Here is my code:
map(lambda i: 3*i, map(lambda x: x, [[1, 2],[3, 4]]))
It outputs: [[1, 2, 1, 2, 1, 2], [3, 4, 3, 4, 3, 4]]
How do I modify it to print 3, 6, 9, 12?
I do not want to use append. Just map and lambda.
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2 Answers
The problem you observe arises from the fact that the inner map returns a sequence of lists, not a sequence of numbers. This is because that map is iterating over a list containing smaller lists. So, if you use itertools.chain.from_iterable to flatten that list, you'll get a sequence of numbers. You can then use the outer map to triplicate them
In [67]: L = [[1, 2],[3, 4]]
In [68]: for i in map(lambda i: 3*i, itertools.chain.from_iterable(L)): print(i)
3
6
9
12
Comments
Your code didn't work as expected because in Python the * operator is polymorphic. Thus if x and n are numbers, x*n returns the product of both numbers, but if one of the operands (say n) is an integer and the other is a sequence (.e. x is a list, a tuple or a string) the expression x*n returns the sequence which results of concatenating x with itself n times. Examples:
>>> .1 * 2.5
0.25
>>> [1, 2] * 3
[1, 2, 1, 2, 1, 2]
>>> "beep" * 2
beepbeep
In order to get the expected result you need to flatten out your list before applying map():
>>> your_list = [[1, 2], [3, 4]]
>>> flat_list = [item for sublist in your_list for item in sublist]
>>> flat_list
[1, 2, 3, 4]
>>> triples = map(lambda x: 3*x, flat_list)
>>> triples
[3, 6, 9, 12]
Actually you don't need to use map() and lambda. I think using a one-liner list comprehension would be more pythonic:
>>> triples = [3*item for sublist in your_list for item in sublist]
>>> triples
[3, 6, 9, 12]
