flow of for loop in python

Question

I just new in python, I code for fetch array value from user, for this reason I asked a question yesterday in stackoverflow. Darius Morawiec and Austin give me the best salutation, but I don't understand the flow of for loop, I google it, but I don't understand those explanation.can any body explain the control of "for" loop for given code below. Thank you

arr = [[int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1))) 
       for c in range(n_cols)] 
       for r in range(n_rows)]

yesterday conversation Link

You could've just asked for this clarification on the answer you got the code from. — Abion47
– Abion47, Commented Feb 13, 2019 at 19:03
I already asked him through comment, but I need the explanation as soon as possible — DevWebTk
– DevWebTk, Commented Feb 13, 2019 at 19:08
If you search on the phrase "Python list comprehension", you’ll find resources that can explain it much better than we can in an answer here. — Prune
– Prune, Commented Feb 13, 2019 at 19:19

Tacratis · Accepted Answer · 2019-02-13 19:11:09Z

0

A list comprehension is a compressed syntax in python to generate a list. If you rewrite this, it will probably be very clear.

The general syntax is: [expression for element in list (optional: if condition)] and it returns a list

This is exactly the same as writing:

new_list = []
for element in original_list:
    if (condition):   
        new_list.append(expression)

in your case, you can rewrite the two list comprehensions (they are nested) like so:

arr = [[second comprehension] for r in range(n_rows)] ->

arr = []
for r in range(n_rows):
    arr.append(second list)

now, for the second list comprehension:

second_list = []
for c in range(n_cols):
    entry = int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1)))
    second_list.append(entry)

And the entire flow is:

arr = []
for r in range(n_rows):
    second_list = []
    for c in range(n_cols):
            entry = int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1)))
            second_list.append(entry)
    arr.append(second list)

answered Feb 13, 2019 at 19:11

Tacratis

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1 Comment

DevWebTk Over a year ago

Thank you @Tacratis i got my best salutation thanks a lot :)

chepner · Accepted Answer · 2019-02-13 19:08:25Z

Despite share the same keywords, that's not a for loop; it's a list comprehension nested in another list comprehension. As such, you need to evaluate the inner list first:

[
  [int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1))) 
     for c in range(n_cols)
  ] 
    for r in range(n_rows)
]

If you were to "unroll" the inner one, it would look like

[
  [
    int(input("Enter value for {}. row and {}. column: ".format(r + 1, 1))),
    int(input("Enter value for {}. row and {}. column: ".format(r + 1, 2))),
    # ...
    int(input("Enter value for {}. row and {}. column: ".format(r + 1, n_cols-1))),
  ]
    for r in range(n_rows)
]

and further unrolling the outer one would result in

[
  [
    int(input("Enter value for {}. row and {}. column: ".format(1, 1))),
    int(input("Enter value for {}. row and {}. column: ".format(1, 2))),
    # ...
    int(input("Enter value for {}. row and {}. column: ".format(1, n_cols-1))),
  ],
  [
    int(input("Enter value for {}. row and {}. column: ".format(2, 1))),
    int(input("Enter value for {}. row and {}. column: ".format(2, 2))),
    # ...
    int(input("Enter value for {}. row and {}. column: ".format(2, n_cols-1))),
  ],
  # ...
  [
    int(input("Enter value for {}. row and {}. column: ".format(n_rows-1, 1))),
    int(input("Enter value for {}. row and {}. column: ".format(n_rows-1, 2))),
    # ...
    int(input("Enter value for {}. row and {}. column: ".format(n_rows-1, n_cols-1))),
  ]
]

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