Related
def play_game(word_list):
hand = deal_hand(HAND_SIZE) # random init
while True:
cmd = raw_input('Enter n to deal a new hand, r to replay the last hand, or e to end game: ')
if cmd == 'n':
hand = deal_hand(HAND_SIZE)
play_hand(hand.copy(), word_list)
print
elif cmd == 'r':
play_hand(hand.copy(), word_list)
print
elif cmd == 'e':
break
else:
print "Invalid command."
While WHAT is True?
I reckon saying 'while true' is shorthand, but for what? While the variable 'hand' is being assigned a value? And what if the variable 'hand' is not being assigned a value?
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 True is true -- ie always. This is an infinite loop
Note the important distinction here between True which is a keyword in the language denoting a constant value of a particular type, and 'true' which is a mathematical concept.
my question: while WHAT is True?
Everything inside the () of the while statement is going to be evaluated as a boolean. Meaning it gets converted into either true or false.
Consider in the statement while(6 > 5)
It first evaluates the expression 6 > 5 which is true so is the same as saying while(true)
Anything that is not FALSE, 0, an emptry string "", null, or undefined is likely to be evaluated to true.
When I first started programming I used to do things like if(foo == true), I didn't realise that was virtually the same thing as if(foo).
So when you say while(true) its like are saying while(true == true)
So to answer you question: While TRUE is True.
In this context, I suppose it could be interpreted as
do
...
while cmd != 'e'
True is always True, so while True will loop forever.
The while keyword takes an expression, and loops while the expression is true. True is an expression that is always true.
As a possibly clarifying example, consider the following:
a = 1
result = a == 1
Here, a == 1 will return True, and hence put True into result. Hence,
a = 1
while a == 1:
...
is equivalent to:
while True:
...
provided you don't alter the value of a inside the while loop.
Formally, True is a Python built-in constant of bool type.
You can use Boolean operations on bool types (at the interactive python prompt for example) and convert numbers into bool types:
>>> print not True
False
>>> print not False
True
>>> print True or False
True
>>> print True and False
False
>>> a=bool(9)
>>> print a
True
>>> b=bool(0)
>>> print b
False
>>> b=bool(0.000000000000000000000000000000000001)
>>> print b
True
And there are "gotcha's" potentially with what you see and what the Python compiler sees:
>>> n=0
>>> print bool(n)
False
>>> n='0'
>>> print bool(n)
True
>>> n=0.0
>>> print bool(n)
False
>>> n="0.0"
>>> print bool(n)
True
As a hint of how Python stores bool types internally, you can cast bool types to integers and True will come out to be 1 and False 0:
>>> print True+0
1
>>> print True+1
2
>>> print False+0
0
>>> print False+1
1
In fact, Python bool type is a subclass of Python's int type:
>>> type(True)
<type 'bool'>
>>> isinstance(True, int)
True
The more important part of your question is "What is while True?" is 'what is True', and an important corollary: What is false?
First, for every language you are learning, learn what the language considers 'truthy' and 'falsey'. Python considers Truth slightly differently than Perl Truth for example. Other languages have slightly different concepts of true / false. Know what your language considers to be True and False for different operations and flow control to avoid many headaches later!
There are many algorithms where you want to process something until you find what you are looking for. Hence the infinite loop or indefinite loop. Each language tend to have its own idiom for these constructs. Here are common C infinite loops, which also work for Perl:
for(;;) { /* loop until break */ }
/* or */
while (1) {
return if (function(arg) > 3);
}
The while True: form is common in Python for indefinite loops with some way of breaking out of the loop. Learn Python flow control to understand how you break out of while True loops. Unlike most languages, for example, Python can have an else clause on a loop. There is an example in the last link.
A while loop takes a conditional argument (meaning something that is generally either true or false, or can be interpreted as such), and only executes while the condition yields True.
As for while True? Well, the simplest true conditional is True itself! So this is an infinite loop, usually good in a game that requires lots of looping. (More common from my perspective, though, is to set some sort of "done" variable to false and then making that true to end the game, and the loop would look more like while not done: or whatever.)
While most of these answers are correct to varying degrees, none of them are as succinct as I would like.
