Question

Can someone kindly explain how the following python code is working? This code is reading a series of integers, which are arranged as 3-tuples.

inverted_list = map(lambda i: (int(numbers[2 + 3*i]),
                           float(numbers[3 + 3*i]),
                           float(numbers[4 + 3*i]))
                    ,range(0, (len(numbers) - 2)/3))

Exactly how map and lambda works? Is the lambda really required here?

Thanks.

Answer 1

No, lambda is not really required here, this could also be written as:

inverted_list = [(int(numbers[2 + 3*i]),
                  float(numbers[3 + 3*i]),
                  float(numbers[4 + 3*i])) for i in range(0, (len(numbers) - 2)/3]

I’m not going into the math bit of the indexers, since i’m only on my first cup of coffee today, but what map does, is apply a function to a sequence, generating a new sequence with the result of the function for each element. The lambda is the function, in this case creating a tuple from your numbers. The sequence is the range expression, that gives you a list of i values that you can use in your indexing expressions (e.g. 3 + 3*i).

The version of your code snippet @gnibbler and I are showing you is modern python. This wasn’t around in, say, python 1.5 – so we used map, reduce, zip and other “functional” high order operations. I personally like them a lot, but I guess the’re doomed to be replaced for most tasks – the list comprehensions are more expressive in cases like this one!