An Introduction to Pythonby Guido van Rossum and Fred L. Drake, Jr. Paperback (6"x9"), 124 pages ISBN 0954161769 RRP £12.95 ($19.95) Sales of this book support the Python Software Foundation! Get a printed copy>>> |

### 5.1.3 Functional Programming Tools

There are three built-in functions that are very useful when used with
lists: `filter()`

, `map()`

, and `reduce()`

.

`‘filter( function, sequence)’` returns a sequence
consisting of those items from the
sequence for which

`function`(`item`)

is true.
If `sequence`is a

`string`

or `tuple`

, the result will
be of the same type; otherwise, it is always a `list`

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

`‘map( function, sequence)’` calls

`function`(`item`)

for each of the items in the sequence and
returns a list of the return values. For example, to compute some
cubes:
>>> def cube(x): return x*x*x ... >>> map(cube, range(1, 11)) [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]

More than one sequence may be passed; the function must then have as
many arguments as there are sequences and is called with the
corresponding item from each sequence (or `None`

if some sequence
is shorter than another). For example:

>>> seq = range(8) >>> def add(x, y): return x+y ... >>> map(add, seq, seq) [0, 2, 4, 6, 8, 10, 12, 14]

`‘reduce( function, sequence)’` returns a single value
constructed by calling the binary function

`function`on the first two items of the sequence, then on the result and the next item, and so on. For example, to compute the sum of the numbers 1 through 10:

>>> def add(x,y): return x+y ... >>> reduce(add, range(1, 11)) 55

If there's only one item in the sequence, its value is returned; if the sequence is empty, an exception is raised.

A third argument can be passed to indicate the starting value. In this case the starting value is returned for an empty sequence, and the function is first applied to the starting value and the first sequence item, then to the result and the next item, and so on. For example,

>>> def sum(seq): ... def add(x,y): return x+y ... return reduce(add, seq, 0) ... >>> sum(range(1, 11)) 55 >>> sum([]) 0

Don't use this example's definition of `sum()`

: since summing
numbers is such a common need, a built-in function
`sum(`

is already provided, and works exactly like
this.`sequence`)* (Added in Python version 2.3)*

ISBN 0954161769 | An Introduction to Python | See the print edition |