Single Line Nested For Loops

Question

Wrote this function in python that transposes a matrix   def transpose m       height   len m      width   len m 0       return     m i  j  for i in range 0  height    for j in range 0  width      In the process I realized I don t fully understand how single line nested for loops execute  Please help me understand by answering the following questions    What is the order in which this for loop executes  If I had a triple nested for loop  what order would it execute  What would be equal the equal unnested for loop    Given     function i j  for i j in object      What type must object be in order to use this for loop structure  What is the order in which i and j are assigned to elements in object  Can it be simulated by a different for loop structure  Can this for loop be nested with a similar or different structure for loop  And how would it look    Additional information is appreciated as well

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You might be interested in itertools product  which returns an iterable yielding tuples  of values from all the iterables you pass it  That is  itertools product A  B  yields all values of the form  a  b   where the a values come from A and the b values come from B  For example   import itertools  A    50  60  70  B    0 1  0 2  0 3  0 4   print  a   b for a  b in itertools product A  B     This prints    50 1  50 2  50 3  50 4  60 1  60 2  60 3  60 4  70 1  70 2  70 3  70 4    Notice how the final argument passed to itertools product is the  inner  one  Generally  itertools product a0  a1      an  is equal to   i0  i1      in  for in in an for in-1 in an-1     for i0 in a0

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First of all  your first code doesn t use a for loop per se  but a list comprehension    Would be equivalent to  for j in range 0  width       for i in range 0  height           m i  j  Much the same way  it generally nests like for loops  right to left  But list comprehension syntax is more complex  I m not sure what this question is asking      Any iterable object that yields iterable objects that yield exactly two objects  what a mouthful - i e   1 2   ab   would be valid   The order in which the object yields upon iteration  i goes to the first yield  j the second  Yes  but not as pretty  I believe it is functionally equivalent to    l   list   for i j in object      l append function i j     or even better use map   map function  object    But of course function would have to get i  j itself  Isn t this the same question as 3

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You can use two for loops in same line by using zip function  Code   list1     Abbas    Ali    Usman   list2     Kamran    Asgar    Hamza    Umer   list3      for i j in zip list1 list2       list3 append i      list3 append j  print list3    Output     Abbas    Kamran    Ali    Asgar    Usman    Hamza    So  by using zip function  we can use two for loops or we can iterate two lists in same row

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Below code for best examples for nested loops  while using two for loops please remember the  output of the first loop is input for the second loop   Loop termination also important while using the nested loops    for x in range 1  10  1        for y in range 1 x                print y          print OutPut   1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8

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The best source of information is the official Python tutorial on list comprehensions   List comprehensions are nearly the same as for loops  certainly any list comprehension can be written as a for-loop  but they are often faster than using a for loop   Look at this longer list comprehension from the tutorial  the if part filters the comprehension  only parts that pass the if statement are passed into the final part of the list comprehension  here  x y      gt  gt  gt    x  y  for x in  1 2 3  for y in  3 1 4  if x    y    1  3    1  4    2  3    2  1    2  4    3  1    3  4     It s exactly the same as this nested for loop  and  as the tutorial says  note how the order of for and if are the same     gt  gt  gt  combs       gt  gt  gt  for x in  1 2 3           for y in  3 1 4               if x    y                  combs append  x  y        gt  gt  gt  combs   1  3    1  4    2  3    2  1    2  4    3  1    3  4     The major difference between a list comprehension and a for loop is that the final part of the for loop  where you do something  comes at the beginning rather than at the end   On to your questions      What type must object be in order to use this for loop structure    An iterable  Any object that can generate a  finite  set of elements  These include any container  lists  sets  generators  etc      What is the order in which i and j are assigned to elements in object    They are assigned in exactly the same order as they are generated from each list  as if they were in a nested for loop  for your first comprehension you d get 1 element for i  then every value from j  2nd element into i  then every value from j  etc       Can it be simulated by a different for loop structure    Yes  already shown above      Can this for loop be nested with a similar or different structure for loop  And how would it look    Sure  but it s not a great idea  Here  for example  gives you a list of lists of characters     ch for ch in word  for word in   apple    banana    pear    the    hello

[python] Single Line Nested For Loops

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