def pos_neg(a, b, negative):
if negative:
return (a < 0 and b < 0)
else:
return ((a < 0 == b > 0) or (a > 0 == b < 0))
so basically I tried some basic problems.
I just started out and went to https://codingbat.com/prob/p162058 to try it and don't understand why if I were to replace the '==' with 'and' it would work? Thanks.
Oh, I got it now, thanks guys. :D
(this python community is fire!!!)
Since you're learning, you might be interested in seeing that this is an equivalent function.
basically it does a bit wise & and returns true if the result is < 0
else a bit wise exclusive or and returns true if the result is < 0
def pos_neg(a , b, negative):
if negative:
return (a & b) < 0 # both sign bits set (i.e. neg), return true
else:
return (a ^ b) < 0 # different signs, return true
In both cases, it is the sign bit of the result that is of concern. If the bit is set then the value will be negative (hence the comparison to < 0)
and is a logical operator and returns true only when both the expressions we are using it on are true.
== is used for comparisons and returns true when both expressions are equal; they don't need to be true.
To give you an example False == False will return True but False and False will return False.
This "==" means equals to, belonging to Python Comparison Operators, used to compare two values.
and language keyword "and" it is for Python Logical Operators used to combine conditional statements.
You should check out this, it may solve others doubts you have.
W3Schools
All your relational operator usages (i.e. a < 0, b < 0, etc.) result to a boolean value and thus these are known as boolean expressions.
When you put a < 0, think of it like a test of: "is a less than 0, true or false?".
So, if both a and b are negative (i.e. less than zero), their expressions will return true.
So on the line return a < 0 and b < 0, replacing and with == is like saying return true == true. Without the change it'd be return true and true.
Note: This does not mean == is the same as and. == checks for equality of the left-hand side to the right-hand side (e.g. 1 == 1) and gives a true or false value depending on the result of equality. and checks for if the left-hand side results to a true statement and if the right-hand side results to a true statement in order to result to a true expression.
Related
All comparison operations in Python have the same priority, which is lower than that of any arithmetic, shifting or bitwise operation. Thus "==" and "<" have the same priority, why would the first expression in the following evaluate to True, different from the 2nd expression?
>>> -1 < 0 == False
True
>>> (-1 < 0) == False
False
I would expect both be evaluated to False. Why is it not the case?
Python has a really nice feature - chained comparison, like in math expressions, so
-1 < 0 == False
is actually a syntactic sugar for
-1 < 0 and 0 == False
under the hood.
I typed
print([1,2] and 3)
and the result is
3
How did this happen? Why is the result 3?
I guess [1,2] is considered as True. But I don't know how the backend process works.
When Boolean "and" operator is "True" it return the last/left most "True" value in expression.
>>> print(3 and 5)
5
>>> print((3 and [1,4] and [1,2] ))
[1, 2]
Boolean operator give "False" for 0, when "and" operator result is "False" it return the firstly "False" value in expression.
>>> print([1,2] and 0)
0
>>> z = False
>>> print((3 and z and 0 ))
False
Boolean Operations — and, or, not
These are the Boolean operations, ordered by ascending priority:
Operation
Result
Notes
x or y
if x is false, then y, else x
(1)
x and y
if x is false, then x, else y
(2)
not x
if x is false, then True, else False
(3)
Notes:
This is a short-circuit operator, so it only evaluates the second argument if the first one is false.
This is a short-circuit operator, so it only evaluates the second argument if the first one is true.
not has a lower priority than non-Boolean operators, so not a == b is interpreted as not (a == b), and a == not b is a syntax error.
Please check this official python doc for more information
This Article From Quora says it best:
Python uses and and or operators in a slightly different way.
It doesn’t return pure boolean values (i.e. True or False) as someone
working with a language like Java might expect.
It returns one of the operand based on conditions, and the truth value
of that operand determines the boolean value if used in a conditional
statement.
and:
Return the first Falsy value if there are any, else return the last
value in the expression.
or:
Return the first Truthy value if there are any, else return the last
value in the expression.
and is a Boolean Operator.
It returns the first operand that evaluates to False or the last one if all evaluate to True.
In this case it will return 3. If the list was empty it would return the empty list, as you can see in this example:
>>> [] and 3
[]
I just learned there are truthy and falsy values in python which are different from the normal True and False.
Can someone please explain in depth what truthy and falsy values are? Where should I use them? What is the difference between truthy and True values and falsy and False values?
We use "truthy" and "falsy" to differentiate from the bool values True and False. A "truthy" value will satisfy the check performed by if or while statements. As explained in the documentation, all values are considered "truthy" except for the following, which are "falsy":
None
False
Numbers that are numerically equal to zero, including:
0
0.0
0j
decimal.Decimal(0)
fraction.Fraction(0, 1)
Empty sequences and collections, including:
[] - an empty list
{} - an empty dict
() - an empty tuple
set() - an empty set
'' - an empty str
b'' - an empty bytes
bytearray(b'') - an empty bytearray
memoryview(b'') - an empty memoryview
an empty range, like range(0)
objects for which
obj.__bool__() returns False
obj.__len__() returns 0, given that obj.__bool__ is undefined
As the comments described, it just refers to values which are evaluated to True or False.
For instance, to see if a list is not empty, instead of checking like this:
if len(my_list) != 0:
print("Not empty!")
You can simply do this:
if my_list:
print("Not empty!")
This is because some values, such as empty lists, are considered False when evaluated for a boolean value. Non-empty lists are True.
Similarly for the integer 0, the empty string "", and so on, for False, and non-zero integers, non-empty strings, and so on, for True.
The idea of terms like "truthy" and "falsy" simply refer to those values which are considered True in cases like those described above, and those which are considered False.
