Python: Loops for simultaneous operation, Two or possibly more?

Question

This question closely relates to How do I run two python loops concurrently?

I'll put it in a clearer manner: I get what the questioner asks in the above link, something like

for i in [1,2,3], j in [3,2,1]:
    print i,j
    cmp(i,j) #do_something(i,j)

But

L1: for i in [1,2,3] and j in [3,2,1]: doesnt work

Q1. but this was amusing what happened here:

    for i in [1,2,3], j in [3,2,1]:
    print i,j


[1, 2, 3] 0
False 0

Q2. How do I make something like L1 work?

Not Multithreading or parallelism really. (It's two concurrent tasks not a loop inside a loop) and then compare the result of the two.

Here the lists were numbers. My case is not numbers:

for i in f_iterate1() and j in f_iterate2():

UPDATE: abarnert below was right, I had j defined somewhere. So now it is:

>>> for i in [1,2,3], j in [3,2,1]:
    print i,j



Traceback (most recent call last):
  File "<pyshell#142>", line 1, in <module>
    for i in [1,2,3], j in [3,2,1]:
NameError: name 'j' is not defined

And I am not looking to zip two iteration functions! But process them simultaneously in a for loop like situation. and the question still remains how can it be achieved in python.

UPDATE #2: Solved for same length lists

>>> def a(num):
    for x in num:
        yield x


>>> n1=[1,2,3,4]
>>> n2=[3,4,5,6]
>>> x1=a(n1)
>>> x2=a(n2)
>>> for i,j in zip(x1,x2):
    print i,j


1 3
2 4
3 5
4 6
>>> 

[Solved]

Q3. What if n3=[3,4,5,6,7,8,78,34] which is greater than both n1,n2. zip wont work here.something like izip_longest? izip_longest works good enough.

Answer 1

It's hard to understand what you're asking, but I think you just want zip:

for i, j in zip([1,2,3], [3,2,1]):
    print i, j

for i, j in zip(f_iterate1(), f_iterate2()):
    print i, j

And so on…

This doesn't do anything concurrently as the term is normally used, it just does one thing at a time, but that one thing is "iterate over two sequences in lock-step".

---

Note that this extends in the obvious way to three or more lists:

for i, j, k in zip([1,2,3], [3,2,1], [13, 22, 31]):
    print i, j, k

(If you don't even know how many lists you have, see the comments.)

---

In case you're wondering what's going on with this:

for i in [1,2,3], j in [3,2,1]:
    print i,j

Try this:

print [1,2,3], j in [3,2,1]

If you've already defined j somewhere, it will print either [1, 2, 3] False or [1, 2, 3] True. Otherwise, you'll get a NameError. That's because you're just creating a tuple of two values, the first being the list [1,2,3], and the second being the result of the expression j in [3,2,1].

So:

j=0
for i in [1,2,3], j in [3,2 1]:
    print i, j

… is equivalent to:

j=0
for i in ([1,2,3], False):
    print i, 0

… which will print:

[1, 2, 3] 0
False 0

Answer 2

You want to use the zip() function:

for i, j in zip([1, 2, 3], [3, 2, 1]):
    #

for i, j in zip(f_iterate1(), f_iterate2()):
    #

zip() pairs up the elements of the input lists, letting you process them together.

If your inputs are large or are iterators, use future_builtins.zip(), or, if you don't care about forward compatibility with Python 3, use itertools.izip() instead; these yield pairs on demand instead of creating a whole output list in one go:

from future_builtins import zip

for i, j in zip(f_iterate1(), f_iterate2()):

Your generators fall in this scenario.

Last but not least, if your input lists have different lengths, zip() stops when the shortest list is exhausted. If you want to continue with the longest list instead, use itertools.izip_longest(); it'll use a fill value when the shorter input sequence(s) are exhausted:

>>> for i, j, k in izip_longest(range(3), range(3, 5), range(5, 10), fillvalue=42):
...     print i, j, k
... 
0 3 5
1 4 6
2 42 7
42 42 8
42 42 9

The default for fillvalue is None.

---

Your attempt:

for i in [1,2,3], j in [3,2,1]:

is really interpreted as:

for i in ([1,2,3], j in [3,2,1]):

where the latter part is interpreted as a tuple with two values, one a list, the other a boolean; after testing j in [3,2,1], is either True or False. You had j defined as 0 from a previous loop experiment and thus 0 in [3, 2, 1] is False.

Answer 3

For same-length arrays, you can use the index to refer to corresponding locations in respective lists, like so:

a = [1, 2, 3, 4, 5]
b = [2, 4, 6, 8, 10]
for i in range(len(a)):
    print(a[i])
    print(b[i])

This accesses same indices of both lists at the same time.