3, 4])