Find out which condition breaks a logical and expression

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I want to find an elegant way to get the component of the logical and expression below that is responsible if the if block is not being executed.

if test1(value) and test2(value) and test3(value):
   print 'Yeeaah'
else:
   print 'Oh, no!', 'Who is the first function that return false?'

In case the else block is entered, how do I find out whether test1, test2 or test3 is responsible by returning the first falsy value?

Salvo.

You can use next and a generator expression:

breaker = next((test.__name__ for test in (test1, test2, test3) if not test(value)), None)

Demo:

>>> def test1(value): return True
>>> def test2(value): return False
>>> def test3(value): return True
>>> value = '_' # irrelevant for this demo
>>>
>>> tests = (test1, test2, test3)
>>> breaker = next((test.__name__ for test in tests if not test(value)), None)
>>> breaker
'test2'
>>> if not breaker:
...:    print('Yeeaah')
...:else:
...:    print('Oh no!')
...:    
Oh no!

Note that test3 is never called in this code.

(Super corner case: use if breaker is not None over if not breaker if for reasons I cannot fathom pranksters reassigned the function's __name__ attribute to ''.)

~edit~

In case you want to know whether the first, second, or n'th test returned something falsy, you can use a similar generator expression with enumerate.

>>> breaker = next((i for i, test in enumerate(tests, 1) if not test(value)), None)
>>> breaker
2

If you want to count from zero, use enumerate(tests) and check if breaker is not None for entering the if block (because 0 is falsy like None).

Conditionals and logic | Think Java, Relational operators are used to check conditions like whether two values are equal, Negating a logical expression is the same as negating each term and The purpose of this exercise is to take a problem and break it into smaller  Condition, in logic, a stipulation, or provision, that needs to be satisfied; also, something that must exist or be the case or happen in order for something else to do so (as in “the will to live is a condition for survival”). In logic, a sentence or proposition of the form “If A then B” [in

We could store that test values in list and then check if they all are True and if not print index of first value that is False.

def test1(x):
    return True
def test2(x):
    return False
def test3(x):
    return True

value = 'a'
statements = [test1(value), test2(value), test3(value)]

if all(statements):
   print('Yeeaah')
else:
   print('Oh, no! Function that first return False is {}'.format(statements.index(False)))

Output:

Oh, no! Function that first return False is 1

Chapter 3: Logicals and Loops, To write useful programs, we almost always need to check conditions and react accordingly. The condition in parentheses can be any boolean expression. Even worse, someone (i.e., a hacker) may intentionally try to break into your  Called Logical AND operator. If both the operands are non-zero, then the condition becomes true. (A && B) is false. || Called Logical OR Operator. If any of the two operands is non-zero, then the condition becomes true. (A || B) is true.! Called Logical NOT Operator. It is used to reverse the logical state of its operand.

Split it three-ways:

Using python3 syntax:

b1 = test1(value)
b2 = test2(value)
b3 = test3(value)

if b1 and b2 and b3:
    print("Yeah")
else:
    print("Nope! 1: {}, 2: {}, 3: {}".format(b1, b2, b3))

Logical Condition Criteria, Returns a vector of logical values for each item in vector z if both conditions are Beware using & and && in more complex expressions for the meanings will vary. check to exit the loop i <- i + 1 #Increasing the counter if( i == 30) break  Therefore, the result of logical expression a .OR. bis .FALSE.if and only if both operands aandbare .FALSE.. In all other cases, the result is always.TRUE. In other words, if one of the two operands of the .OR.operator is .TRUE., the result is .TRUE. The following is the truth table of .EQV.

[PDF] Indefinite Loops and Boolean Expressions, For information on comparing arrays in a logical expression, see Processing of  http://adampanagos.org Starting with a "complicated" logical expression, we apply logical law's (.e.g. DeMorgan's law, Tautology law, etc) to simplify the logical

while loop to repeat when condition is true, not as obvious. • impossible to determine before the loop begins statements in the body, and go back to step 1 next statement true false condition body of the loop test When we encounter the sentinel, we break out of the loop. • example​  point of view is that the expression (false & expr2) might be optimised at JIT-time and the second expression will not be evaluated. In my opinion this would break the contract of the C# language if the JIT compiler were to remove the call to expr2 entirely. I'm not saying that it's good programming practice to depend on the side effects of expr2's

While loop with multiple conditions - MATLAB Answers, This MATLAB function evaluates an expression, and repeats the execution of a group of statements in a loop Then, exit the loop using a break statement. Logical operators are used in portions of programs known as flow control (for example if . . . else . . . end blocks). Vector or matrix logical operations return vector or matrix output and that flow control blocks require scalar logical expressions.

Comments
  • This seems to me the most pythonic answer, and most easily generalized
  • Maybe at least some text would benefit your answer
  • Note that this does evaluate all the tests, while the chained and operations are lazy and short circuit. So in theory if the tests have side effects, this code is not strictly equivalent.