Put simply, using while True: is just a way of running a loop that will continue to run until you explicitly break out of it using break or return. Since True will always evaluate to True, you have to force the loop to end when you want it to.
while True:
# do stuff
if some_condition:
break
# do more stuff - code here WILL NOT execute when `if some_condition:` evaluates to True
While normally a loop would be set to run until the while condition is false, or it reaches a predefined end point:
do_next = True
while do_next:
# do stuff
if some_condition:
do_next = False
# do more stuff - code here WILL execute even when `if some_condition:` evaluates to True
Those two code chunks effectively do the same thing
If the condition your loop evaluates against is possibly a value not directly in your control, such as a user input value, then validating the data and explicitly breaking out of the loop is usually necessary, so you'd want to do it with either method.
The while True format is more pythonic since you know that break is breaking the loop at that exact point, whereas do_next = False could do more stuff before the next evaluation of do_next.
In some languages True is just and alias for the number. You can learn more why this is by reading more about boolean logic.
while True mean infinite loop, this usually use by long process.
you can change
while True:
with
while 1:
To answer your question directly: while the loop condition is True. Which it always is, in this particular bit of code.
while loops continue to loop until the condition is false. For instance (pseudocode):
i = 0
while i < 10
i++
With each iteration of the loop, i will be incremented by 1, until it is 10. At that point, the condition i < 10 is no longer true, and the loop will complete.
Since the condition in while True is explicitly and always true, the loop will never end (until it is broken out of some other way, usually by a construct like break within the loop body).
Nothing evaluates to True faster than True. So, it is good if you use while True instead of while 1==1 etc.
while True:
...
means infinite loop.
The while statement is often used of a finite loop. But using the constant 'True' guarantees the repetition of the while statement without the need to control the loop (setting a boolean value inside the iteration for example), unless you want to break it.
In fact
True == (1 == 1)
While True means loop will run infinitely is no condition is mentioned inside the while loop that breaks it.
You can break the code using 'break' or 'return'
>>> a = ['foo', 'bar', 'baz']
>>> while True:
... if not a:
... break
... print(a.pop(-1))
...
baz
bar
foo
Code copied from the realpython.com
How to use while True in Python?
# Python program to demonstrate
# while loop with True
while True:
pass
If we run the above code then this loop will run infinite number of times. To come out of this loop we will use the break statement explicitly.
With Break Statement
weekSalary = 0
dayOfWeek = 1
week = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
while(True):
if(week[dayOfWeek] == "Sunday"):
print("Week Over, Its holiday!!")
break
weekSalary += 2000
dayOfWeek += 1
print(str(weekSalary))
With Return Statement
Since True always evaluates to True , the loop will run indefinitely, until something within the loop return.
class Solution:
def minEatingSpeed(self, piles: List[int], h: int) -> int:
k = 1
while True:
total_time = 0
for i in piles:
total_time += ceil(i / k)
if total_time > h:
k += 1
else:
return k
Anything can be taken as True until the opposite is presented. This is the way duality works. It is a way that opposites are compared. Black can be True until white at which point it is False. Black can also be False until white at which point it is True. It is not a state but a comparison of opposite states. If either is True the other is wrong. True does not mean it is correct or is accepted. It is a state where the opposite is always False. It is duality.
I have an if statement has many conditions, for example:
if(0 > 1 or 9 < 10 or 2 == 1):
print('Hello World!')
so i wanna know which is the right condition that let the if statement continues to print hello world? "without using another if statement or elif"
In my code I have lots of conditions so it's difficult to use a lot of else statements to just know what is the right condition was.
In general, it's impossible - each condition is evaluated, we can only get a result.
However, if instead of ors, we have them stored* in a list like this:
conditions = [0>1, 9<10, 2==1] # it gets evaluated here!*
if any(conditions):
print('Hello World!')
we could get the index of the True element by doing conditions.index(True).
[*] But be aware that conditions doesn't consist of pure conditions but of Trues and Falses because they got evaluated. That's why I said it's impossible to get the condition itself.
I don't know why you would ever want to use this but okay...
You need to return a value which has a special __bool__ so I would define a class.
The class will have one instance variable, index to indecate the first True condition, or None if there's no True condition.
The __bool__ function then only needs to check whether index is None:
class Any:
def __init__(self, *conditions):
self.index = None
for i, cond in enumerate(conditions):
if cond:
self.index = i
break
def __bool__(self):
return self.index is not None
Example usage:
if o := Any(0 > 1, 9 < 10, 2 == 1):
print(f'The index of the first True condition is {o.index}')
For hard coded conditions like in your example, a good IDE should have an indicator and propose that you simplify the condition.
If you have variables in the condition, this will of course not be possible. In such a case, I would refactor the code and introduce additional semantics by using a variable name for the individual boolean parts of the condition.
In PyCharm, the shortcut Ctrl+Alt+V extracts a condition into a variable.
