CODEWITHSUNDEEP
python-2.7
altarbza
altarbza

Reputation: 437

For loops with lists?

I'm starting to code in python and I came across this code snippet:

for [x, y] in L:
    for ix in range(-1, 2):
        for iy in range(-1, 2):
            cir = pylab.Circle((x + ix, y + iy), radius=sigma,  fc='r')
            pylab.gca().add_patch(cir)

at the line 1 I can not understand what is happening because I had never seen anything like it in another programming language. How this works?

for [x, y] in L:

[x, y] is a list? i dont know.

Upvotes: 1

Views: 56

Answers (2)

Pabitra Pati
Pabitra Pati

Reputation: 477

L must be a sequence of lists (or tuples) with two elements, which can be iterated over. So whenever for [x,y] in L: is executed, it picks each item in th sequence one by one and enters into the loop.

let the sequence be L = [[2,3], [4,5], ['Jeff', 7]] Now here what will happen when for [x,y] in L: will be executed is :- first list in the sequence [2,3] will be picked up and assigned as x and y respectively. And in the next iteration x and y will get the value 4 & 5 respectively. Like wise in third iteration x will be Jeff and y will be 7.

L = [[2,3], [4,5], ['Jeff', 7]]
count = 0
for [x,y] in L:
    count += 1
    print " Iteration :- %d, \t x :- %s, \t y:- %s" %(count, str(x), str(y))

Upvotes: 1

Bill the Lizard
Bill the Lizard

Reputation: 405735

Yes, [x, y] is a list with two elements. For your loop to work, L must be a list (or other iterable data structure) that contains a bunch of lists with two elements. Each time through the loop one of those lists is copied into [x, y], then the individual values of x and y are used in the body of the loop.

Try this and see if it makes sense:

L = [ [1, 2], [3, 4] ]

for [x, y] in L:
    print x
    print y

Upvotes: 0

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