Example:
for x in iterable1:
expression
The map
form would be:
map(lambda x: expression, iterable1)
How do I extend this to a nested for loop using only map
and without list comprehensions?
Example:
for x in itr1:
for y in itr2:
expr
You could use itertools.product
to build the cartesian product of your two nested sequences, and map
your expression to the that list of 2-tuples:
from itertools import product
map(lambda (x, y): expression, product(itr1, itr2))
Example with some actual values:
seq = map(lambda (x, y): '%s:%s' % (x, y), product(itr1, itr2))
for item in seq:
print item
Note that the lambda (x, y)
is necessary to unpack each 2-tuple from the sequence to the separate x
and y
arguments used in the expression.
Bear with me on this one. Not an explanation but this worked after 2 days. Using only map and list. It's bad code. Suggestions to shorten the code are welcome. Python 3 solution
Example using list comprehension:
>>> a=[x+y for x in [0,1,2] for y in [100,200,300]]
>>> a
[100,200,300,101,201,301,102,202,302]
Example using for:
>>>a=[]
>>>for x in [0,1,2]:
... for y in [100,200,300]:
... a.append(x+y)
...
>>>a
[100,200,300,101,201,301,102,202,302]
Now example using only map:
>>>n=[]
>>>list(map(lambda x:n.extend(map(x,[100,200,300])),map(lambda x:lambda y:x+y,[0,1,2])))
>>>n
[100,200,300,101,201,301,102,202,302]
Much smaller python2.7 solution:
>>>m=[]
>>>map(lambda x:m.extend(x),map(lambda x:map(x,[100,200,300]),map(lambda x:lambda y:x+y,[0,1,2])))
>>>m
[100,200,300,101,201,301,102,202,302]
Another variation : I emailed to Mark Lutz and this was his solution. This doesn't use closures and is the closest to nested for loops functionality.
>>> X = [0, 1, 2]
>>> Y = [100, 200, 300]
>>> n = []
>>> t = list(map(lambda x: list(map(lambda y: n.append(x + y), Y)),X))
>>> n
[100,200,300,101,201,301,102,202,302]
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