How to avoid two loops here in python

Question

Here is my problem: I have a dict in python such as:

a = {1:[2, 3], 2:[1]}

I would like to output:

1, 2
1, 3
2, 1

what I am doing is

for i in a:
    for j in a[i]:
        print i, j 

so is there any easier way to do that avoiding two loops here or it is the easiest way already?

Answer 1

The code you have is about as good as it gets. One minor improvement might be iterating over the dictionary's items in the outer loop, rather than doing indexing:

for i, lst in a.items() # use a.iteritems() in Python 2
    for j in lst:
        print("{}, {}".format(i, j))

Answer 2

Couple of alternatives using list comprehensions, if you want to avoid explicit for loops.

# 1 method

# Python2.7
for key, value in a.iteritems():    # Use a.items() for python 3
    print "\n".join(["%d, %d" % (key, val) for val in value])

# 2 method - A more fancy way with list comprehensions

print "\n".join(["\n".join(["%d, %d" % (key, val) for val in value]) for key, value in a.iteritems()])

Both will output

1, 2
1, 3
2, 1

Answer 3

Remember in Python, Readability counts., so ideally @Blckknght's solution is what you should look forward, but just looking at your problem, technically as a POC, that you can rewrite your expression as a single loop, here is a solution.

But caveat, if you wan;t your code to be Readable, remember Explicit is better than implicit.

>>> def foo():
    return '\n'.join('{},{}'.format(*e) for e in chain(*(izip(cycle([k]),v) for k,v in a.items())))

>>> def bar():
    return '\n'.join("{},{}".format(i,j) for i in a for j in a[i])

>>> cProfile.run("foo()")
         20 function calls in 0.000 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <pyshell#240>:1(foo)
        5    0.000    0.000    0.000    0.000 <pyshell#240>:2(<genexpr>)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
       10    0.000    0.000    0.000    0.000 {method 'format' of 'str' objects}
        1    0.000    0.000    0.000    0.000 {method 'items' of 'dict' objects}
        1    0.000    0.000    0.000    0.000 {method 'join' of 'str' objects}


>>> cProfile.run("bar()")
         25 function calls in 0.000 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <pyshell#242>:1(bar)
       11    0.000    0.000    0.000    0.000 <pyshell#242>:2(<genexpr>)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
       10    0.000    0.000    0.000    0.000 {method 'format' of 'str' objects}
        1    0.000    0.000    0.000    0.000 {method 'join' of 'str' objects}