For example, an empty list ([]) is considered "falsy", and a non-empty list (for example, [1]) is considered "truthy".
See also this section of the documentation.
Python determines the truthiness by applying bool() to the type, which returns True or False which is used in an expression like if or while.
Here is an example for a custom class Vector2dand it's instance returning False when the magnitude (lenght of a vector) is 0, otherwise True.
import math
class Vector2d(object):
def __init__(self, x, y):
self.x = float(x)
self.y = float(y)
def __abs__(self):
return math.hypot(self.x, self.y)
def __bool__(self):
return bool(abs(self))
a = Vector2d(0,0)
print(bool(a)) #False
b = Vector2d(10,0)
print(bool(b)) #True
Note: If we wouldn't have defined __bool__ it would always return True, as instances of a user-defined class are considered truthy by default.
Example from the book: "Fluent in Python, clear, concise and effective programming"
Truthy values refer to the objects used in a boolean context and not so much the boolean value that returns true or false.Take these as an example:
>>> bool([])
False
>>> bool([1])
True
>>> bool('')
False
>>> bool('hello')
True
Where should you use Truthy or Falsy values ?
These are syntactic sugar, so you can always avoid them, but using them can make your code more readable and make you more efficient.
Moreover, you will find them in many code examples, whether in python or not, because it is considered good practice.
As mentioned in the other answers, you can use them in if tests and while loops. Here are two other examples in python 3 with default values combined with or, s being a string variable. You will extend to similar situations as well.
Without truthy
if len(s) > 0:
print(s)
else:
print('Default value')
with truthy it is more concise:
print(s or 'Default value')
In python 3.8, we can take advantage of the assignment expression :=
without truthy
if len(s) == 0:
s = 'Default value'
do_something(s)
with truthy it is shorter too
s or (s := 'Default value')
do_something(s)
or even shorter,
do_something(s or (s := 'Default value'))
Without the assignment expression, one can do
s = s or 'Default value'
do_something(s)
but not shorter. Some people find the s =... line unsatisfactory because it corresponds to
if len(s)>0:
s = s # HERE is an extra useless assignment
else:
s = "Default value"
nevertheless you can adhere to this coding style if you feel comfortable with it.
Any object in Python can be tested for its truth value. It can be used in an if or while condition or as operand of the Boolean operations.
The following values are considered False:
None
False
zero of any numeric type, for example, 0, 0L, 0.0, 0j.
any empty sequence, for example, '', (), [].
any empty mapping, for example, {}.
instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.
All other values are considered True -- thus objects of many types are always true.
Operations and built-in functions that have a Boolean result always return 0 or False for false and 1 or True for true, unless otherwise stated.
In case of if (!id) {}
!expr returns false if its single operand can be converted to true; otherwise, returns true.
If a value can be converted to true, the value is so-called truthy. If a value can be converted to false, the value is so-called falsy.
Examples of expressions that can be converted to false are:
null;
NaN;
0;
empty string ("" or '' or ``);
undefined.
Even though the ! operator can be used with operands that are not Boolean values, it can still be considered a boolean operator since its return value can always be converted to a boolean primitive. To explicitly convert its return value (or any expression in general) to the corresponding boolean value, use a double NOT operator or the Boolean constructor.
Example:
n1 = !null // !t returns true
n2 = !NaN // !f returns true
n3 = !'' // !f returns true
n4 = !'Cat' // !t returns false
While in case of if (id != null) {} it will only check if the value in id is not equal to null
reference https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_NOT
Falsy means something empty like empty list,tuple, as any datatype having empty values or None.
Truthy means :
Except are Truthy
I was solving some beginner problem at "codebat". Then I came up with this problem. I couldn't solve this so I viewed the solution. They showed me this.
Can anyone explain the code?
Problem:Given 2 int values, return True if one is negative and one is positive. Except if the parameter "negative" is True, then return True only if both are negative.
def pos_neg(a, b, negative):
if negative:
return (a < 0 and b <0)
else:
return((a < 0 and b >0) or (a > 0 and b < 0))
The condition if negative checks whether negative is true or false:
If negative is true, then the function returns the result of (a < 0 and b <0), which is true if and only if both values are negative.
If negative is false, then the function returns the result of (a < 0 and b >0) or (a > 0 and b < 0), which is true if one is negative and the other positive.
i dont understand why the code bellow doesnt work properly. If both variables a and b < 0 it should print that both numbers are negative,else the last message. But it just dont work so, what am i doing wrong? please help!
import random
while True:
input()
a=random.randint(-9,9)
b=random.randint(-9,9)
print(a,b)
if a and b < 0:
print("2 negative numbers:",a,b)
else:
print("one or both of the numbers are positive!")
I'm running this on python 3.4.
I think you're a little confused about how operators distribute.
When you have
if a and b < 0
it doesn't mean
if (both a and b) < 0
but instead
if (a) and (b < 0)
which is equivalent to
if (a != 0) and (b < 0)
since "numeric zero of all types ... evaluates to false" (see the reference on booleans on docs.python.org)
Instead, you want
if a < 0 and b < 0
which will tell you if both a and b are less than zero.
Evaluating Both Operands will resolve the issue. Here both Operands are expressions which results in true or false, so if both result in true; you will get your required result.
if ((a < 0) and (b < 0)):
You are evaluating just a, not it's relation to 0:
if a < 0 and b < 0:
This:
a and b < 0:
Is equivalent to this:
(a) and (b < 0):
(a) is False when a equals 0 and True otherwise. Therefore, due to short-circuiting b < 0 isn't even evaluated.
As a fix you may use all method:
all(i < 0 for i in (a, b))