If list index exists do X

Question

In my program  user inputs number n  and then inputs n number of strings  which get stored in a list   I need to code such that if a certain list index exists  then run a function   This is made more complicated by the fact that I have nested if statements about len my list    Here s a simplified version of what I have now  which isn t working   n   input   Define number of actors      count   0  nams       while count  lt  n      count   count   1     print  Define name for actor    count          name   raw input        nams append name   if nams 2    I am trying to say  if nams 2  exists  do something depending on len nams      if len nams   gt  3          do something     if len nams   gt  4         do something else  if nams 3    etc

User · Answer

A lot of answers  not the simple one   To check if a index  id  exists at dictionary dict   dic      dic  name      joao  dic  age       39   if  age  in dic   returns true if  age  exists

User · Answer

Oneliner   do X   if len your list   gt  your index else do something else       Full example   In  10   def do X                  print 1                                                                                                                                                                                                                                                   In  11   def do something else                  print 2                                                                                                                                                                                                                                                   In  12   your index   2                                                                                                                                                                                                                         In  13   your list    1 2 3                                                                                                                                                                                                                     In  14   do X   if len your list   gt  your index else do something else                                                                                                                                                                         1   Just for info  Imho  try     except IndexError is better solution

User · Answer

len nams  should be equal to n in your code  All indexes 0  lt   i  lt  n  exist

User · Answer

ok  so I think it s actually possible  for the sake of argument     gt  gt  gt  your list    5 6 7   gt  gt  gt  2 in zip  enumerate your list   0  True  gt  gt  gt  3 in zip  enumerate your list   0  False

User · Answer

If you want to iterate the inserted actors data   for i in range n       if len nams i    gt  3          do something     if len nams i    gt  4          do something else

User · Answer

I need to code such that if a certain list index exists  then run a function    You already know how to test for this and in fact are already performing such tests in your code   The valid indices for a list of length n are 0 through n-1 inclusive   Thus  a list has an index i if and only if the length of the list is at least i   1

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Do not let any space in front of your brackets   Example   n   input                 Tip  You should add comments over and or under your code  Not behind your code     Have a nice day

User · Answer

It can be done simply using the following code   if index  lt  len my list       print index   exists in the list   else      print index   doesn t exist in the list

User · Answer

You can try something like this   list     a    b    C    d    e    f    r    for i in range 0  len list   2       print list i      if len list    2    1 and  i    len list -1          break     print list i 1

User · Answer

Could it be more useful for you to use the length of the list len n  to inform your decision rather than checking n i  for each possible length

User · Answer

I need to code such that if a certain list index exists  then run a function  This is the perfect use for a try block  ar  1 2 3   try      t ar 5  except IndexError      print  sorry  no 5         Note  this only is a valid test in this context    with absolute  ie  positive  index   a relative index is only showing you that a value can be returned   from that relative index from the end of the list     However  by definition  all items in a Python list between 0 and len the list -1 exist  i e   there is no need for a try block if you know 0  lt   index  lt  len the list    You can use enumerate if you want the indexes between 0 and the last element  names   barney   fred   dino    for i  name in enumerate names       print i         name      if i in  3 4             do your thing with the index  i  or value  name  for each item     If you are looking for some defined  index  thought  I think you are asking the wrong question  Perhaps you should consider using a mapping container  such as a dict  versus a sequence container  such as a list   You could rewrite your code like this  def do something name       print  some thing 1 done with     name           def do something else name       print  something 2 done with     name               def default name       print  nothing done with     name            something to do         3  do something              4  do something else                            n   input   quot Define number of actors   quot   count   0 names       for count in range n       print  quot Define name for actor     quot  format count 1       name   raw input        names append name       for name in names      try          something to do len name   name      except KeyError          default name   Runs like this  Define number of actors  3 Define name for actor 1  bob Define name for actor 2  tony Define name for actor 3  alice some thing 1 done with bob something 2 done with tony nothing done with alice  You can also use  get method rather than try except for a shorter version   gt  gt  gt  something to do get 3  default   bob   some thing 1 done with bob  gt  gt  gt  something to do get 22  default   alice   nothing done with alice

User · Answer

Using the length of the list would be the fastest solution to check if an index exists   def index exists ls  i       return  0  lt   i  lt  len ls   or  -len ls   lt   i  lt  0    This also tests for negative indices  and most sequence types  Like ranges and strs  that have a length   If you need to access the item at that index afterwards anyways  it is easier to ask forgiveness than permission  and it is also faster and more Pythonic  Use try  except    try      item   ls i        Do something with item except IndexError        Do something without the item   This would be as opposed to   if index exists ls  i       item   ls i        Do something with item else        Do something without the item

[python] If list index exists, do X

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