[Tutor] Unintentionally manipulating a list

[Steven D'Aprano]

ranjan das wrote:
> I am facing the following problem
> 
> 
> I have a list of the form
> 
> INPUT= [ [ ['A1-N','A2-Y','A3-N' ],['B1-Y','B2-N','B3-N' ] ], [.........] ]

> and I want an output of the form (selecting only those elements which have a
> Y associated with them)
> 
> OUTPUT=[ ['A2-Y', 'B1-Y'],[....] ]

Can the Y be anywhere, or must it be at the end of the string? Should 
'Y2-N' be included or excluded? I will assume excluded.

What about lowercase 'y'? Should 'A3-y' be included? I assume it should be.



> I wrote the following code. Although it gives me the desired output, it
> CHANGES the list INPUT

Start with a simple function that extracts the strings you want from the 
inner-most list, Here's the hard way:

def select(items, suffix):
     suffix = suffix.upper()
     result = []
     for s in items:
         if s.upper().endswith(suffix):
             result.append(s)
     return result


This can be simplified to:

def select(items, suffix):
     suffix = suffix.upper()
     return [s for s in items if s.upper().endswith(suffix)]


Now you can use it:



OUTPUT = []
# INPUT looks like:
# [ [a1_list, b1_list, ...], [a2_list, b2_list, ...], ... ]
for middle in INPUT:
     # middle looks like [a99_list, b99_list, c99_list, ...]
     temp = []
     for inner in middle:
         # inner looks like [astr-N, bstr-Y, cstr-Y, ...]
         temp.extend(select(inner, '-Y'))
     OUTPUT.append(temp)


There is no need to make a copy of the input list.



-- 
Steven