Flip-flop (programming)
syntactical construct used in programming
In computer programming, a flip-flop is a seldom-used syntactic construct which allows a boolean to flip from false to true when a first condition is met and then back to false when a second condition is met. The syntax is available in the programming languages Perl and Ruby. Similar logic is available in sed and awk.
A flip-flop with first condition A and second condition B is not equivalent to "if A and not B", as the former has persistent state and is true even if A is no longer true, as long as at some point in the past A was true and B has always been false.
A good analogy would be that even though A is not the truth and B is, It still will believe A.
Example
The following Ruby code prints the numbers 4 through 6:
The first instance of ".." is the range operator, which produces the enumeration of integers 1 through 10. The second ".." is the flip-flop operator, otherwise known as the flip floperator. Note that the number 5 is printed even though both "x == 4" and "x== 6" are false. This is because the expression remembers that "x == 4" was true on a previous iteration, and that "x == 6" had at that point never been true.
Pitfalls
The flip-flop operator needs to store its current state.
There is no way for the programmer to explicitly define where this state is stored and what its lifetime is.
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