CHAPTER 7

STATEMENT-LEVEL CONTTROL STRUCTURE

• Two factors are necessary to make the computations in programs flexible and powerful: some means of selecting among alternative control flow paths (of statement execution) and some means of causing the repeated execution of certain collections of statements. Statements that provide these kinds of capabilities are called control statements
• It was proven that all algorithms that can be expressed by flowcharts can be coded in a programming language with only two control statements: one of choosing between two control flow paths and one for logically controlled iterations.
• Writeability and readability are very important. All languages that have become very widely used contain more control statements than the two that are minimally required, because writeability is enhanced by a larger number of control statements.
• Control structure is a control statement and the collection of statements whose execution it controls. Control structures should have single entries and single exits.

COMPOUND STATEMENTS

• One feature that makes control statement design easier is a method of forming statement collections.
• Algol 60 introduced the first statement collection structure, the compound statement:

begin

statement_1;

…

statement_n

end

• Compound statements allow a collection of statements to be abstracted to a single statement – which can be used to great advantage in control statement design. Data declarations can be added to the beginning of a compound statement in several languages, making it a block.
• C, C++, and Java use braces to delimit both compound statements and blocks.

SELECTION STATEMENTS

• Selection statements provides the means of choosing between two or more execution paths in a program. They fall in two general categories, two-way and n-way, or multiple selection.

Two-Way Selection Statements

• The simplest design issue for two-way selectors is the type of expression that controls the selector.
• All imperitative languages include a single-way selector, in most cases as a sub-form of a two-way selector

if(flag .ne. 1) go to 20

i = 1

j = 2

20 continue

if (Boolean expression) then

begin

statement _1;

…

statement-n

end

• Most of the languages that followed Algol 60, including FORTRAN 77 and 90, provide a single-way selectors that can select a compound statement or a sequence of statements.
• Algol introduced the first two-way selector:

if (Boolean expression) then

statement

else

statement

Nesting Selectors

if sum = 0 then

if count = 0 then

result := 0

else

result := 1

• The designers of Algol 60 chose to use syntax, rather than a rule, to connect else clauses. An if statement is not allowed to be nested directly in a then clause. If an if must be nested in a then clause, it must be placed in a compound statement.

if sum = 0 then

begin

if count = 0 then

result := 0

end

else

result := 1

MULTIPLE SELECTION CONSTRUCTS

• The multiple selection construct allows one of any number of statements or statement groups, it is a generalization of a group. In fact, a single-way and a two-way selectors can be built with a multiple selector.
• Multiple selectors can be build from two-way selectors and gotos, but the resulting structure is difficult to write and read.
• FORTRAN’S three-way selector, which is called an arithmetic IF, is a special case of multiple selection statements. Because only three or fewer statement collections can be selected.

IF (arithmetic expression) N1, N2, N3

• The general form of Hoare’s multiple selector:

case integer_expression of

begin

statement_1;

…

statement-n

end

-- Pascal’s case

case expression of

constant_list_1: statement_1:

…

constant_list_n: statement_n

end

-- The C multiple selector construct, switch, which also appears i C++ and Java:

switch (expression) {

case expression_1: statement_1;

…

case expression_n: statement_n;

[default: statement_n + 1]

}

ITERATIVE STATEMENTS

An iterative statement is one that causes a statement or collection of statements to be executed 0 or 1 or more times. Every programming language has included some method of repeating the execution of segments of code. Iteration is the very essence of the power of the computer. If iteration were not possible, programmers would be required to state every action in sequence; useful programs would be large and inflexible and take a huge amount of time to write.

• The first iterative constructs in programming languages were directly related to arrays.
• The body of a loop is the collection of statements whose execution is controlled by the iteration statement.
• The term pretest means that the test for loop completion occurs before the loop body is executed.
• The term posttest means that it occurs after the loop body is executed.
• The iteration statement and the associated loop body together form an iteration construct.

COUNTER-CONTROLLED LOOPS

• A counting iterative control statement has a variable, called the loop variable, in which the count value is maintained. This includes some means of specifying the initial and terminal values of the loop variable and the difference between sequential loop variable values, often called the step-size.

THE DO STATEMENT OF FORTRAN 77 & FORTRAN 90

• The distinctive feature of this statement was that it was a posttest, making it different from the counting iterative statements of all other programming languages.
• The general form of this statement is;

DO label variable = initial, terminal, [step size]

Where the label is the last statement in the loop body and the step size, when absent, defaults to 1.

LOGICALLY CONTROLLED LOOPS

• In many cases, collections of statements mist be repeatedly executed, but the repetition control is based on a Boolean expression rather than a counter. For these situation, a logically controlled loop is convenient.

ITERATION BASED ON DATA STRUCTURES

• Rather than have a counter or Boolean expression control iterations, these loops are controlled by the number of elements in a data structure.
• COMMON LISP and Perl have such statements.
• In COMMON LISP the dolist function iterates on simple lists, which are the most common data structures in LISP programs.
• Perl's for each statement is similar, it iterates on the elements of lists or arrays.

UNCONDITIONAL BRANCHING

• An unconditional branch statement transfers execution control to a specified place in the program.
• The unconditional branch or goto is the most powerful statement for controlling the flow of execution of a program's statements.
• Problems follow directly from a goto's capability of forcing any program statement to follow any other in execution sequence, regardless of whether that statement precedes or follows the first in textual order.
• Modula-2 and Java are designed without goto's
• Most currently used languages include a goto statement.