samrap - 1 year ago 104
Python Question

# Python: Difference between filter(function, sequence) and map(function, sequence)

I'm reading through the Python documentation to really get in depth with the Python language and came across the filter and map functions. I have used filter before, but never map, although I have seen both in various Python questions here on SO.

After reading about them in the Python tutorial, I'm confused on the difference between the two. For example, from 5.1.3. Functional Programming Tools:

``````>>> def f(x): return x % 2 != 0 and x % 3 != 0
...
>>> filter(f, range(2, 25))
[5, 7, 11, 13, 17, 19, 23]
``````

and

``````>>> def cube(x): return x*x*x
...
>>> map(cube, range(1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
``````

These looked almost exactly the same in function to me, so I went into terminal to run Python interactively and tested out my own case. I used
`map`
for both the first and second instances above, and for the first one (
`return x % 2 != 0 and x % 3 != 0`
) it returned a list of booleans rather than numbers.

Why does
`map`
sometimes return a boolean and other times the actual return value?

Can someone explain to me exactly the difference between
`map`
and
`filter`
?

``````map(cube, range(1, 11))
``````

is equivalent to

``````[cube(1), cube(2), ..., cube(10)]
``````

While the list returned by

``````filter(f, range(2, 25))
``````

is equivalent to `result` after running

``````result = []
for i in range(2, 25):
if f(i):
result.append(i)
``````

Notice that when using `map`, the items in the result are values returned by the function `cube`.

In contrast, the values returned by `f` in `filter(f, ...)` are not the items in `result`. `f(i)` is only used to determine if the value `i` should be kept in `result`.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download