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Noob: Loops and the 'else' construct

 
 
Ixiaus
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Posts: n/a
 
      10-19-2007
I have just come across a site that discusses Python's 'for' and
'while' loops as having an (optional) 'else' structure.

At first glance I interpreted it as being a bit like the 'default'
structure in PHP's switch block... But the switch block isn't a loop,
so, I am now confused as to the reason for using 'else' with the for
and while loops...

A few quick tests basically show that statements in the else structure
are executed at the fulfillment of the loop's expression (ie, no
break).

Example:

for i in range(10):
print i
else:
print 'the end!'

0
1
2
3
4
5
6
7
8
9
10
the end!

 
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Gabriel Genellina
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Posts: n/a
 
      10-19-2007
En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <(E-Mail Removed)>
escribió:

> I have just come across a site that discusses Python's 'for' and
> 'while' loops as having an (optional) 'else' structure.
>
> At first glance I interpreted it as being a bit like the 'default'
> structure in PHP's switch block... But the switch block isn't a loop,
> so, I am now confused as to the reason for using 'else' with the for
> and while loops...
>
> A few quick tests basically show that statements in the else structure
> are executed at the fulfillment of the loop's expression (ie, no
> break).


A `while` loop tests a condition: if it evaluates to true, keep cycling;
if it is false, stop. The `else` clause is executed when the condition is
false, as in any `if` statement. If you exit the loop by using `break`,
the `else` part is not executed (because you didn't get out of the loop by
determining the condition falseness)

You can think of a `for` loop as meaning `while there are remaining
elements to be iterated, keep cycling` and the `else` clause applies when
there are no more elements. A `break` statement does not trigger the else
clause because the iteration was not exhausted.

Once you get the idea, it's very simple.

--
Gabriel Genellina

 
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Thorsten Kampe
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      10-19-2007
* Gabriel Genellina (Fri, 19 Oct 2007 00:11:18 -0300)
> En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <(E-Mail Removed)>
> escribió:
> > I have just come across a site that discusses Python's 'for' and
> > 'while' loops as having an (optional) 'else' structure.
> >
> > At first glance I interpreted it as being a bit like the 'default'
> > structure in PHP's switch block... But the switch block isn't a loop,
> > so, I am now confused as to the reason for using 'else' with the for
> > and while loops...
> >
> > A few quick tests basically show that statements in the else structure
> > are executed at the fulfillment of the loop's expression (ie, no
> > break).

>
> A `while` loop tests a condition: if it evaluates to true, keep cycling;
> if it is false, stop. The `else` clause is executed when the condition is
> false, as in any `if` statement. If you exit the loop by using `break`,
> the `else` part is not executed (because you didn't get out of the loop by
> determining the condition falseness)


So a for/else loop is exactly the same thing as a for loop with the
else clause outside the loop (except for "break")? Guess that's why I
never used that...

Thorsten
 
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Dustan
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      10-19-2007
On Oct 19, 3:12 am, Thorsten Kampe <(E-Mail Removed)> wrote:
> So a for/else loop is exactly the same thing as a for loop with the
> else clause outside the loop (except for "break")?


Am I missing something here? It sounds to me like you just described
two identical constructs.

> Guess that's why I
> never used that...
>
> Thorsten



 
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Paul Boddie
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Posts: n/a
 
      10-19-2007
On 19 Okt, 13:39, Dustan <(E-Mail Removed)> wrote:
> On Oct 19, 3:12 am, Thorsten Kampe <(E-Mail Removed)> wrote:
>
> > So a for/else loop is exactly the same thing as a for loop with the
> > else clause outside the loop (except for "break")?

>
> Am I missing something here? It sounds to me like you just described
> two identical constructs.


Think of the loop-plus-else construct as behaving like this:

while 1:
# Get next element (in a for loop)
if loop_condition: # eg. whether we have an element
# Loop body statement
else:
# Loop else statement
break

Taking the example...

for i in range(10):
print i
else:
print 'the end!'

