The while loop in my code refuses to terminate when I use the break statement, and I'm really confused why this is happening.
point = 0
while point < len(list_of_line_lists) - 1:
sentence = list_of_line_lists[point]["text"]
datapoint = point
while True:
try:
if list_of_line_lists[datapoint]["author"] == list_of_line_lists[datapoint + 1]["author"]:
sentence += list_of_line_lists[datapoint + 1]["text"]
datapoint += 1
print(datapoint)
else:
print("this isn't breaking")
break
except IndexError:
break
In my code above, the break statement within the else statement refuses to trigger. The code within it is executed, as there's a flurry of "this isn't breaking" in the output, but the loop itself doesn't terminate. This is how my output looks(until I manually stop it myself):
this isn't breaking
this isn't breaking
this isn't breaking
this isn't breaking
this isn't breaking
this isn't breaking
this isn't breaking
Any ideas to why this is happening?
The inner loop is breaking, but the outer loop isn't. The reason is because you are using the point variable in the outer while loop, but that (point) variable value is never changed (incremented/decremented).
This question already has answers here:
Why does python use 'else' after for and while loops?
(24 answers)
Closed 5 years ago.
Why is the break needed in the following code to skip processing of the else statement?
Why doesn't the if evaluation exit the program by itself, without continuing to process the else statement as well?
Is it because the if is nested in a for loop?
If so, why is that? I can't figure out why this works this way.
code:
for number in range(1,10):
if number == 5:
print "I counted to %s" % number
else:
print "I counted from 1 to 10"
output:
I counted to 5 I counted from 1 to 10
code:
for number in range(1,10):
if number == 5:
print "I counted to %s" % number
break
else:
print "I counted from 1 to 10"
output:
I counted to 5
You need to check Python's official document on: "break and continue Statements, and else Clauses on Loops" which says:
The break statement, like in C, breaks out of the innermost enclosing
for or while loop.
Loop statements may have an else clause; it is executed when the loop
terminates through exhaustion of the list (with for) or when the
condition becomes false (with while), but not when the loop is
terminated by a break statement.
I wrote a program in Python to generate prime numbers
here is the program
def genPrimes(n):
primes = [2] # primes generated so far
last = 3 # last number tried
while last <= n:
for p in primes:
if last % p == 0 and math.sqrt(p) <= last:
break
else:
primes.append(last)
last += 2
return primes
http://codepad.org/d33tsQyT
This program is producing a right answer. If you see the indentation for else: statement it is wrongly placed. if i try to place the else statement in if block interpreter is showing memory error. Can anyone tell why this is happening.
Thanks in advance
Maries
The else is actually attached to the for loop, and executes if the program doesn't break out of the loop. In your case, it executes if none of the primes divide into the number, so the number is prime and gets appended to the list.
See also the documentation.
It's not placed incorrectly, python is assuming you're using a for-else loop.
From the docs:
When used with a loop, the else clause has more in common with the
else clause of a try statement than it does that of if statements: a
try statement’s else clause runs when no exception occurs, and a
loop’s else clause runs when no break occurs.
I have a while loop which checks for the rising edge of the variable Accessory2. My code is the following:
# Check for rising edge of ACC2
while (currentTime.Value - StartTime) < TimeOutValue:
if Accessory2.Value == 1:
Acc2StartTime = currentTime.Value
print rttPrefix + "ACC2 output to ON"
break
yield None
else:
print rttPrefix + "No ACC2 output"
DynamicFlag.Value = -2
However, when the variable Accessory2 goes up, the break condition stops the if statement but the else statement is also executed. Any idea why ?
The output is the following:
*RTT:* ACC2 output to ON
*RTT:* No ACC2 output
Maybe you're just executing (or continuing) the while loop multiple times, and the first execution prints your first line and a subsequent execution prints the second.
Your else statement is indented at the same level as your while loop, so thus is not inside it.
