nested list comprehension and if clauses

[Jyotirmoy Bhattacharya]

I'm a newcomer to Python. I have just discovered nested list
comprehensions and I need help to understand how the if-clause
interacts with the multiple for-clauses. I have this small program:

def multab(n):
print 'multab',n
return [n*i for i in range(5)]

print [(m,n) for m in range(5) for n in multab(m) if m>2]

It produces the output:
multab 0
multab 1
multab 2
multab 3
multab 4
[(3, 0), (3, 3), (3, 6), (3, 9), (3, 12), (4, 0), (4, 4), (4, 8), (4,
12), (4, 16)]

I was wondering if there is some way to write the if-clause so that it
is 'hoisted' out of the inner loop and the multab function is not
called at all for m=0,1,2. That would seem to be important if multab
were an expensive function.

 

[Paul Rubin]

print [(m,n) for m in range(5) for n in multab(m) if m>2]

I was wondering if there is some way to write the if-clause so that it
is 'hoisted' out of the inner loop and the multab function is not
called at all for m=0,1,2. That would seem to be important if multab
were an expensive function.

Maybe you mean

print [(m,n) for m in [a for a in range(5) if a > 2] for n in multab(m)]

You'd probably really write

print [[(m,n) for m in range(3,5)] for n in multab(m)]

You should also use xrange instead of range, if the range might be
large. And you could use a generator expression instead of a listcomp
for the inner list.

 

[Alex Martelli]

I'm a newcomer to Python. I have just discovered nested list
comprehensions and I need help to understand how the if-clause
interacts with the multiple for-clauses. I have this small program:

def multab(n):
print 'multab',n
return [n*i for i in range(5)]

print [(m,n) for m in range(5) for n in multab(m) if m>2]

It produces the output:
multab 0
multab 1
multab 2
multab 3
multab 4
[(3, 0), (3, 3), (3, 6), (3, 9), (3, 12), (4, 0), (4, 4), (4, 8), (4,
12), (4, 16)]

I was wondering if there is some way to write the if-clause so that it
is 'hoisted' out of the inner loop and the multab function is not
called at all for m=0,1,2. That would seem to be important if multab
were an expensive function.

Sure, just place the if clause where it needs to apply (between the two
for clauses) [apart from the fact that this example is best expressed by
using range(3,5), as somebody already said;-)].

Generally, the semantics of:

x = [<expr> for <F1> if <I2> for <F3>]

are exactly those of

x = []
for <F1> :
if <I2> :
for<F3> :
x.append(expr)

and similarly for other mixes of for and if clauses (except that the
first clause must always be a for clause) -- you can always, simply and
mechanically conceptually translate them into an equivalent nest of for
and if statements, ending in a somelist.append(...) [where somelist may
be a "temporary anonymous" list if you're just going to use the listcomp
further rather than just assigning it to a name].


Alex

 

[Paul Rubin]

print [(m,n) for m in range(5) for n in multab(m) if m>2]

Sure, just place the if clause where it needs to apply (between the two
for clauses) [apart from the fact that this example is best expressed by
using range(3,5), as somebody already said;-)].

You mean

print [(m,n) for m in range(5) if m > 2 for n in multab(m)]

Heh. I didn't realize you could do that. Thanks.

 

[Alex Martelli]

print [(m,n) for m in range(5) for n in multab(m) if m>2]

Sure, just place the if clause where it needs to apply (between the two
for clauses) [apart from the fact that this example is best expressed by
using range(3,5), as somebody already said;-)].

You mean

print [(m,n) for m in range(5) if m > 2 for n in multab(m)]

Heh. I didn't realize you could do that. Thanks.

You're welcome (though a range(3,5) would still be better here.


Alex

 

[Jyotirmoy Bhattacharya]

print [(m,n) for m in range(5) for n in multab(m) if m>2]
I was wondering if there is some way to write the if-clause so that it
is 'hoisted' out of the inner loop and the multab function is not
called at all for m=0,1,2. That would seem to be important if multab
were an expensive function.

Generally, the semantics of:

x = [<expr> for <F1> if <I2> for <F3>]

are exactly those of

x = []
for <F1> :
if <I2> :
for<F3> :
x.append(expr)

Thanks. That makes things completely clear to me.