What does enumerate mean

Question

What does for row number  row in enumerate cursor   do in Python   What does enumerate mean in this context

User · Answer

I am reading a book  Effective Python  by Brett Slatkin and he shows another way to iterate over a list and also know the index of the current item in the list but he suggests that it is better not to use it and to use enumerate instead   I know you asked what enumerate means  but when I understood the following  I also understood how enumerate makes iterating over a list while knowing the index of the current item easier  and more readable     list of letters     a    b    c   for i in range len list of letters        letter   list of letters i      print  i  letter    The output is   0 a 1 b 2 c   I also used to do something  even sillier before I read about the enumerate function    i   0 for n in list of letters      print  i  n      i    1   It produces the same output    But with enumerate I just have to write   list of letters     a    b    c   for i  letter in enumerate list of letters       print  i  letter

User · Answer

The enumerate function works as follows   doc      I like movie  But I don t like the cast  The story is very nice    doc1   doc split      for i in enumerate doc1        print i    The output is   0   I like movie    1    But I don t like the cast    2    The story is very nice

User · Answer

The enumerate   function adds a counter to an iterable   So for each element in cursor  a tuple is produced with  counter  element   the for loop binds that to row number and row  respectively   Demo    gt  gt  gt  elements     foo    bar    baz    gt  gt  gt  for elem in elements          print elem      foo bar baz  gt  gt  gt  for count  elem in enumerate elements           print count  elem      0 foo 1 bar 2 baz   By default  enumerate   starts counting at 0 but if you give it a second integer argument  it ll start from that number instead    gt  gt  gt  for count  elem in enumerate elements  42           print count  elem      42 foo 43 bar 44 baz   If you were to re-implement enumerate   in Python  here are two ways of achieving that  one using itertools count   to do the counting  the other manually counting in a generator function   from itertools import count  def enumerate it  start 0         return an iterator that adds a counter to each element of it     return zip count start   it    and  def enumerate it  start 0       count   start     for elem in it          yield  count  elem          count    1   The actual implementation in C is closer to the latter  with optimisations to reuse a single tuple object for the common for i      unpacking case and using a standard C integer value for the counter until the counter becomes too large to avoid using a Python integer object  which is unbounded

User · Answer

It s a builtin function that returns an object that can be iterated over  See the documentation   In short  it loops over the elements of an iterable  like a list   as well as an index number  combined in a tuple   for item in enumerate   a    b    c         print item   prints   0   a    1   b    2   c     It s helpful if you want to loop over a sequence  or other iterable thing   and also want to have an index counter available  If you want the counter to start from some other value  usually 1   you can give that as second argument to enumerate

User · Answer

I am assuming that you know how to iterate over elements in some list  for el in my list        do something  Now sometimes not only you need to iterate over the elements  but also you need the index for each iteration  One way to do it is  i   0 for el in my list        do somethings  and use value of  quot i quot  somehow     i    1  However  a nicer way is to user the function  quot enumerate quot   What enumerate does is that it receives a list  and it returns a list-like object  an iterable that you can iterate over  but each element of this new list itself contains 2 elements   the index and the value from that original input list  So if you have arr     a    b    c    Then the command enumerate arr  returns something like    0  a     1  b     2  c     Now If you iterate over a list  or an iterable  where each element itself has 2 sub-elements  you can capture both of those sub-elements in the for loop like below  for index  value in enumerate arr       print index value   which would print out the sub-elements of the output of enumerate  And in general you can basically  quot unpack quot  multiple items from list into multiple variables like below  idx value    2  c   print idx  print value   which would print 2 c  This is the kind of assignment happening in each iteration of that loop with enumerate arr  as iterable

User · Answer

As other users have mentioned  enumerate is a generator that adds an incremental index next to each item of an iterable   So if you have a list say l     test 1    test 2    test 3    the list enumerate l   will give you something like this    0   test 1     1   test 2     2   test 3      Now  when this is useful  A possible use case is when you want to iterate over items  and you want to skip a specific item that you only know its index in the list but not its value  because its value is not known at the time    for index  value in enumerate joint values      if index    3         continue       Do something with the other  value    So your code reads better because you could also do a regular for loop with range but then to access the items you need to index them  i e   joint values i     Although another user mentioned an implementation of enumerate using zip  I think a more pure  but slightly more complex  way without using itertools is the following   def enumerate l  start 0       return zip range start  len l    start   l    Example   l     test 1    test 2    test 3   enumerate l  enumerate l  10    Output        0   test 1     1   test 2     2   test 3           10   test 1     11   test 2     12   test 3      As mentioned in the comments  this approach with range will not work with arbitrary iterables as the original enumerate function does

[python] What does enumerate() mean?

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