Noob: Loops and the 'else' construct

[Ixiaus]

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!

[Gabriel Genellina]

En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <pa*******@comcast.net>
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

[Thorsten Kampe]

* Gabriel Genellina (Fri, 19 Oct 2007 00:11:18 -0300)

En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <pa*******@comcast.net>
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

[Dustan]

On Oct 19, 3:12 am, Thorsten Kampe <[e-mail]:

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

[Paul Boddie]

On 19 Okt, 13:39, Dustan <[e-mail]:

On Oct 19, 3:12 am, Thorsten Kampe <[e-mail]:

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

[MRAB]

On Oct 19, 4:11 am, "Gabriel Genellina" <[e-mail]>
wrote:

En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <[e-mail]>
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"

[Thorsten Kampe]

* Dustan (Fri, 19 Oct 2007 11:39:04 -0000)

On Oct 19, 3:12 am, Thorsten Kampe <[e-mail]:

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!'
#

[Diez B. Roggisch]

MRAB schrieb:

On Oct 19, 4:11 am, "Gabriel Genellina" <[e-mail]>
wrote:

>En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <[e-mail]>
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