Some people use a special short-circuiting and expression to replace
if blocks in Python. For instance, instead of
if x >= 0.5:
f(x)
one can use
x >= 0.5 and f(x)
The point is that because multi-term logical conditions are evaluated in
series, f(x) is only evaluated after x >= 0.5 returns True, making
x >= 0.5 a pre-condition of f(x).
You cannot use an outright assignment (here y = x) as the second part
of the and statement; only a function will do.
In any case, this syntax is unsatisfying to me because it is less
explicit than using if outright. But I've seen it defended by a gifted
programmer as being "explicit once you recognize the syntax", and I
wondered if there was any timing advantage to it.
The results of timing, on 10^8 iterations, are (in seconds):
time using if: 43.7663021088
time using and: 44.545058012
It doesn't look as though the and statement saves time, particularly.
I suspect it is a case of coding craftiness — surely a pleasurable thing
in itself. But it now seems clear to me that using if is better; it is
both explicit and involves no loss of speed.
Addendum, later in the evening:
There is another way to use short-circuiting to avoid the use of if:
with == instead of and; for instance:
x >= 0.5 == f(x)
Just as with and, == causes f(x) to be evaluated after x >= 0.5;
we can show (though I have not done so here) that f(x) is in fact
evaluated. But it turns out that this trick saves even less time:
time using ==: 50.7892489433
Best to stick with if, it seems.
The program I wrote to compare processing times follows:
import time
import random
def f(x):
y = x
rounds = 100000000
r = random.random
start_time = time.time()
for i in xrange(rounds):
x = r()
if x >= 0.5:
f(x)
print 'time using if: ', time.time() - start_time
start_time = time.time()
for i in xrange(rounds):
x = r()
x >= 0.5 and f(x)
print 'time using and:', time.time() - start_time
start_time = time.time()
for i in xrange(rounds):
x = r()
x >= 0.5 == f(x)
print 'time using ==: ', time.time() - start_time
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