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Further Python Features (worked examples)

Adapted from,,,

Iterable Objects

Several of these examples involve iterable objects - variables that can be used to access a sequence of values. A simple example is

x=[0, 1, 2, 3, 4]

Transposing a Matrix

mat = [[1, 2, 3], [4, 5, 6]]


mat = [[1, 2], [3, 4, [5, 6]]
  • Why the *? -
    list(zip([1, 2, 3], [4, 5, 6]))
    would give the right answer, but mat is a single item. Using * breaks it into a list of items, which is what zip needs.
  • Why use zip? - zip takes all the 1st elements of the items given and puts them together, then all the 2nd elements and so on. In this case, 1 and 4 will be put together, then 2 and 5, then 3 and 6. It's like using a real zip, zipping together the corresponding teeth on each side.
  • Why use list? - zip returns an iterable object. If you try to print the output of zip you'll get something like
    <zip object at 0x7f7abed7eac8>
    So you need to use list

Taking a string input

Suppose you want the user to input some numbers to put into a list, and you wanted to do it with one line of code. If you run this

result = map(int ,input().split())

and type

5 6 7 8

you'll get

[5, 6, 7, 8]
  • Why use split? input() gets a string from the user. split() breaks it into strings.
  • Why use map?
    result = list(input().split())

    would output a list of strings

    ['5', '6', '7', '8']

    map(func, *iterables) make an iterator that runs the function func using arguments from each of the iterables. In this case int converts each string created by split into an integer.

Instead you could use list comprehension

result = (int(x) for x in list(input().split()))

and type

5 6 7 8

which would get the same output.