What is correct python syntax for this kind of list comprehension?

Question

task: {x*y such that x belongs to S & y is iteration count } where S is some other set something like this:

j=0
[i*j for j++ and i in S]

[s1*1, s2*2, s3*3...]

Answer 1

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for your edited question, you want

[i * j for j, i in enumerate(S)]

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python doesn't have ++ because it keeps a clear distinction between statements and expressions. use

[(i + 40) * i for i in xrange(60)]

another way to do this is

[i * j for i, j in enumerate(xrange(60), start=40)]

and yet another way is

[i * j for i, j in zip(xrange(40, 100), xrange(60))]

I think that the first is the best way to do it because it reduces function calls and is the most readable.

Also, if you don't know that you absolutely need a list, use a generator expression

((i + 40) * i for i in xrange(60))

This will allow you to process the results one at a time and never store a whole list in memory. You can pass a generator expression to stuff like sum, max, min and most other builtins.

Answer 2

S = range(40,100)
[i*j for i,j in enumerate(S)]

Answer 3

One way to do this would be to use enumerate in conjunction with range:

[x * (count + 1) for count, x in enumerate(range(40, 100))]

Look at other answers for (a lot of) other ways to do this :) :)

Answer 4

I'd do it in a generator:

def fooGen(S):
    j = 1
    for i in S:
        yield i * j
        J += 1

Answer 5

"""
task: {x*y such that x belongs to S & y is iteration count } where S is some other set
[snip unfortunate introduction of i and j]
[s1*1, s2*2, s3*3...]
"""

a very simple translation:

[x * y for y, x in enumerate(S, start=1)]

or

[x * (y + 1) for y, x in enumerate(S)]

Answer 6

you can use the current element index as an incrementor

>>> S=[2,4,6,8]
>>> [i*(S.index(i)+1) for i in S]
[2, 8, 18, 32]
>>> 

so your c-ish "j++" is S.index(i)+1