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Conditional Logic

Conditional logic makes a program tick.

Sections of code can be skipped or repeated multiple times. Code can be executed in every repetition of a loop or every several repetitions. Complex decision trees can be created which execute code only under very specific conditions. Lasso supports a variety of operations for performing conditional logic.

  • If/Else Conditional tests one or more conditions selectively executing code

  • Match Conditional compares a value to one or more choices selectively executing code

  • Looping repeatedly executing based on a condition

If/Else Conditional

An if/else conditional is a special construct that allows code to be executed only if a particular expression results to true. The if/else conditional differs from the conditional operator in that it permits multiple condition tests as well as multiple lines of code within the condition bodies (the conditional operator allows only single expressions). The if/else conditional supports one default "else" which will execute if none of the condition expressions are true.

The if/else conditional can take two forms. The following example shows the first form. The "..." in the example shows where the body expressions for that particular condition would occur.

   if (expression)
  ...
  else (expression)
  ...
  else
  ...
  /if

Each expression is evaluated in order and the first value which is not false will have its corresponding condition body executed. Once completed, no further conditions will be tested and execution will resume at the end of the if/else conditional.

The second form operates like the first, but permits the if/else to be used with the association/givenBlock syntax.

  if (expression) => {
  ...
  else (expression)
  ...
  else
  ...
  }

Either form is accepted. An if/else conditional produces no value.

Match Conditional

A match conditional allows code to be selectively executed based upon the logical equivalence of two or more objects. Match conditionals are given an initial test value and a series of case values and condition bodies. The initial value is tested against each case value using onCompare. The first case value that matches the initial test value will have its condition body executed. Each case can have more than one value to be tested against. If no case values match, then the default case, if present, has its condition body executed.

Like the if/else conditional, a match conditional has two forms. The following example shows the first form with several case values and a default case.

  match (expression)
  case (c1, c2)
  ...
  case(c3)
  ...
  case
  ...
  /match

The second form uses the association/givenBlock syntax. Either form is accepted.

  match (expression) => {
  case (c1, c2)
  ...
  case(c3)
  ...
  case
  ...
  }

Looping

Lasso offers several operations that loop, executing a body of code repeatedly, based upon some criteria. This criteria can be a boolean true/false expression, a number counting to a pre-defined point, or the count of the number of elements in a composite object. Each method of looping supports skipping to the top of the next iteration, aborting the loop process entirely and retrieving the current count of the number of loops that have occurred. A looping operation does not produce a value.

Each of these looping operations support the two forms shown for if/else and match. Most examples are shown in both froms. In each example, "..." is used to indicate where body code would be placed.

While Loop

A while loop executes its body as long as its test expression is true. The test expression is evaluated before the beginning of each loop.

  while (expression)
  ...
  /while

  while (expression) => {
  ...
  }

Counting Loop

A counting loop loops from one integer number to another, either counting up or down each iteration, until the counter reaches the end value. The most common usage of a counting loop is to give it a number indicating how many times it is to execute its body. Other usages involve giving the counting loop a specific beginning number, a specific ending number, and an increment value, by which the counter will be incremented for each iteration.

In the following example, the body will be executed 5 times.

  loop(5)
  ...
  /loop

  loop(5) => {
  ...
  }

Counting loops also support providing an explicit beginning and an ending value for the counter. This is done by providing the to, from and by parameters, or by providing -to, -from and -by keyword parameters. Either method is accepted.

  loop(5, -10, 10)
  ...
  /loop  loop to 5 starting from -10 incrementing by 10

  loop(-to=5, -from= -10, -by=10)
  ...
  /loop  loop to 5 starting from -10 incrementing by 10

In the case of using integer parameters, the order is significant. In the case of using keywords, either the -from or -by may be omitted and all keywords may be supplied in any order.

Iterate

An iterate loop is applied to objects that contain other objects, such as arrays or maps. Iterate will execute the body once for each element contained in such an object. Iterate makes the individual elements available through the loop_value method. When iterating objects that store their elements associatively as keys and values, iterate makes the key value available through the loop_key method.

The following example creates a staticarray and iterates its contents.

  local(lv = staticarray(2, 4, 6, 8, 10))

  iterate(#lv)
  loop_value the current value from #lv
  /iterate

  iterate(#lv) => {
  ...
  }

Loop_abort

The loop_abort method can be used within the body of any of the looping operations mentioned in this chapter. When loop_abort is called, all loopings will cease and execution will begin following the looping operation.

Loop_continue

The loop_continue method can be used within the body of a looping operation to cause the current loop to cease executing. Looping begins again at the top with the testing of the loop condition.

Loop_count

All of the loop operations keep track of the current loop number. The loop_count method can be called to retrieve this number. For while and iterate, the loop count always begins with 1 on the first loop, advancing for each loop thereafter.

In a counting loop, the loop count begins with the loop's from value and advances either forward or backward depending on how the loop was constructed.

Note: Query Expressions do not support loop_abort, loop_continue or loop_count.

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