Use break to exit a loop:
while True:
ser = serial.Serial("/dev/ttyUSB0", 4800, timeout =1)
checking = ser.readline();
if checking.find(",,,,"):
print "not locked yet"
else:
print "locked and loaded"
break
The True and False line didn't do anything in your code; they are just referencing the built-in boolean values without assigning them anywhere.
Use break to exit a loop:
while True:
ser = serial.Serial("/dev/ttyUSB0", 4800, timeout =1)
checking = ser.readline();
if checking.find(",,,,"):
print "not locked yet"
else:
print "locked and loaded"
break
The True and False line didn't do anything in your code; they are just referencing the built-in boolean values without assigning them anywhere.
You can use a variable as condition for your while loop instead of just while True. That way you can change the condition.
So instead of having this code:
while True:
...
if ...:
True
else:
False
... try this:
keepGoing = True
while keepGoing:
ser = serial.Serial("/dev/ttyUSB0", 4800, timeout =1)
checking = ser.readline();
if checking.find(",,,,"):
print "not locked yet"
keepGoing = True
else:
keepGoing = False
print "locked and loaded"
EDIT:
Or as another answerer suggests, you can just break out of the loop :)
Videos
for example, why does this code work:
user = input("Enter marital status: ")
user = user.upper()
valid = False
while not valid:
if user == "M" or user == "S":
valid = True
else:
print ("invalid input")
user = input("Enter marital status: ")
user = user.upper()but this doesn't:
user = input("Enter marital status: ")
user = user.upper()
valid = True
while not valid:
if user == "M" or user == "S":
valid = False
else:
print ("invalid input")
user = input("Enter marital status: ")
user = user.upper()isn't the boolean condition changing in both cases?
can someone just give me some guidelines for using while True/False,e.g. when is it best to use this type of a while loop? that I can follow on my test tomorrow (yes, tomorrow).
Thanks in advance!
When you say the code doesn't work, I assume you mean the code inside the while loop doesn't execute in the 2nd example.
While loops in python have format
while BOOLEAN:
CODEwhere BOOLEAN is an expression which evaluates to either True or False. If the BOOLEAN evaluates to True, then CODE is executed. If not, CODE is skipped over.
In your second example, BOOLEAN is the expression
not validvalid has value True so,
not validis not True which evalutes to False. Therefore the CODE doesn't execute.
Evaluate valid = True; not valid in your head.
If you want 'while false' functionality, you need not. Try while not fn: instead.
The condition is the loop is actually a "pre-" condition (as opposed to post-condition "do-while" loop in, say, C). It tests the condition for each iteration including the first one.
On first iteration the condition is false, thus the loop is ended immediately.
what does the True refer to?
eg if i write:
while True:
print("hi")i know that "hi" will be repeated infinitely, but what is it thats True thats making "hi" get printed?
also, whatever it is thats True, im assuming its always true, so is that why if i typed 'while False', none of the code below would ever run?
sorry if this is a stupid question
edit: also, besides adding an if statement, is there anything else that would break this infinite while loop?
and how would i break the loop, lets say after "hi" has been printed 10 times, using an if statement or whatever else there is that can break it. thanks!
https://pastebin.com/NWkKQc1P
It's from Automate the Boring Stuff. I tried running it through the python tutor and it didn't clarify. Is the basic point that there actually isn't anything it's checking to be True, so it's an infinite loop until you trigger the `break`?
PS I figure this is something simple from much earlier that I didn't internalize. I plan to go through all the basic curriculum again to make sure there aren't any gaps in super basic knowledge.
while True means loop forever. The while statement takes an expression and executes the loop body while the expression evaluates to (boolean) "true". True always evaluates to boolean "true" and thus executes the loop body indefinitely. It's an idiom that you'll just get used to eventually! Most languages you're likely to encounter have equivalent idioms.
Note that most languages usually have some mechanism for breaking out of the loop early. In the case of Python it's the break statement in the cmd == 'e' case of the sample in your question.
my question: while WHAT is True?
While True is True.
The while loop will run as long as the conditional expression evaluates to True.
Since True always evaluates to True, the loop will run indefinitely, until something within the loop returns or breaks.
while X repeats when X equals to True so in while True it's always True. It only breaks with break statement.
In your code, you only check the value inside the while loop with if so neither you break the while loop nor you change True to False in while True.
If you want to use while:
i = 0
while i < 10:
i += 1
print(i)
Or
i = 0
while True:
if i < 10:
i += 1
print(i)
else:
break
Without while:
for i in range(10):
print(i)
while True
will make the loop run forever because "true" always evaluates to true. You can exit the loop with a break.
To achieve what you want to do, I would use
while i < 10:
print (i)
i++
I feel like it should work, but Python doesn't seem to care about my feelings. So I'm playing around with functions and I feel like I should be able to create a function that returns a boolean value that breaks a while loop in the body of the code. So here's what I've got, sadly, it doesn't seem to actually read what's being returned and just continues to ask for a username. Please, ignore my stupid usernames, I am neither smart no clever. :
def username_checker(username, users,):
"""Checks list of users to find out if username is already taken"""
if username in users:
print("\nSorry that name is already taken.")
return True
else:
print (f"\nWelcome, {username}")
return False
users = ["bbrad", "poopinajimmyhat7", "coolkid28",]
while True:
user_name = input ("\nPlease enter a username: ")
username_checker (user_name, users)