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]

• Samwise: 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.
• user6276743: Please recheck/reformat your code. It does not return anything because the while loop never runs, since i != 2.
• David: When reformatting your code as recommended above, please also make sure the indentation level is correct for each line
• Rashik: You probably want an if statement inside another for loop instead of the while loop.

Answer 1

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]).

• ademola adelakun: 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: 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]

Answer 2

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