Numpy/Python variable assignment

Question

I have been trying to get this program to work using Numpy in Python. Here is my code.

import numpy as np

def function(inNum):
    return inNum - 1


i = 1
n = 50000000
AllOutputs = np.array([])
while(i < n):
    SingularOutput = np.array([])
    num = i

    while(num > 1):
        print(num)
        num = function(num)
        np.append(SingularOutput,num)
    np.append(AllOutputs,SingularOutput)

    i = i + 1
    print(i)

The output in the console gives decreasing numbers, here is a snippet.

It seems as if is being assigned the values returned from function(inNum), but as it can be seen i is not ever assigned any value other than i = i + 1, thus I do not know why this is happening. Lastly, if I modify the num = i assignment to nums = 1 (or I assume anything else) I get the expected output of

1
2
3
4
5
...(etc)

What am I doing wrong?

You have two sequences being printed. On each iteration of the outer loop, you print a descending sequence from num to 2, followed by the value of i. The i values do increase with each iteration, but most of the output comes from the decreasing sequence. — chepner
– chepner, Commented Jan 31, 2022 at 19:07
When you set num = 1, you prevent the inner loop from executing at all, so all you see is the increasing values of i produced by the outer loop. — chepner
– chepner, Commented Jan 31, 2022 at 19:11
The statement np.append(SingularOutput,num) doesn't modify SingularOutput. It creates a new array--which you never use. See numpy.org/doc/stable/reference/generated/numpy.append.html — jjramsey
– jjramsey, Commented Jan 31, 2022 at 19:12

hpaulj · Accepted Answer · 2022-01-31 19:50:29Z

Don't try to imitate list methods with numpy:

AllOutputs = []
while(i < n):
    SingularOutput = []
    num = i

    while(num > 1):
        print(num)
        num = function(num)
        SingularOutput.append(num)
    AllOutputs.append(SingularOutput)

print(AllOutputs)
arr = np.array(AllOutputs)
print(arr)
i = i + 1
print(i)

np.empty([]) and np.append are not clones of [] and list.append. Please read the docs with care!

Running the code I see that you are making sublists that differ in length. There's no point to using numpy for this.

In [39]: i=1; n=6
    ...: AllOutputs = []
    ...: while(i < n):
    ...:     SingularOutput = []
    ...:     num = i
    ...:     while(num > 1):
    ...:         print('num',num)
    ...:         num = fun(num)
    ...:         SingularOutput.append(num)
    ...:     AllOutputs.append(SingularOutput)
    ...:     i = i+1
    ...:     print('i',i)
    ...: 
i 2
num 2
i 3
num 3
num 2
i 4
num 4
num 3
num 2
i 5
num 5
num 4
num 3
num 2
i 6
In [40]: AllOutputs
Out[40]: [[], [1], [2, 1], [3, 2, 1], [4, 3, 2, 1]]

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