statement level flow of control iteration structures copyright © 2003-2014 by curt hill

Statement Level Flow of Control

Iteration Structures

Copyright © 2003-2014 by Curt Hill

Looping statements

• Iteration is fundamental• Repeating a statement or

statements is present in all languages– Even Ada Lovlace wrote a loop in

the nineteenth century– Plankakul also had a loop

• The use of loops is a large amount of the power of the computers


Looping• The early loops were often

connected to processing array data, since early programming was mostly numerical

• Just because the while is the most general looping construct does not mean that it is desirable to have only it– Writeability is improved by

several loops


Design issues• How is the iteration controlled?

– Counting, Boolean testing or combination

• Where shall the test and exit mechanism be?

• Two separate considerations– Physical placement of the

mechanism•This is generally negligible

– Logical placement before the loop body or after loop body


Location

• Tests may be:• Before

– Pretest or leading decision

• After – Posttest or trailing decision

• Anywhere in the middle


Counter controlled• This is the classic array

manipulation loop• It is less general than a boolean

controlled loop but very handy

• There is a control variable, usually integer– It is given an initial value and

tested against a terminal value– There may be a step size value to

allow counting by other than 1– These are the loop parameters

• Supported by machine instructions


Design issues

• What is the type of the control variable?

• What is the scope of the control variable?

• May the loop parameters be changed in the loop?

• How many times should the loop parameters be evaluated


The DO of FORTRAN I-IV

• Posttest• DO label variable = initial,

terminal [,step]• Support for this in the hardware• Only posttest count controlled

loop• Parameters must be unsigned

constants or positive integers, not expressions


FORTRAN 77 and 90• Loop retains same form, but

becomes leading decision• The type of the parameters is

enlarged to that of integer or real• The parameters may now be

expressions as well• They are evaluated once at the

beginning of the loop to produce the iteration count, the number of times to loop

• When loop terminates it has its most recent value


FORTRAN 90

• F90 also introduces an END DO option instead of statement label

• Control variable only allows integer


ALGOL 60• This is one of the most complicated

loops because it tried to do it all• Count controlled• List controlled• Boolean controlled

– Quit when true– Quit when false

• Stuff before the body• Stuff after the body

– See the book for some examples

• It evaluated everything every time• The control variable was declared

external to the loop


COBOL Perform varying

• COBOL has a weird and delightful loop for subscripting through arrays that may be at most 3 dimensions

• The Perform is both a pseudo function call and loop– Several forms of it

• See example on next screen


COBOL Perform


Perform ABC Varying ndx From 1 by q

Until ndx = 50 After sub from 10 by 2 Until sub > 30

After N from 12 by -1 Until sub < 2.

The Pascal for• A delightfully simple pretest count

controlled loop• Control variable is any ordinal type• Increment can only be 1 or -1• Loop variable is declared externally• Loop variable is undefined at the

end of the loop• Parameters are only evaluated

once• Assignment to the control variable

in the loop is illegal


Ada for• Somewhat similar to Pascal in

concept but different in form• for var in [reverse] range loop …

end loop• for x in 1..A loop … end loop• The control variable is declared

in the loop and assumes the same type as the range

• No assignment is allowed on the control variable


C family for• Not actually count controlled loops

since the test does not have to match the initialization or step

• Very powerful and occasionally unruly

• Book has a nice example using the comma operator

• Java the test must be boolean• C++ and Java restrict any defined

variables to the loop body


Logically controlled loops• Design issues

– Pretest or posttest?– Separate loop or special form of

counting loop?

• Examples– There are several languages that

have both a pretest and posttest loop: C, C++, Java, Pascal

– FORTRAN 77 & 90 had no pretest or posttest logical loop

– Ada has only a pretestCopyright © 2003-2014 by Curt Hill

User located test and exits• The programmer may choose

locations other than beginning and end– Modula-2 and Ada have a loop statement

which is infinite– There is a conditional exit statement

that can be put anywhere including multiple sites

– C, C++, Java have the break which can accomplish the same thing• Which is a goto to first statement following

loop

– C and C++ also have a continue which is exit the current iteration but stay in loop


Iteration based on data structures

• Several languages allow a loop similar to a counting loop but based on something other than an integer– LISP has a dolist function that executes

a piece of code for each item on the list– Perl and Lambda MOO have a similar

construct that does something for each item on the list

• This has become popular enough that Java (5) and C++ (2011) have added


Examples


Lambda MOO:for x in (list) . . . endfor

Java (5 and after) :Picture p … ;Pixel [] px = p.getPixels();for(Pixel ap: px){ // process ap }

Ada


subtype MyRange is Integer range 0.99;MyArray: array (MyRange) of Integer;for Index in MyRange loop ...MyArray(Index) ... end loop;-- Really a Boolean loop

Finally

• The next presentation will consider is the forms of the GOTO and other control flow oddities


statement level flow of control iteration structures copyright © 2003-2014 by curt hill

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