Array loop not returning expected output in Python

Question

pressures = [[5,6,7],
             [8,9,10],
             [11,12,13],
             [14,15,16]]
i = 0
while (i == 2):
    for row in pressures[i:]:
        cycle[i] = row[i]
row[1]

Please any idea why the above code is returning just the last value of the array which is 15

I expect the output to be ;

[ 6,
  9,
 12,
 15]

This code doesn't return anything, and the while loop is a no-op since there is no way for i to be 2. You might need to include more of your actual code. — Samwise
– Samwise, Commented May 16, 2020 at 15:20
Please recheck/reformat your code. It does not return anything because the while loop never runs, since i != 2. — user6276743
– user6276743, Commented May 16, 2020 at 15:20
When reformatting your code as recommended above, please also make sure the indentation level is correct for each line — David
– David, Commented May 16, 2020 at 15:21
You probably want an if statement inside another for loop instead of the while loop. — Rashik
– Rashik, Commented May 16, 2020 at 15:22

David · Accepted Answer · 2020-05-16 15:32:38Z

3

If all you want is the middle column returned, then you can do that by iterating through the rows and taking the 1 element:

pressures = [[5,6,7],
             [8,9,10],
             [11,12,13],
             [14,15,16]]


middle_col = []
for row in pressures:
    middle_col.append(row[1])

Equivalently, you can do this in one line:

pressures = [[5,6,7],
             [8,9,10],
             [11,12,13],
             [14,15,16]]

middle_col = [row[1] for row in pressures]

The best way, though, is probably to use NumPy with array indexing:

import numpy as np

pressures = np.array([[5,6,7],
                      [8,9,10],
                      [11,12,13],
                      [14,15,16]])

middle_col = pressures[:, 1]

which will return a numpy array which looks like np.array([6,9,12,15]).

edited May 16, 2020 at 15:32

answered May 16, 2020 at 15:27

David

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2 Comments

ademola adelakun Over a year ago

Thank you this solves my problem. But what if I need to loop through I.e to return any column I specify not just one at a time. That will be very helpful. Thanks

David Over a year ago

you just loop through the indices of the columns. instead of row[1], it would be row[i] where i is your iteration variable. Likewise for pressures[:,i]

Renaud · Accepted Answer · 2020-05-16 15:22:15Z

0

i = 0 and just after while (i == 2): are not compatible

answered May 16, 2020 at 15:22

Renaud

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