Statement-Level Control Structures

1
Statement-Level Control Structures

• Levels of flow control
• Control Statements
• 1. Sequence
• 2. Selection
• 3. Iteration
• Unconditional branching
• Guarded commands
• Sebesta Chapter 8

2
Levels of Program Flow Control

• 1. Within expressions Ch. 7
• 2. Among program statements Ch. 8
• 3. Among program units Ch. 9-10
• 4. Exceptions Ch. 14

3
Evolution of Flow Control

• FORTRAN I
• control statements based directly on IBM 704
  hardware
• 1960s
• A lot of research and argument about different
  types of flow control

4
Control Structures

• A control statement and the statements whose
  execution it controls
• Sequence
• Selection
• Iteration/Repetition
• Overall Design Question
• What control statements should a language have,
  beyond selection and pretest logical loops?

5
Flow Chart Elements
decision
block of statements
start/end
control flow
6
Sequence and Selection

Sequence
true
false

Selection (multi-way)
Selection (if/else)
7
Iteration/Repetition Pre and Post Test
Pre-test repetition
Post-test repetition
(while lttestgt do ltstuffgt)
(do ltstuffgt while lttestgt)
8
Minimum Control Structures

• Proven early
• all control structures can be coded with only
• 1. two-way selection, and
• 2. pretest logical loops!
• Lecture-specific question
• What is the relation to assembly languages?

9
Selection Statements

• Alternatives between two or more execution paths
• 1. Two-way selectors
• 2. Multiple-way selectors

10
Two-Way Selection Statements

• General form
• if ltcontrol_expressiongt
• then ltthen_clausegt
• else ltelse_clausegt
• Design Issues
• What is the form and type of the control
  expression?
• How are then_clause and else_clause specified?
• How should the meaning of nested selectors be
  specified?

11
Two-Way Selection Fortran

• FORTRAN
• IF (ltboolean_exprgt) ltstatementgt
• Problem
• only a single statement can be selected
• GOTO must be used to select more, e.g.
• IF (.NOT. ltconditiongt) GOTO 20
• ...
• 20 CONTINUE
• GOTOs must also be used for the else clause
• Negative logic is bad for readability
• These problems were solved in FORTRAN 77
• Most later languages
• allow compound statements to be selected
• support else clause

12
Two Way-Selection in Common Lisp

• Syntax
• (if lttestgt ltdo_if_truegt
• ltdo_if_falsegt)
• Single/nested functions for ltdo_if_...gt
• May contain multiple statements
• if surrounded by a block structure
• prog, let, do, etc.

13
Lisp Two Way Selection

• (when ( x 0)
• (terpri)
• (princ "It's now 0")
• )
• (if (gt (aref a x) (aref b x))
• (progn block, returns last value
• (terpri) (princ "Fixing")
• (setf (aref a x) (aref b x)))
• (if (lt (aref a x) (aref b x))
• (format t "It's Less")
• (format t "It's Equal")))

14
Nesting Selectors

• Java example
• if (sum 0)
• if (count 0)
• result 0
• else result 1
• To which if does the else belong to?
• Java's static semantics rule
• else matches with the nearest if

15
Nesting Selectors (cont.)

• Compound statements must be used for alternative
  semantics
• if (sum 0)
• if (count 0)
• result 0
• else result 1
• are used in C, C, and C
• Perl all then and else clauses must be compound

16
Multiple-Way Selection Statements

• Select among one of many statements
• Design issues
• Form and type of the control expressions
• How are the selectable segments specified?
• Will only a single segment be executed or does
  control continue checking other control
  expressions?
• What happens if no control expressions occurs?

17
Multiple-Way Selection Examples

• Early multiple selectors
• FORTRAN arithmetic IF (a three-way selector)
• IF (arithmetic expression) N1, N2, N3
• Segments require GOTOs (not structured
  programming)
• Not encapsulated (selectable segments could be
  anywhere)
• Result "spaghetti code"

18
Multiple-Way Selection Examples

• Modern multiple selectors
• switch statement in C, C, Java
• switch (ltexpressiongt) case ltconst_expr_1gt
  ltstmt_1gt
• case ltconst_expr_ngt ltstmt_ngt
• default ltstmt_n1gt

19
Multiple-Way Selection Examples

• Design choices for Cs switch statement
• Control expression can be only an integer type
• Selectable segments can be statement sequences
• Any number of segments can be executed in one
  execution of the construct
• no implicit branch at the end of selectable
  segments
• default clause is for unrepresented values
• if there is no default, the whole statement does
  nothing
• This leads to many bugs

20
Lisp Multiple Selection cond

• (defun test (x)
• (cond
• ((lt x 0) test
• (terpri) (princ Its negative"))
• actions to perform if test is true
• ((gt x 100)
• (terpri) (princ "It's huge"))
• (t
• (terpri) (princ Its reasonable"))))
• Only the 1st true test and following expressions
  within the corresponding clause are evaluated
• Safe - like ADA
• GOTOs not needed

21
Multiple-Way Selection With if

• Multiple Selectors can appear as direct
  extensions of two-way selectors, using else-if
  clauses, e.g., in Ada
• if ...
• then ...
• elsif ...
• then ...
• elsif ...
• then ...
• else ...
• endif