Find Items & Indices In A List...

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  • andrea.gavana@agip.it
    #1

    Find Items & Indices In A List...

    Hello NG,

    I was wondering if there is a faster/nicer method (than a for loop)
    that will allow me to find the elements (AND their indices) in a list that
    verify a certain condition. For example, assuming that I have a list like:

    mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]

    I would like to find the indices of the elements in the list that are equal
    to 1 (in this case, the 1,2,3,4,9 elements are equal to 1). I could easily
    use a for loop but I was wondering if there is a faster method...

    Thanks for every suggestion.

    Andrea.
    ------------------------------------------------------------------------------------------------------------------------------------------
    Message for the recipient only, if received in error, please notify the
    sender and read http://www.eni.it/disclaimer/


    Tags: None
    • kaerbuhez
      #2
      Re: Find Items & Indices In A List...

      <andrea.gavana@ agip.it> a écrit dans le message de news:
      mailman.7461.11 02691050.5135.p ython-list@python.org...[color=blue]
      > For example, assuming that I have a list like:
      >
      > mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
      >
      > I would like to find the indices of the elements in the list that are
      > equal
      > to 1 (in this case, the 1,2,3,4,9 elements are equal to 1).[/color]

      List comprehension is your friend:

      PythonWin 2.4 (#60, Nov 30 2004, 11:49:19) [MSC v.1310 32 bit (Intel)] on
      win32.
      Portions Copyright 1994-2004 Mark Hammond (mhammond@skipp inet.com.au) - see
      'Help/About PythonWin' for further copyright information.[color=blue][color=green][color=darkred]
      >>> mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
      >>> [i for i, j in enumerate(mylis t) if j==1][/color][/color][/color]
      [1, 2, 3, 4, 9][color=blue][color=green][color=darkred]
      >>>[/color][/color][/color]

        Comment

        • Ola Natvig
          #3
          Re: Find Items &amp; Indices In A List...

          andrea.gavana@a gip.it wrote:[color=blue]
          > Hello NG,
          >
          > I was wondering if there is a faster/nicer method (than a for loop)
          > that will allow me to find the elements (AND their indices) in a list that
          > verify a certain condition. For example, assuming that I have a list like:
          >
          > mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
          >
          > I would like to find the indices of the elements in the list that are equal
          > to 1 (in this case, the 1,2,3,4,9 elements are equal to 1). I could easily
          > use a for loop but I was wondering if there is a faster method...
          >
          > Thanks for every suggestion.
          >
          > Andrea.
          > ------------------------------------------------------------------------------------------------------------------------------------------
          > Message for the recipient only, if received in error, please notify the
          > sender and read http://www.eni.it/disclaimer/
          >
          >[/color]

          You could do a list comprehension /generator expression. Like this:
          [i for i in range(len(mylis t)) if mylist[i] == 1]

          --
          --------------------------------------
          Ola Natvig <ola.natvig@inf osense.no>
          infoSense AS / development

            Comment

            • Bengt Richter
              #4
              Re: Find Items &amp; Indices In A List...

              On Fri, 10 Dec 2004 16:01:26 +0100, andrea.gavana@a gip.it wrote:
              [color=blue]
              >Hello NG,
              >
              > I was wondering if there is a faster/nicer method (than a for loop)
              >that will allow me to find the elements (AND their indices) in a list that
              >verify a certain condition. For example, assuming that I have a list like:
              >
              >mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
              >
              >I would like to find the indices of the elements in the list that are equal
              >to 1 (in this case, the 1,2,3,4,9 elements are equal to 1). I could easily
              >use a for loop but I was wondering if there is a faster method...
              >
              >Thanks for every suggestion.
              >[/color]
              One way:
              [color=blue][color=green][color=darkred]
              >>> mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
              >>> [i for i,v in enumerate(mylis t) if v==1][/color][/color][/color]
              [1, 2, 3, 4, 9]

              Regards,
              Bengt Richter

                Comment

                • Steven Bethard
                  #5
                  Re: Find Items &amp; Indices In A List...