A more realistic example (before):
class Customer:
def __init__(self, age, totalsales, paymenttype):
self.age = age
self.totalsales = totalsales
self.paymenttype = paymenttype
c = Customer(21, 3000, 2)
if c.age > 18 or c.totalsales > 5000 or c.paymenttype == 1:
print('Hello World!')
After:
c = Customer(21, 3000, 2)
is_adult = c.age > 18
is_vip = c.totalsales > 5000
is_payed_in_advance = c.paymenttype == 1
if is_adult or is_vip or is_payed_in_advance:
print('Hello World!')
When you reach the if-statement, you can inspect each part of the condition in the debugger.
Note that this may change the behavior of your program, because with the changed code, each condition is evaluated, whereas short circuiting may have been applied before. However, I never ran into a situation where this caused a problem.
Chained boolean expressions will be evaluated from left to right. If one of the chained statements is evaluated as being True, the remaining conditions will not be checked.
Assuming second_condition is fulfilled and hence will be evaluated as True, the following pseudo-code snipped would evaluate first_condition as False and then enter the if statement because of second_condition being True. However, third_condition will not be checked as another check before was already evaluated as True and thus the complete statement will become True:
if (first_condition or second_condition or third_condition):
pass
Knowing which condition was evaluated as True is not possible with the approach shown above. Therefore, I would suggest rewriting your checks as follows:
def handle_true(condition):
pass
if first_condition:
handle_true('first')
elif second_condition:
handle_true('second')
elif third_condition:
handle_true('third')
else:
pass
The if/elif will be evaluated in the same way as your chained or expression. If one condition fails, the next will be checked. If one of the given conditions is evaluated as True the branch will be entered. If none of the given conditions is fulfilled, the default else will be entered.
Combining this with the small helper function handle_true() should do the trick and not only provide a way to check which condition fired, but also provide a single place for handling any True condition.
I think that a good option will be to create a list of condition and to check you the item of your list in a loop.
cond=[True, False, True, 5>10,True,False,1==1,3<-1,'a' == 'a'] # assume this is your list
for i in range(len(cond)):
if cond[i]:
print(i) # will return the Item adress correspending to True
You can do:
print(0 > 1) print(9 < 10)
It will print true or false
Why only if statement is executed & not else statement if we write an if-else with if having constant value. For example this code in python
x=5
if 5:
print("hello 5")
else:
print("bye")
Also the point to be noted is that in second line even if I replace 5 with 500 or any number, if statement will be only executed.Can anyone please explain.
You have to apply some condition to execute the if-else statement such as (if x==5) or (if x==500). In this case, if the condition is justified then it will follow if statement else it will follow else statement. In your scenario, there is no condition to check with a constant number so it will be automatically converted to true.
Python treats all integers greater than 0 as true. Mostly, you'd use this just as a binary choice, checking if an integer is 1. In this case 5 is being found to be true, which means else is not called, as the if was activated.
if 0:
print("Hello")
will not print hello because 0 is false.
if-else requires a boolean response. Any number(positive/negative) other than 0 implies True.
Try:
if 5:
print("hello 5")
else:
print("bye")
output: hello 5
#
if 0:
print("hello 5")
else:
print("bye")
output: bye
Perhaps an additional reason. It used to be the case that python, like c, had no bool type. So instead of saying
boolean = False # or True
you would say
boolean = 0 # or 1 or any other int
Python later added the bool type with True and False constants, but they are really just wrappers of the ints 1 and 0 (infact bool is a subclass of int). This can be seen by
print(True + True)
print(False * 123)
print(True == 1)
print(False == 0)
Think of if-else as a fork.
You go either this way (statements with the if) or that way (statements with else). The decision which way to turn is done by the condition-expression following the keyword if.
If the condition-expression evaluates to
any non-zero value the if-branch will be taken
zero the else-branch will be taken
varA = 1
varB = 2
Code w/ Correct Result:
if type(varA) == type('a') or type(varB) == type('a'):
print "string involved (either varA or varB is a string)"
else:
print "varA and varB are not strings"
Code w/ Incorrect Result:
if type(varA) or type(varB) == type('a'):
print "string involved (either varA or varB is a string)"
else:
print "varA and varB are not strings"
Why exactly does the 2nd set of code not return the expected result (i.e. "varA and varB are not strings")? What is the step-by-step breakdown of what Python is doing with the 2nd set of code? I found a similar question had already been answered but did not entirely understand the explanation. Python: If-else statements.
