Need help understanding this while loop
As a beginner how do I understand while loops?
While Loops, how do they work?
loops - When to use "while" or "for" in Python - Stack Overflow
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While loops is kinda frustrating I'm 20 days into python and I'm stuck on loops since last 4 days
Yes, there is a huge difference between while and for.
The for statement iterates through a collection or iterable object or generator function.
The while statement simply loops until a condition is False.
It isn't preference. It's a question of what your data structures are.
Often, we represent the values we want to process as a range (an actual list), or xrange (which generates the values) (Edit: In Python 3, range is now a generator and behaves like the old xrange function. xrange has been removed from Python 3). This gives us a data structure tailor-made for the for statement.
Generally, however, we have a ready-made collection: a set, tuple, list, map or even a string is already an iterable collection, so we simply use a for loop.
In a few cases, we might want some functional-programming processing done for us, in which case we can apply that transformation as part of iteration. The sorted and enumerate functions apply a transformation on an iterable that fits naturally with the for statement.
If you don't have a tidy data structure to iterate through, or you don't have a generator function that drives your processing, you must use while.
while is useful in scenarios where the break condition doesn't logically depend on any kind of sequence. For example, consider unpredictable interactions:
while user_is_sleeping():
wait()
Of course, you could write an appropriate iterator to encapsulate that action and make it accessible via for – but how would that serve readability?¹
In all other cases in Python, use for (or an appropriate higher-order function which encapsulate the loop).
¹ assuming the user_is_sleeping function returns False when false, the example code could be rewritten as the following for loop:
for _ in iter(user_is_sleeping, False):
wait()
Here is your code, improved:
number = 50
while number >= 0:
if number % 5 == 0:
print(number)
number = number - 1
I'm confused about that too, but here are my thoughts: you only subtracted when number % 5 == 1, but it can also be 2, 3, 4 or 5.
num = 50
while num != 0:
if num % 5 == 0:
print(num)
num -= 1
would work for this as it will only print the number if it is divisible by 5, and always subtract.
I am not sure what you are trying to do. You can implement a do-while loop like this:
while True:
stuff()
if fail_condition:
break
Or:
stuff()
while not fail_condition:
stuff()
What are you doing trying to use a do while loop to print the stuff in the list? Why not just use:
for i in l:
print i
print "done"
Update:
So do you have a list of lines? And you want to keep iterating through it? How about:
for s in l:
while True:
stuff()
# use a "break" instead of s = i.next()
Does that seem like something close to what you would want? With your code example, it would be:
for s in some_list:
while True:
if state is STATE_CODE:
if "//" in s:
tokens.add( TOKEN_COMMENT, s.split( "//" )[1] )
state = STATE_COMMENT
else :
tokens.add( TOKEN_CODE, s )
if state is STATE_COMMENT:
if "//" in s:
tokens.append( TOKEN_COMMENT, s.split( "//" )[1] )
break # get next s
else:
state = STATE_CODE
# re-evaluate same line
# continues automatically
Here's a very simple way to emulate a do-while loop:
condition = True
while condition:
# loop body here
condition = test_loop_condition()
# end of loop
The key features of a do-while loop are that the loop body always executes at least once, and that the condition is evaluated at the bottom of the loop body. The control structure show here accomplishes both of these with no need for exceptions or break statements. It does introduce one extra Boolean variable.