                  andrea.gavana@a gip.it wrote:[color=blue]
                  > Hello NG,
                  >
                  > I was wondering if there is a faster/nicer method (than a for loop)
                  > that will allow me to find the elements (AND their indices) in a list that
                  > verify a certain condition. For example, assuming that I have a list like:
                  >
                  > mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
                  >
                  > I would like to find the indices of the elements in the list that are equal
                  > to 1 (in this case, the 1,2,3,4,9 elements are equal to 1). I could easily
                  > use a for loop but I was wondering if there is a faster method...[/color]

                  Everyone has already given you the answer (enumerate in a LC or GE), I'd
                  just comment that it's easy enough to extend their answers to any given
                  condition:
                  [color=blue][color=green][color=darkred]
                  >>> def getindices(sequ ence, predicate):[/color][/color][/color]
                  .... return [i for i, v in enumerate(seque nce) if predicate(v)]
                  ....[color=blue][color=green][color=darkred]
                  >>> getindices([0,1,1,1,1,5,6,7 ,8,1,10], bool)[/color][/color][/color]
                  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10][color=blue][color=green][color=darkred]
                  >>> def equalsone(v):[/color][/color][/color]
                  .... return v == 1
                  ....[color=blue][color=green][color=darkred]
                  >>> getindices([0,1,1,1,1,5,6,7 ,8,1,10], equalsone)[/color][/color][/color]
                  [1, 2, 3, 4, 9][color=blue][color=green][color=darkred]
                  >>> def f(v):[/color][/color][/color]
                  .... return pow(v, 3, 4) == 3
                  ....[color=blue][color=green][color=darkred]
                  >>> getindices([0,1,1,1,1,5,6,7 ,8,1,10], f)[/color][/color][/color]
                  [7]

                  Steve

                    Comment

                    • Bengt Richter
                      #6
                      Re: Find Items &amp; Indices In A List...

                      On Fri, 10 Dec 2004 16:27:29 GMT, Steven Bethard <steven.bethard @gmail.com> wrote:
                      [color=blue]
                      >andrea.gavana@ agip.it wrote:[color=green]
                      >> Hello NG,
                      >>
                      >> I was wondering if there is a faster/nicer method (than a for loop)
                      >> that will allow me to find the elements (AND their indices) in a list that
                      >> verify a certain condition. For example, assuming that I have a list like:
                      >>
                      >> mylist = [0, 1, 1, 1, 1, 5, 6, 7, 8, 1, 10]
                      >>
                      >> I would like to find the indices of the elements in the list that are equal
                      >> to 1 (in this case, the 1,2,3,4,9 elements are equal to 1). I could easily
                      >> use a for loop but I was wondering if there is a faster method...[/color]
                      >
                      >Everyone has already given you the answer (enumerate in a LC or GE), I'd
                      >just comment that it's easy enough to extend their answers to any given
                      >condition:
                      >[color=green][color=darkred]
                      > >>> def getindices(sequ ence, predicate):[/color][/color]
                      >... return [i for i, v in enumerate(seque nce) if predicate(v)]
                      >...[color=green][color=darkred]
                      > >>> getindices([0,1,1,1,1,5,6,7 ,8,1,10], bool)[/color][/color]
                      >[1, 2, 3, 4, 5, 6, 7, 8, 9, 10][color=green][color=darkred]
                      > >>> def equalsone(v):[/color][/color]
                      >... return v == 1
                      >...[color=green][color=darkred]
                      > >>> getindices([0,1,1,1,1,5,6,7 ,8,1,10], equalsone)[/color][/color]
                      >[1, 2, 3, 4, 9][color=green][color=darkred]
                      > >>> def f(v):[/color][/color]
                      >... return pow(v, 3, 4) == 3
                      >...[color=green][color=darkred]
                      > >>> getindices([0,1,1,1,1,5,6,7 ,8,1,10], f)[/color][/color]
                      >[7]
                      >[/color]
                      Conclusion:
                      Python is programmer's Lego ;-)

                      Regards,
                      Bengt Richter

                        Comment

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