In the second code snippet, the condition of the if-statement is being interpreted by Python like this:
if (type(varA)) or (type(varB) == type('a')):
Moreover, it will always evaluate to True.
This is because, no matter what the value of varA is, type(varA) evaluates to True:
>>> varA = 'a'
>>> bool(type(varA))
True
>>> varA = False
>>> bool(type(varA))
True
>>>
In fact, since Python's logical operators short-circuit (stop evaluating as soon as possible), the type(varB) == type('a') part of the condition will never even be evaluated.
On a separate note, you should be using is to compare types:
if type(varA) is str or type(varB) is str:
or, you can use isinstance:
if isinstance(varA, str) or isinstance(varB, str):
Your second example does not work because it parses as
if (type(varA)) or (type(varB) == type('a')):
and type(varA) will always be a class type which is considered True, so the whole expression will be True
There are better ways to do this
if any(isinstance(v, str) for v in (varA, varB)):
any takes an iterable and evaluates to True if anything in the iterable is true.
isinstance checks to see if the first argument "is a" second argument. Placing the generator expression inside of any reads as "if any v in (varA, varB) is a string): ... "
>>> var = 1
>>> isinstance(var, str) # var is an int, not a str
False
>>> isinstance(var, int)
True
>>> isinstance('a', int)
False
>>> isinstance('a', str) # 'a' is a str
True
iCodez is absolutely correct, but if you really want to do something along the lines of "list all elements and check if one of them is a string":
if str in map(type, [varA, varB]):
print "string involved"
Because in the second case you are not comparing both variables. Any integer above 0 would return True so you are not comparing types here.
if type(varA):
will always be True, because varA is equal to 1. You never even get to the second part of the condition.
def play_game(word_list):
hand = deal_hand(HAND_SIZE) # random init
while True:
cmd = raw_input('Enter n to deal a new hand, r to replay the last hand, or e to end game: ')
if cmd == 'n':
hand = deal_hand(HAND_SIZE)
play_hand(hand.copy(), word_list)
print
elif cmd == 'r':
play_hand(hand.copy(), word_list)
print
elif cmd == 'e':
break
else:
print "Invalid command."
While WHAT is True?
I reckon saying 'while true' is shorthand, but for what? While the variable 'hand' is being assigned a value? And what if the variable 'hand' is not being assigned a value?
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 True is true -- ie always. This is an infinite loop
Note the important distinction here between True which is a keyword in the language denoting a constant value of a particular type, and 'true' which is a mathematical concept.
my question: while WHAT is True?
Everything inside the () of the while statement is going to be evaluated as a boolean. Meaning it gets converted into either true or false.
Consider in the statement while(6 > 5)
It first evaluates the expression 6 > 5 which is true so is the same as saying while(true)
Anything that is not FALSE, 0, an emptry string "", null, or undefined is likely to be evaluated to true.
When I first started programming I used to do things like if(foo == true), I didn't realise that was virtually the same thing as if(foo).
So when you say while(true) its like are saying while(true == true)
So to answer you question: While TRUE is True.
In this context, I suppose it could be interpreted as
do
...
while cmd != 'e'
True is always True, so while True will loop forever.
The while keyword takes an expression, and loops while the expression is true. True is an expression that is always true.
As a possibly clarifying example, consider the following:
a = 1
result = a == 1
Here, a == 1 will return True, and hence put True into result. Hence,
a = 1
while a == 1:
...
is equivalent to:
while True:
...
provided you don't alter the value of a inside the while loop.
Formally, True is a Python built-in constant of bool type.
You can use Boolean operations on bool types (at the interactive python prompt for example) and convert numbers into bool types:
>>> print not True
False
>>> print not False
True
>>> print True or False
True
>>> print True and False
False
>>> a=bool(9)
>>> print a
True
>>> b=bool(0)
>>> print b
False
>>> b=bool(0.000000000000000000000000000000000001)
>>> print b
True
And there are "gotcha's" potentially with what you see and what the Python compiler sees:
>>> n=0
>>> print bool(n)
False
>>> n='0'
>>> print bool(n)
True
>>> n=0.0
>>> print bool(n)
False
>>> n="0.0"
>>> print bool(n)
True
As a hint of how Python stores bool types internally, you can cast bool types to integers and True will come out to be 1 and False 0:
>>> print True+0
1
>>> print True+1
2
>>> print False+0
0
>>> print False+1
1
In fact, Python bool type is a subclass of Python's int type:
>>> type(True)
<type 'bool'>
>>> isinstance(True, int)
True
The more important part of your question is "What is while True?" is 'what is True', and an important corollary: What is false?
