[Tutor] 'for' loops

[Steve Willoughby]

On Mon, Dec 01, 2008 at 04:44:02PM -0800, WM. wrote:
> I recently asked a question about 'for' loops, expecting them to be 
> similar to 'for-next' loops. I have looked at several on-line tutors but 
>  am still in the dark about what 'for' loops do.
> Does anyone have a plain English about the use of 'for' loops?
> Are 'while' loops the only way Python runs a sub-routine over & over?

No, both 'while' and 'for' loops are for running a block of code
(whether subroutine calls or whatever) over and over.  The difference
between the two is that 'while' will continue repeating the block
for however many iterations it takes until the condition is satisfied
('while x is true, for some expression x'), a 'for' loop will run the
block of code a set number of times ('once for each element of some
set of values').

So if you want to execute 'print' for every line of a file, you
would do this:

  for line in file:
    print line

If you wanted to double a value until it exceeded 100, you would
use a while loop:

  while x <= 100:
    x *= 2

If you just want something executed a specific number of times,
(like print "hello" 10 times), you can use a for loop:

  for i in range(10):
    print "hello"

This is, just like any 'for' loop, executing the block once per
element of a list.  The list in this case is range(10) which is
an expression that generates the list (0, 1, 2, ..., 9), so you
get one run through the code for each of those.

Does that help?

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-- 
Steve Willoughby    |  Using billion-dollar satellites
steve at alchemy.com   |  to hunt for Tupperware.