Short-circuit evaluation
Assume functions a and b return boolean values, and further, the execution of function b takes considerable resources and is to be minimised.
If we needed to compute:
x = a() and b()
Then it would be best to not compute the value of b() if the value of a() is computed as False, as the value of x can then only ever be False.
Similarly, if we needed to compute:
y = a() or b()
Then it would be best to not compute the value of b() if the value of a() is computed as true, as the value of x can then only ever be True.
Some languages will stop further computation of boolean equations as soon as the result is known, so-called short-circuit evaluation of boolean expressions
Task Description
The task is to create two functions named a and b, that take and return the same boolean value. The functions should also print their name whenever they are called. calculate and assign the values of the following equations to a variable in such a way that function b is only called when necessary:
x = a(i) and b(j) y = a(j) or b(j)
If the language does not have short-circuit evaluation, this might be achieved with nested if statements.
Python
Pythons and and or binary, infix, boolean operators will not evaluate their right-hand expression if the outcome can be determined from the value of the left-hand expression. <lang python>>>> def a(answer): print(" # Called function a(%r) -> %r" % (answer, answer)) return answer
>>> def b(answer): print(" # Called function b(%r) -> %r" % (answer, answer)) return answer
>>> for i in (False, True): for j in (False, True): print ("\nCalculating: x = a(i) and b(j)") x = a(i) and b(j) print ("Calculating: y = a(i) or b(j)") y = a(i) or b(j)
Calculating: x = a(i) and b(j)
# Called function a(False) -> False
Calculating: y = a(i) or b(j)
# Called function a(False) -> False # Called function b(False) -> False
Calculating: x = a(i) and b(j)
# Called function a(False) -> False
Calculating: y = a(i) or b(j)
# Called function a(False) -> False # Called function b(True) -> True
Calculating: x = a(i) and b(j)
# Called function a(True) -> True # Called function b(False) -> False
Calculating: y = a(i) or b(j)
# Called function a(True) -> True
Calculating: x = a(i) and b(j)
# Called function a(True) -> True # Called function b(True) -> True
Calculating: y = a(i) or b(j)
# Called function a(True) -> True</lang>
Pythons if expression can also be used to the same ends (but probably should not): <lang python>>>> for i in (False, True): for j in (False, True): print ("\nCalculating: x = a(i) and b(j) using x = b(j) if a(i) else False") x = b(j) if a(i) else False print ("Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True") y = b(j) if not a(i) else True
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(False) -> False
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(False) -> False # Called function b(False) -> False
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(False) -> False
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(False) -> False # Called function b(True) -> True
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(True) -> True # Called function b(False) -> False
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(True) -> True
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(True) -> True # Called function b(True) -> True
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(True) -> True</lang>