Statement-Level Control Structures Sections 1-4 Chapter 8 Statement-Level Control Structures Sections 1-4

Levels of Control Flow Within expressions (Chapter 7)‏ controlled by precedence and associativity Among program units function calls Among program statements Selection Repetition Unconditional branching 2

Control Structure A Control structure is control statement plus statements whose execution it controls Fortran control statements were based on those of the underlying hardware branch instructions based on a test jump instructions (unconditional) 3

Control Structure Theory says All algorithms represented by flowcharts can be coded with only two-way selection and pre-test logical loops Having mutiple control statements is just to make things easier for the programmer. 4

Selection Statements A selection statement provides the means of choosing between two or more paths of execution Two-way selectors (if) Multiple-way selectors (switch) 5

Two-Way Selection Statements General form: if control_expression then clause else clause control_expression should evaluate to true or false Either then-clause or else-clause gets done 6

Two-Way Selection Statements Design Issues: What is the form and type of the control expression? How are the then and else clauses specified? How should the meaning of nested selectors be specified? 7

Examples FORTRAN: IF (boolean_expr) statement Problem: Can select only a single statement; to select more, a GOTO must be used, as in the following example IF (.NOT. condition) GOTO 20 ... 20 CONTINUE Negative logic is bad for readability Solution: allow compound statements for the selectable segment of their single-way selectors 8

Syntax for if In Java et al. Consider deriving How to fix it? <ifstmt> -> if (<test>) <stmt> | if (<test>) <stmt> else <stmt> Consider deriving if (p) if (q) s1 else s2 This grammar is ambiguous How to fix it? sematic rule (else goes with nearest if)‏ require compound statements (Perl)‏ keyword at end (Ruby)‏ 9

Multi-Way Selection Allow the selection of one of any number of statements or statement groups Design Issues: What is the form and type of the control expression? How are the selectable segments specified? Is execution flow through the structure restricted to include just a single selectable segment? What is done about unrepresented expression values? 10

Multiple Selection in FORTRAN Arithmetic IF (a three-way selector)‏ IF (arithmetic expression) N1, N2, N3 N1, N2, N3 are statement labels Segments require GOTOs Not encapsulated (selectable segments could be anywhere)‏ Computed goto GOTO (N1, N2, N3, …) arithmetic_expression 11

switch for Multiple Selection switch (expression) { case const_expr_1: stmt_1; … case const_expr_n: stmt_n; [default: stmt_n+1] } 12

Design choices for switch Control expression must be integer Selectable segments can be statement sequences, blocks, or compound statements Any number of segments can be executed in one execution of the construct (fall-through behavior)‏ default clause is for unrepresented values (if there is no default, the whole statement does nothing)‏ 13

Multiple-Way Selection: Scheme cond special form (cond (test1 result1) … (testn resultn) (else defaultResult)) case special form (case expr ((valuelist1) result1) … (valuelistn) resultn) (else defaultResult) ) 14

Multiple-Way Selection Using if Multiple Selectors can appear as direct extensions to two-way selectors, using else-if clauses, for example in Perl: if ... {. . . } elsif ... else { ... } 15

Iterative Statements The repeated execution of a statement or compound statement is accomplished either by iteration or recursion General design issues for iteration control statements: How is iteration controlled? logical expression or counter 2. Where is the control mechanism in the loop? top or bottom of loop 16

Counter-Controlled Loops A counting iterative statement has a loop variable, and a means of specifying the initial and terminal, and stepsize values for statement for (init; test; update) {…} DO in Fortran do 10, I=1,10 17

Counter-Controlled Loops Design Issues: What are the type and scope of the loop variable? What is the value of the loop variable at loop termination? Should it be legal for the loop variable or loop parameters to be changed in the loop body, and if so, does the change affect loop control? Should the loop parameters be evaluated only once, or once for every iteration? 18

C’s for statement for ([expr_1] ; [expr_2] ; [expr_3]) statement The expressions can be whole statements or statement sequences There is no explicit loop variable Everything can be changed in the loop The first expression is evaluated once, but the other two are evaluated with each iteration 19

for in C relatives C++ Java and C# Control expression can be Boolean The initial expression can include variable definitions scope is from the definition to the end of the loop body Java and C# The control expression must be Boolean 20

Logically-Controlled Loops Control is based on a Boolean expression Design issues: Pre-test while (ctrl_expr) loop body post-test do 21

Examples C and C++ also have both pre-test and post-test loops (while-do and do- while) Java is like C, except the control expression must be Boolean Perl has two pre-test logical loops, while and until, but no post-test logical loop 22

User-Located Loop Control Mechanisms Sometimes convenient to have loop control in middle of loop body HP Basic: loop - endloop with exit statement anywhere in the block Simple for single loops (use break) Design issues for nested loops Should the conditional be part of the exit? Should control be transferable out of more than one loop? 23

C , C++, and Java: break statement break and continue C , C++, and Java: break statement Unconditional; for any loop or switch; one level only Java and C# have a labeled break statement: control transfers to the label An alternative: continue statement; it skips the remainder of this iteration, but does not exit the loop 24

Iteration Based on Data Structures Number of elements of in a data structure control loop iteration Control mechanism is a call to an iterator function that returns the next element in some chosen order, if there is one; otherwise terminate loop 25

Iteration Based on Data Structures C's for can be used to build a user-defined iterator: for (p=root; p==NULL; traverse(p)){ } Perl has foreach statement iterates on the elements of arrays and other collections: @strList = {“Bob”, “Carol”, “Ted”}; foreach $name (@strList)‏ {print “Name: $name”;} Java has Iterator interface, foreach in Java 1.5 Python has for-in 26

Unconditional Branching Transfers execution control to a specified place in the program goto statement Represented one of the most heated debates in 1960’s and 1970’s Major concern: Readability Some languages do not support goto statement (e.g., Module-2 and Java)‏ C# offers goto statement (can be used in switch statements)‏ Loop exit statements (break) are restricted goto’s 27