I've noticed the following code is legal in Python. My question is why? Is there a specific reason?
n = 5
while n != 0:
print n
n -= 1
else:
print "what the..."
Many beginners accidentally stumble on this syntax when they try to put an if/else block inside of a while or for loop, and don't indent the else properly. The solution is to make sure the else block lines up with the if, assuming that it was your intent to pair them. This question explains why it didn't cause a syntax error, and what the resulting code means. See also I'm getting an IndentationError. How do I fix it?, for the cases where there is a syntax error reported.
The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won't be executed.
One way to think about it is as an if/else construct with respect to the condition:
if condition:
handle_true()
else:
handle_false()
is analogous to the looping construct:
while condition:
handle_true()
else:
# condition is false now, handle and go on with the rest of the program
handle_false()
An example might be along the lines of:
while value < threshold:
if not process_acceptable_value(value):
# something went wrong, exit the loop; don't pass go, don't collect 200
break
value = update(value)
else:
# value >= threshold; pass go, collect 200
handle_threshold_reached()
The else clause is executed if you exit a block normally, by hitting the loop condition or falling off the bottom of a try block. It is not executed if you break or return out of a block, or raise an exception. It works for not only while and for loops, but also try blocks.
You typically find it in places where normally you would exit a loop early, and running off the end of the loop is an unexpected/unusual occasion. For example, if you're looping through a list looking for a value:
for value in values:
if value == 5:
print "Found it!"
break
else:
print "Nowhere to be found. :-("
Allow me to give an example on why to use this else-clause. But:
my point is now better explained in Leo’s answer
I use a for- instead of a while-loop, but else works similar (executes unless break was encountered)
there are better ways to do this (e.g. wrapping it into a function or raising an exception)
Breaking out of multiple levels of looping
Here is how it works: the outer loop has a break at the end, so it would only be executed once. However, if the inner loop completes (finds no divisor), then it reaches the else statement and the outer break is never reached. This way, a break in the inner loop will break out of both loops, rather than just one.
for k in [2, 3, 5, 7, 11, 13, 17, 25]:
for m in range(2, 10):
if k == m:
continue
print 'trying %s %% %s' % (k, m)
if k % m == 0:
print 'found a divisor: %d %% %d; breaking out of loop' % (k, m)
break
else:
continue
print 'breaking another level of loop'
break
else:
print 'no divisor could be found!'
The else-clause is executed when the while-condition evaluates to false.
From the documentation:
The while statement is used for repeated execution as long as an expression is true:
while_stmt ::= "while" expression ":" suite
["else" ":" suite]
This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the else clause, if present, is executed and the loop terminates.
A break statement executed in the first suite terminates the loop without executing the else clause’s suite. A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.
The else clause is only executed when the while-condition becomes false.
Here are some examples:
Example 1: Initially the condition is false, so else-clause is executed.
i = 99999999
while i < 5:
print(i)
i += 1
else:
print('this')
OUTPUT:
this
Example 2: The while-condition i < 5 never became false because i == 3 breaks the loop, so else-clause was not executed.
i = 0
while i < 5:
print(i)
if i == 3:
break
i += 1
else:
print('this')
OUTPUT:
0
1
2
3
Example 3: The while-condition i < 5 became false when i was 5, so else-clause was executed.
i = 0
while i < 5:
print(i)
i += 1
else:
print('this')
OUTPUT:
0
1
2
3
4
this
My answer will focus on WHEN we can use while/for-else.
At the first glance, it seems there is no different when using
while CONDITION:
EXPRESSIONS
print 'ELSE'
print 'The next statement'
and
while CONDITION:
EXPRESSIONS
else:
print 'ELSE'
print 'The next statement'
Because the print 'ELSE' statement seems always executed in both cases (both when the while loop finished or not run).
Then, it's only different when the statement print 'ELSE' will not be executed.
