Inline for loop

Question

I m trying to learn neat pythonic ways of doing things  and was wondering why my for loop cannot be refactored this way   q     1  2  3  4  1  2  5  1  2  3  4  5  vm    -1  -1  -1  -1   for v in vm      if v in q          p append q index v       else          p append 99999   vm p index max p      i   I tried replacing the for loop with    p append q index v   if v in q else p append 99999  for v in vm    But it doesn t work  The for v in vm  loop evicts numbers from vm based on when they come next in q

User · Accepted Answer

What you are using is called a list comprehension in Python, not an inline for-loop (even though it is similar to one). You would write your loop as a list comprehension like so:

p = [q.index(v) if v in q else 99999 for v in vm]

When using a list comprehension, you do not call list.append because the list is being constructed from the comprehension itself. Each item in the list will be what is returned by the expression on the left of the for keyword, which in this case is q.index(v) if v in q else 99999. Incidentially, if you do use list.append inside a comprehension, then you will get a list of None values because that is what the append method always returns.

User · Answer

your list comphresnion will  work but will return list of None because append return None   demo    gt  gt  gt  a     gt  gt  gt    a append x  for x in range 10     None  None  None  None  None  None  None  None  None  None   gt  gt  gt  a  0  1  2  3  4  5  6  7  8  9    better way to use it like this    gt  gt  gt  a    x for x in range 10     gt  gt  gt  a  0  1  2  3  4  5  6  7  8  9

User · Answer

you can use enumerate keeping the ind index of the elements is in vm  if you make vm a set you will also have 0 1  lookups   vm    -1  -1  -1  -1   print  ind if q in vm else 9999 for ind ele in enumerate vm

User · Answer

q     1  2  3  4  1  2  5  1  2  3  4  5  vm    -1  -1  -1  -1 1 2 3 1   p      for v in vm      if v in q          p append q index v       else          p append 99999   print p p    q index v  if v in q else 99999 for v in vm  print p   Output    99999  99999  99999  99999  0  1  2  0   99999  99999  99999  99999  0  1  2  0    Instead of using append   in the list comprehension you can reference the p as direct output  and use q index v  and 99999 in the LC   Not sure if this is intentional but note that q index v  will find just the first occurrence of v  even tho you have several in q  If you want to get the index of all v in q  consider using a enumerator and a list of already visited indexes  Something in those lines pseudo-code    visited      for i  v in enumerator vm      if i not in visited         p append q index v      else         p append q index v max visited      this line should only check for v in q after the index of max visited     visited append i

[python] Inline for loop

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