List comprehensions

[Dan Schmidt]

mlh at vier.idi.ntnu.no (Magnus L. Hetland) writes:

|   P = [(x, y) for x in X for y in Y]
| 
| Hm... Do the iterators work in parallel? Or would I end up with a
| Cartesian  product here? Logically (or intuitively), the above
| expression would seem (to me) to mean:
| 
|   P = []
|   for x in X:
|       for y in Y:
|           P.append((x,y))
| 
| which is, come to think of it, exactly what it means. Cool! But what
| could the version with "and" mean, then? Since it is a bit more
| verbose, it should probably be something less often used... Hm. It
| seems to me to mean something like:
| 
|   P = []
|   for i in range(min(len(X),len(Y))):
|       x, y = X[i], Y[i]
|       P.append(x,y)

You know, I'm only half kidding when I say that the syntax for that
second type of loop should be

    P = [(x, y) for x in X while y in Y]

-- 
Dan Schmidt | http://www.dfan.org