CODEWITHSUNDEEP
pythonfor-looppython-3.xdata-structures
Bede Constantinides
Bede Constantinides

Reputation: 2569

Pythonic iteration over multiple lists in parallel

I'd like to know an elegant, Pythonic way to iterate over a list of lists (or dict of lists) in parallel in Python 3. The number of lists is not known until runtime, so I believe I cannot simply supply them as arguments to the zip() function.

For example, given the following data structure:

var = [['x1' ,'x2' ,'x3'], ['y1', 'y2', 'y3'], ['z1', 'z2', 'z3'], …]

I would like to be able to access the following values on each iteration:

x1,y1,z1 followed by x2,y2,z2 followed by x3,y3,z3 and so on.

Presumably I could achieve this using list indexes directly, or by using itertools.chain, but I'm looking for a more elegant approach using generators or otherwise.

Upvotes: 1

Views: 860

Answers (2)

Hugh Bothwell
Hugh Bothwell

Reputation: 56714

zip(*var) will automatically unpack your list of lists.

So, for example:

var = [['x1' ,'x2' ,'x3'], ['y1', 'y2', 'y3'], ['z1', 'z2', 'z3'], ['w1', 'w2', 'w3']]

for ltrs in zip(*var):
    print(", ".join(ltrs))

results in

x1, y1, z1, w1
x2, y2, z2, w2
x3, y3, z3, w3

Edit: per comments below, he wants to use the items from a dictionary,

var = {
    'id_172': ['x1', 'x2', 'x3'],
    'id_182': ['y1', 'y2', 'y3'],
    'id_197': ['z1', 'z2', 'z3']
}

I assume we are using the values with the keys in sorted order:

keys = sorted(var.keys())
for ltrs in zip(*(var[k] for k in keys)):
    print(", ".join(ltrs))

which gives

x1, y1, z1
x2, y2, z2
x3, y3, z3

Warning: do note that this sorts the keys in lexocographic order (ie string alphabetical order), so for example "id_93" comes after "id_101". If your labels need to be sorted in numeric order you will need to use a custom key function, something like

keys = sorted(var.keys(), key=lambda k: int(k[3:]))

Upvotes: 8

thefourtheye
thefourtheye

Reputation: 239683

If you look at the input and output data carefully, you are actually transposing a two dimensional array. That can be done easily with Python's builtin zip function, like this

var = [['x1' ,'x2' ,'x3'], ['y1', 'y2', 'y3'], ['z1', 'z2', 'z3']]
print list(zip(*var))
# [('x1', 'y1', 'z1'), ('x2', 'y2', 'z2'), ('x3', 'y3', 'z3')]

You can iterate over it, like this

for x, y, z in zip(*var):
   print x, y, z

Output

x1 y1 z1
x2 y2 z2
x3 y3 z3

Upvotes: 2

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