Python finding an element in a list

Question

What is a good way to find the index of an element in a list in Python  Note that the list may not be sorted   Is there a way to specify what comparison operator to use

User · Answer

I found this by adapting some tutos  Thanks to google  and to all of you     def findall L  test       i 0     indices          while True           try                next value in list passing the test             nextvalue   filter test  L i    0                 add index of this value in the index list                by searching the value in L i                indices append L index nextvalue  i                  iterate i  that is the next index from where to search             i indices -1  1          when there is no further  good value   filter returns               hence there is an out of range exeption         except IndexError              return indices   A very simple use   a    0 0 2 1  ind   findall a  lambda x x gt 0     2  3    P S  scuse my english

User · Answer

The best way is probably to use the list method  index    For the objects in the list  you can do something like   def   eq   self  other       return self Value    other Value   with any special processing you need   You can also use a for in statement with enumerate arr   Example of finding the index of an item that has value   100   for index  item in enumerate arr       if item  gt  100          return index  item   Source

User · Answer

There is the index method  i   array index value   but I don t think you can specify a custom comparison operator  It wouldn t be hard to write your own function to do so  though   def custom index array  compare function       for i  v in enumerate array           if compare function v               return i

User · Answer

I use function for returning index for the matching element  Python 2 6    def index l  f        return next  i for i in xrange len l   if f l i     None    Then use it via lambda function for retrieving needed element by any required equation e g  by using element name   element   mylist index mylist  lambda item  item  name       my name      If i need to use it in several places in my code i just define specific find function e g  for finding element by name   def find name l  name        return l index l  lambda item  item  name      name     And then it is quite easy and readable   element   find name mylist  my name

User · Answer

assuming you want to find a value in a numpy array  I guess something like this might work   Numpy where arr   value   0

User · Answer

how s this one   def global index lst  test       return   pair 0  for pair in zip range len lst    lst  if test pair 1       Usage    gt  gt  gt  global index  1  2  3  4  5  6   lambda x  x gt 3   lt generator object  lt genexpr gt  at     gt   gt  gt  gt  list     3  4  5

User · Answer

The index method of a list will do this for you  If you want to guarantee order  sort the list first using sorted    Sorted accepts a cmp or key parameter to dictate how the sorting will happen   a    5  4  3  print sorted a  index 5    Or   a     one    aardvark    a   print sorted a  key len  index  a

User · Answer

Here is another way using list comprehension  some people might find it debatable   It is very approachable for simple tests  e g  comparisons on object attributes  which I need a lot    el    x for x in mylist if x attr     foo   0    Of course this assumes the existence  and  actually  uniqueness  of a suitable element in the list

User · Answer

If you just want to find out if an element is contained in the list or not    gt  gt  gt  li   a    b    new    mpilgrim    z    example    new    two    elements    gt  gt  gt   example  in li True  gt  gt  gt   damn  in li False

User · Answer

From Dive Into Python    gt  gt  gt  li   a    b    new    mpilgrim    z    example    new    two    elements    gt  gt  gt  li index  example   5

[python] Python: finding an element in a list

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