CHAPTER 8 - Statement Level Control Structures

Control Structure

a control statement and the statements whose execution it controls

2 types of control structures

Selection Statements -- Iterative Statements

Only design issue relevant to all selection and iteration control statements

Should a control structure have multiple entries?

Selection Statement

provides the means of choosing between two or more paths of execution

2 categories of selection statements

Two-way selection -- Multiple-way selection

3 design issues for two-way selection

form and type of the control expression -- how then and else clauses are specified -- how meaning of nested selectors is specified

Control Expression in C89

used arithmetic expressions -- nonzero is true -- zero is false

Control Expression in Java/C#

only Boolean expressions allowed

Clause Form in C-based languages

uses braces to form compound statements

Clause Form in Perl

all then and else clauses must be compound statements even if they contain a single statement

Clause Form in Python

uses indentation to define clauses -- colon used instead of then

Nesting Selectors rule

else clause is always paired with the nearest unpaired then clause -- applies in Java and contemporary languages

C# switch

differs from C-based switch -- static semantic rule disallows implicit execution of more than one segment -- every segment must end with break or goto

Iterative Statement

causes a statement or collection of statements to be executed zero, one, or more times -- often called a loop

Pretest loop

loop completion condition checked before the loop body is executed -- example: while

Posttest loop

loop completion condition checked after the loop body is executed -- example: do-while

Key difference: while vs do-while

do-while always executes the loop body at least once

Counter-Controlled Loop

has a loop variable -- specifies initial value, terminal value, and stepsize

Stepsize

the difference between sequential loop variable values

Loop Parameters

the initial, terminal, and stepsize values

C for loop infinite loop

if the second expression is absent

C for vs Fortran/Ada

C's for is more flexible -- each expression can comprise multiple statements -- allows multiple loop variables of any type

Java/C# for loop

like C++ except loop control expression is restricted to Boolean

Logically Controlled Loop

repetition based on a Boolean expression rather than a counter

User-Located Loop Exits -- C/C++

unconditional unlabeled exits using break

User-Located Loop Exits -- Java/C#/Perl

unconditional labeled exits (break in Java/C# -- last in Perl)

continue statement

skips the rest of the current iteration without terminating the loop -- transfers control to the loop's control mechanism

Iterator

called at the beginning of each iteration -- returns an element from a data structure in a specific order

foreach in C#

iterates on elements of arrays and other collections -- uses built-in iterators of predefined generic collections

Unconditional Branch / goto

the most powerful statement for controlling execution flow -- carelessly used, it makes programs difficult to read and unreliable

Languages without goto

Java -- Python -- Ruby

Guarded Commands (Dijkstra 1975)

alternative control statements -- evaluates all Boolean guards -- if more than one true, chooses non-deterministically -- if none true, runtime error -- basis for CSP concurrency mechanisms

Motivation for Guarded Commands

support program correctness verification during development rather than after completion