This is equivalent to...

while 1:
# Get next element (from the range iterator)
if next element: # yes, it's more complicated than this
print i
else:
print 'the end!'
break

Now consider what happens if you put a break statement inside the for
loop.

Paul

 
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MRAB
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Posts: n/a
 
      10-19-2007
On Oct 19, 4:11 am, "Gabriel Genellina" <(E-Mail Removed)>
wrote:
> En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <(E-Mail Removed)>
> escribió:
>
> > I have just come across a site that discusses Python's 'for' and
> > 'while' loops as having an (optional) 'else' structure.

>
> > At first glance I interpreted it as being a bit like the 'default'
> > structure in PHP's switch block... But the switch block isn't a loop,
> > so, I am now confused as to the reason for using 'else' with the for
> > and while loops...

>
> > A few quick tests basically show that statements in the else structure
> > are executed at the fulfillment of the loop's expression (ie, no
> > break).

>
> A `while` loop tests a condition: if it evaluates to true, keep cycling;
> if it is false, stop. The `else` clause is executed when the condition is
> false, as in any `if` statement. If you exit the loop by using `break`,
> the `else` part is not executed (because you didn't get out of the loop by
> determining the condition falseness)
>
> You can think of a `for` loop as meaning `while there are remaining
> elements to be iterated, keep cycling` and the `else` clause applies when
> there are no more elements. A `break` statement does not trigger the else
> clause because the iteration was not exhausted.
>
> Once you get the idea, it's very simple.
>

It's useful when you want to search for an item and to do something if
you don't find it, eg:

for i in items:
if is_wanted(i):
print "Found it"
break
else:
print "Didn't find ir"

 
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Thorsten Kampe
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Posts: n/a
 
      10-20-2007
* Dustan (Fri, 19 Oct 2007 11:39:04 -0000)
> On Oct 19, 3:12 am, Thorsten Kampe <(E-Mail Removed)> wrote:
> > So a for/else loop is exactly the same thing as a for loop with the
> > else clause outside the loop (except for "break")?

>
> Am I missing something here? It sounds to me like you just described
> two identical constructs.


#
for i in range(10):
print i
else:
print 'the end!'
#

is the same else

#
for i in range(10):
print i
print 'the end!'
#
 
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Diez B. Roggisch
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Posts: n/a
 
      10-20-2007
MRAB schrieb:
> On Oct 19, 4:11 am, "Gabriel Genellina" <(E-Mail Removed)>
> wrote:
>> En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <(E-Mail Removed)>
>> escribió:
>>
>>> I have just come across a site that discusses Python's 'for' and
>>> 'while' loops as having an (optional) 'else' structure.
>>> At first glance I interpreted it as being a bit like the 'default'
>>> structure in PHP's switch block... But the switch block isn't a loop,
>>> so, I am now confused as to the reason for using 'else' with the for
>>> and while loops...
>>> A few quick tests basically show that statements in the else structure
>>> are executed at the fulfillment of the loop's expression (ie, no
>>> break).

>> A `while` loop tests a condition: if it evaluates to true, keep cycling;
>> if it is false, stop. The `else` clause is executed when the condition is
>> false, as in any `if` statement. If you exit the loop by using `break`,
>> the `else` part is not executed (because you didn't get out of the loop by
>> determining the condition falseness)
>>
>> You can think of a `for` loop as meaning `while there are remaining
>> elements to be iterated, keep cycling` and the `else` clause applies when
>> there are no more elements. A `break` statement does not trigger the else
>> clause because the iteration was not exhausted.
>>
>> Once you get the idea, it's very simple.
>>

> It's useful when you want to search for an item and to do something if
> you don't find it, eg:
>
> for i in items:
> if is_wanted(i):
> print "Found it"
> break
> else:
> print "Didn't find ir"


Wrong. It's not:

for i in []:
print i
else:
print "I'm reached, too"

prints out "I'm reached, too"

The else will ONLY not get executed when the loop is left prematurely
through a break:

for i in [1]:
print i
break
else:
print "I'm reached, too"

won't print the "I'm ..."


Diez
 
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