Interaction operators in sequence diagrams

In sequence diagrams, an interaction operator defines the semantics of a combined fragment and determines how to use the interaction operands in the combined fragment.
The following table lists the three most commonly used interaction operators:
Interaction operator Description
Alternative (alt) An alternative interaction operator represents the logic equivalent of an if-then-else statement. Only one of the offered alternatives runs on any pass through the interaction. However, as is true of any operand, the selected operand in the alternative structure runs only if the guard condition tests true. If there is no guard, the operand always runs when it is selected. The else clause of the alternative combined fragment runs when no other option is selected.
Option (opt) An option interaction operator represents the logic equivalent of an if statement. To be used, the guard condition must be satisfied. If the guard condition fails, the behavior is ignored. The graphic representation of an option combined fragment looks like an alternative that offers only one alternative.
Loop A loop interaction operator indicates that the interaction fragment runs repeatedly. The number of times the fragment runs is determined by the minint and maxint parameters of the operator. The syntax of the loop operator is loop (minint, maxint) where maxint can also be infinity (*). After the minimum number of iterations is satisfied, a Boolean expression is tested on each pass. When the Boolean expression tests false, the loop ends.

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