5.4.2.6 Do Statement

A <do-statement> executes a sequence of statements as long as a specified pre/post-condition is True.

 do-statement = "Do" [condition-clause] EOS statement-block 

                 "Loop" [condition-clause] 

 condition-clause = while-clause / until-clause 
  
 while-clause = "While" boolean-expression 
 until-clause = "Until" boolean-expression 

Static Semantics.  

§ Only one <condition-clause> can be specified after the keyword Do or the keyword Loop, not both. If an <until-clause> is specified, the effect is as if it were a <while-clause> with the value of the <boolean-expression> set to "Not (<boolean-expression>)".

§ If no <condition-clause> is specified (either after Do or Loop), the effect is as if a <condition-clause> containing a <while-clause> with the expression "True" were specified after Do.

 

Runtime Semantics.

§ A <do-statement> repeatedly evaluates its <condition-clause> and executes the <statement-block> if it evaluates to the data value True. The ordering of the of the evaluation of the <condition-clause> and the execution of the <statement-block> is defined by the following table:

Location of <condition-clause>	Result
None specified	Execution of the loop continues until an <exit-do-statement> is executed.
Immediately following "Do"	<condition-clause> is evaluated prior to executing <statement-block>. If it evaluates to the data value False then execution of the <statement-block> and the current statement immediately completes.
Immediately following "Loop"	The <statement-block> is executed before evaluation of the <condition-clause>. If it evaluates to the data value True, then the <statement-block> is again executed and the process is repeated. If it evaluates to the data value False then execution of the <statement-block> and the current statement immediately completes.