[Tutor] Difference between filter and map

[vanam]

Yes i did a mistake in expressing my problem below are the instances of the
script and its corresponding output,for each instance its giving contrasting
result i want explanation for that
[1]:def squ(n):
           return n*n
     filter(squ,range(3))---->output is not seen on the interpreter
     map(squ,range(3))----->output  not seen on the interpreter
print filter(squ,range(3))----->output is [1,2]
print map(squ,range(3))------>output is [0,1,4]

[2]:def squ(n):
          y = n*n
          print y
      filter(squ,range(3))-->Below is the output
      0
      1
      4
      map(squ,range(3))-->Below is the output
      0
      1
      4
      print filter(squ,range(3))--->Below is the output
      0
      1
      4
      []
      print map(squ,range(3))-->Below is the output
      0
      1
      4
      [None,None,None]
I want to know why in each case its giving different results and diff
between filter and map
On 1/23/07, Kent Johnson <kent37 at tds.net> wrote:
>
> vanam wrote:
> > ya i am sure about that i am using python editor which has python
> > intrepreter attached to it i got the same output for both filter and map
> > def squ(n):
> >    y = n*n
> >   print y
> > filter(y,range(3))->0  1 4
> > map(y,range(3))->0 1 4
>
> This is quite different that what you posted the first time. This
> function squ() *prints* n*n but *returns* None (since it has no explicit
> return statement). The previous squ() actually returned n*n. But the
> results are still different if you look carefully:
>
> In [2]: def sq(n):
>     ...:     y=n*n
>     ...:     print y
>     ...:
>     ...:
>
> In [3]: map(sq, range(3))
> 0
> 1
> 4
> Out[3]: [None, None, None]
>
> The function sq() is called for each element of range(3) and prints the
> square. This is why 0, 1, 4 are printed. But the value returned from
> map() is the list [None, None, None] which is the accumulated return
> values from calling sq().
>
> In [4]: filter(sq, range(3))
> 0
> 1
> 4
> Out[4]: []
>
> Here, sq() is still called for each element of range(3). Since the
> printing is from sq(), 0, 1 and 4 are still printed. But the return
> value is an empty list [] because None is not true so sq(n) is not true
> for any elements of range(3).
>
> Kent
>
>


-- 

                                           Vanam
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