break/continue - newbe

[Ann]

I have trouble sometimes figuring out where
break and continue go to. Is there some easy
way to figure it out, or a tool?
TIA
Ann

 

[Jeff Shannon]

I have trouble sometimes figuring out where
break and continue go to. Is there some easy
way to figure it out, or a tool?

Break and continue always operate on the most-nested loop that's
currently executing. To show an example, let's add some line numbers to
some code...

1) while spam:
2) if foo(spam):
3) continue
4) for n in range(spam):
5) if bar(n):
6) break
7) results.append(baz(spam, n))
8) spam = spam - 1

Now, when this loop runs, when foo(spam) evaluates as True we execute
the continue at line 3. At this time, we're running code that's at the
loop-nesting level just inside 'while spam' (line 1), so line 1 is the
loop statement that's affected by the continue on line 3. If line 3 is
triggered, then we skip lines 4-8 and go back to line 1.

If line 3 is *not* triggered, then we enter a for loop at line 4.
There's a break at line 6; if this gets triggered, then we look
backwards to find the most-current (i.e. most nested) loop, which is now
that for loop on line 4. So we break out of that for loop (which
comprises lines 4-7), and drop down to line 8.

So, in general, the way to determine how break and continue will affect
program flow is to look backwards (up) for the most recent loop
statement; the break/continue will be inside that statement's dependent
body. Break will drop you down to the next line after the loop body,
and continue will bring you back up to the top of the loop body and the
start of the next loop iteration.

Jeff Shannon
Technician/Programmer
Credit International

 

[Ann]

Break and continue always operate on the most-nested loop that's
currently executing. To show an example, let's add some line numbers to
some code...

1) while spam:
2) if foo(spam):
3) continue
4) for n in range(spam):
5) if bar(n):
6) break
7) results.append(baz(spam, n))
8) spam = spam - 1

Now, when this loop runs, when foo(spam) evaluates as True we execute
the continue at line 3. At this time, we're running code that's at the
loop-nesting level just inside 'while spam' (line 1), so line 1 is the
loop statement that's affected by the continue on line 3. If line 3 is
triggered, then we skip lines 4-8 and go back to line 1.

If line 3 is *not* triggered, then we enter a for loop at line 4.
There's a break at line 6; if this gets triggered, then we look
backwards to find the most-current (i.e. most nested) loop, which is now
that for loop on line 4. So we break out of that for loop (which
comprises lines 4-7), and drop down to line 8.

So, in general, the way to determine how break and continue will affect
program flow is to look backwards (up) for the most recent loop
statement; the break/continue will be inside that statement's dependent
body. Break will drop you down to the next line after the loop body,
and continue will bring you back up to the top of the loop body and the
start of the next loop iteration.

Jeff Shannon
Technician/Programmer
Credit International

Thanks Jeff, that solves my problem. BTW: is there an easy way to
break/continue out more than one level?

 

[Fredrik Lundh]

Thanks Jeff, that solves my problem. BTW: is there an easy way to
break/continue out more than one level?

not really; to continue more than one level, you have to break out of the
inner loop, and make sure the logic in that loop body brings you back to
the top.

to break more than one level, you can:

1) use an exception

class LeaveLoop(Exception): pass

def myfunction():
try:
for a in ...:
for b in ...:
if c:
raise LeaveLoop # break out of both loops
do something
except LeaveLoop:
pass

2) organize your code so you can use return rather than "multi-break"

def myfunction():
calculate()

def calculate():
for a in ...:
for b in ...:
if c:
return
do something

3) use flags

def myfunction():
run = 1
for a in ...:
for b in ...:
if c:
run = 0
if not run:
break
if not run:
break

Solution 2 is almost always the best way to do this; solution 3 is often the worst.

</F>

 

[John Machin]

I have trouble sometimes figuring out where
break and continue go to. Is there some easy
way to figure it out, or a tool?
TIA
Ann

You need a tool called py2ftn. It would convert statements like "if a >
b: break" to e.g. "IF(A.GT.B)GOTO12345".

A longer example; this:

while 1:
blah1
if cond1: break
blah2
if cond2: continue
blah3

becomes:

10000 IF(1)10001,10003,10001
10001 CONTINUE
BLAH1
IF(COND1)GOTO10003
BLAH2
IF(COND2)GOTO10002
BLAH3
10002 CONTINUE
GOTO10000
10003 CONTINUE

HTH,
John

 

[John Machin]

I have trouble sometimes figuring out where
break and continue go to. Is there some easy
way to figure it out, or a tool?
TIA
Ann

You need a tool called py2ftn. It would convert a sequence of
statements like:

while 1:
blah1
if cond1: break
blah2
if cond2: continue
blah3

to:

10000 IF(1)10001,10003,10001
10001 CONTINUE
BLAH1
IF(COND1)GOTO10003
BLAH2
IF(COND2)GOTO10002
BLAH3
10002 CONTINUE
GOTO10000
10003 CONTINUE

HTH,
John