It's when there is a breakinside the code block under while
In [17]: i = 0
In [18]: while i < 5:
print i
if i == 2:
break
i = i +1
else:
print 'ELSE'
print 'The next statement'
....:
0
1
2
The next statement
If differ to:
In [19]: i = 0
In [20]: while i < 5:
print i
if i == 2:
break
i = i +1
print 'ELSE'
print 'The next statement'
....:
0
1
2
ELSE
The next statement
return is not in this category, because it does the same effect for two above cases.
exception raise also does not cause difference, because when it raises, where the next code will be executed is in exception handler (except block), the code in else clause or right after the while clause will not be executed.
I know this is old question but...
As Raymond Hettinger said, it should be called while/no_break instead of while/else.
I find it easy to understeand if you look at this snippet.
n = 5
while n > 0:
print n
n -= 1
if n == 2:
break
if n == 0:
print n
Now instead of checking condition after while loop we can swap it with else and get rid of that check.
n = 5
while n > 0:
print n
n -= 1
if n == 2:
break
else: # read it as "no_break"
print n
I always read it as while/no_break to understand the code and that syntax makes much more sense to me.
thing = 'hay'
while thing:
if thing == 'needle':
print('I found it!!') # wrap up for break
break
thing = haystack.next()
else:
print('I did not find it.') # wrap up for no-break
The possibly unfortunately named else-clause is your place to wrap up from loop-exhaustion without break.
You can get by without it if
you break with return or raise → the entire code after the call or try is your no-break place
you set a default before while (e.g. found = False)
but it might hide bugs the else-clause knows to avoid
If you use a multi-break with non-trivial wrap-up, you should use a simple assignment before break, an else-clause assignment for no-break, and an if-elif-else or match-case to avoid repeating non-trival break handling code.
Note: the same applies to for thing in haystack:
Else is executed if while loop did not break.
I kinda like to think of it with a 'runner' metaphor.
The "else" is like crossing the finish line, irrelevant of whether you started at the beginning or end of the track. "else" is only not executed if you break somewhere in between.
runner_at = 0 # or 10 makes no difference, if unlucky_sector is not 0-10
unlucky_sector = 6
while runner_at < 10:
print("Runner at: ", runner_at)
if runner_at == unlucky_sector:
print("Runner fell and broke his foot. Will not reach finish.")
break
runner_at += 1
else:
print("Runner has finished the race!") # Not executed if runner broke his foot.
Main use cases is using this breaking out of nested loops or if you want to run some statements only if loop didn't break somewhere (think of breaking being an unusual situation).
For example, the following is a mechanism on how to break out of an inner loop without using variables or try/catch:
for i in [1,2,3]:
for j in ['a', 'unlucky', 'c']:
print(i, j)
if j == 'unlucky':
break
else:
continue # Only executed if inner loop didn't break.
break # This is only reached if inner loop 'breaked' out since continue didn't run.
print("Finished")
# 1 a
# 1 b
# Finished
The else: statement is executed when and only when the while loop no longer meets its condition (in your example, when n != 0 is false).
So the output would be this:
5
4
3
2
1
what the...
Suppose you've to search an element x in a single linked list
def search(self, x):
position = 1
p =self.start
while p is not None:
if p.info == x:
print(x, " is at position ", position)
return True
position += 1
p = p.link
else:
print(x, "not found in list")
return False
So if while conditions fails else will execute, hope it helps!
The better use of 'while: else:' construction in Python should be if no loop is executed in 'while' then the 'else' statement is executed. The way it works today doesn't make sense because you can use the code below with the same results...
n = 5
while n != 0:
print n
n -= 1
print "what the..."
As far as I know the main reason for adding else to loops in any language is in cases when the iterator is not on in your control. Imagine the iterator is on a server and you just give it a signal to fetch the next 100 records of data. You want the loop to go on as long as the length of the data received is 100. If it is less, you need it to go one more times and then end it. There are many other situations where you have no control over the last iteration. Having the option to add an else in these cases makes everything much easier.