A practical introduction to Pascal programming language - GIARA

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Chapter 3Iterative constructions3.1 Contents of this chapterThis chapter covers the following contents:1.a Iterative structure for using to and downto clauses.1.b Iterative structure while.1.c Iterative structure repeat.3.2 Brief reviewComputer programs are usually based on the repetition of a sequence of instructions. It wouldbe very inconvenient for a programmer to rewrite the instructions casting such sequences severaltimes. Moreover, the programmer might not know in advance how many times the sequencesmust be repeated. Hence, most of the modern programming languages allow for the use of theso-called iterative constructions, that is, constructions that force the program to repeat (iterate)the same sequences several times. Moreover, the number of times the code is to be repeated canbe dependent upon the data input by the user so that there is no need for the programmer tohave that knowledge in advance.There are three iterative constructions considered in this chapter: for, while and repeatstructures. Very often, two or even three of them can be applied to solve a given solution.However, their semantic is different, and hence very often one of the solutions is slightly moreappropriate than the others. The syntax of these structures is included in Figures 3.1-3.3.The first of the iterative structures covered in this book is the for structure. This structureallow for the repetition of some code as many time as indicated by a pair of integer values. Thekey concept in this structure is that of the counting variable (var, in Fig. 3.1). This variable takesan initial value (initVal), and is then modified by one unit each time the code in the instructionblock is repeated. Eventually, when the counter goes over (or under) a predefined limit, theiteration stops.In a for structure, we can use either a to or a downto clause. The only difference betweenthem is whether the counter variable (var) is increased or decreased at each iteration. When theto clause is used, the iteration is kept until the counter is greater than finalVal. Alternatively,if downto is used instead, the iteration is hold until the counter is lower than finalVal. Notethat, if using the to clause, we should use a finalVal greater than initVal. Otherwise, as seen35
A Practical Introduction to Pascal Programming Language...[Precedent code]❄[Precedent code]for var:=initVal to finalVal doInstruction Block[Following code]...var:=var+1✲var:=initVal❄var ≤ finalValtrue❄Instruction blockfalse❄[Following code](a) for structure with to clause...[Precedent code]❄[Precedent code]for var:=initVal downto finalVal doInstruction Block[Following code]...var:=var-1✲var:=initVal❄var ≥ finalValtrue❄Instruction blockfalse❄[Following code](b) for structure with downto clauseFigure 3.1: Schematic representation of the if-then-else constructions36
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