First, for every language you are learning, learn what the language considers 'truthy' and 'falsey'. Python considers Truth slightly differently than Perl Truth for example. Other languages have slightly different concepts of true / false. Know what your language considers to be True and False for different operations and flow control to avoid many headaches later!
There are many algorithms where you want to process something until you find what you are looking for. Hence the infinite loop or indefinite loop. Each language tend to have its own idiom for these constructs. Here are common C infinite loops, which also work for Perl:
for(;;) { /* loop until break */ }
/* or */
while (1) {
return if (function(arg) > 3);
}
The while True: form is common in Python for indefinite loops with some way of breaking out of the loop. Learn Python flow control to understand how you break out of while True loops. Unlike most languages, for example, Python can have an else clause on a loop. There is an example in the last link.
A while loop takes a conditional argument (meaning something that is generally either true or false, or can be interpreted as such), and only executes while the condition yields True.
As for while True? Well, the simplest true conditional is True itself! So this is an infinite loop, usually good in a game that requires lots of looping. (More common from my perspective, though, is to set some sort of "done" variable to false and then making that true to end the game, and the loop would look more like while not done: or whatever.)
While most of these answers are correct to varying degrees, none of them are as succinct as I would like.
Put simply, using while True: is just a way of running a loop that will continue to run until you explicitly break out of it using break or return. Since True will always evaluate to True, you have to force the loop to end when you want it to.
while True:
# do stuff
if some_condition:
break
# do more stuff - code here WILL NOT execute when `if some_condition:` evaluates to True
While normally a loop would be set to run until the while condition is false, or it reaches a predefined end point:
do_next = True
while do_next:
# do stuff
if some_condition:
do_next = False
# do more stuff - code here WILL execute even when `if some_condition:` evaluates to True
Those two code chunks effectively do the same thing
If the condition your loop evaluates against is possibly a value not directly in your control, such as a user input value, then validating the data and explicitly breaking out of the loop is usually necessary, so you'd want to do it with either method.
The while True format is more pythonic since you know that break is breaking the loop at that exact point, whereas do_next = False could do more stuff before the next evaluation of do_next.
In some languages True is just and alias for the number. You can learn more why this is by reading more about boolean logic.
while True mean infinite loop, this usually use by long process.
you can change
while True:
with
while 1:
To answer your question directly: while the loop condition is True. Which it always is, in this particular bit of code.
while loops continue to loop until the condition is false. For instance (pseudocode):
i = 0
while i < 10
i++
With each iteration of the loop, i will be incremented by 1, until it is 10. At that point, the condition i < 10 is no longer true, and the loop will complete.
Since the condition in while True is explicitly and always true, the loop will never end (until it is broken out of some other way, usually by a construct like break within the loop body).
Nothing evaluates to True faster than True. So, it is good if you use while True instead of while 1==1 etc.
while True:
...
means infinite loop.
The while statement is often used of a finite loop. But using the constant 'True' guarantees the repetition of the while statement without the need to control the loop (setting a boolean value inside the iteration for example), unless you want to break it.
In fact
True == (1 == 1)
While True means loop will run infinitely is no condition is mentioned inside the while loop that breaks it.
You can break the code using 'break' or 'return'
>>> a = ['foo', 'bar', 'baz']
>>> while True:
... if not a:
... break
... print(a.pop(-1))
...
baz
bar
foo
Code copied from the realpython.com
How to use while True in Python?
# Python program to demonstrate
# while loop with True
while True:
pass
If we run the above code then this loop will run infinite number of times. To come out of this loop we will use the break statement explicitly.
With Break Statement
weekSalary = 0
dayOfWeek = 1
week = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
while(True):
if(week[dayOfWeek] == "Sunday"):
print("Week Over, Its holiday!!")
break
weekSalary += 2000
dayOfWeek += 1
print(str(weekSalary))
With Return Statement
Since True always evaluates to True , the loop will run indefinitely, until something within the loop return.
class Solution:
def minEatingSpeed(self, piles: List[int], h: int) -> int:
k = 1
while True:
total_time = 0
for i in piles:
total_time += ceil(i / k)
if total_time > h:
k += 1
else:
return k
Anything can be taken as True until the opposite is presented. This is the way duality works. It is a way that opposites are compared. Black can be True until white at which point it is False. Black can also be False until white at which point it is True. It is not a state but a comparison of opposite states. If either is True the other is wrong. True does not mean it is correct or is accepted. It is a state where the opposite is always False. It is duality.