1 / 29

Chapter 8: Control Structures

Chapter 8: Control Structures. Lectures # 15. Chapter 8 Topics. Introduction Compound Statements Selection Statements Iterative Statements Unconditional Branching Conclusions. Chapter 8: Control Structures. 2. Levels of Control Flow. Within expressions

chouser
Télécharger la présentation

Chapter 8: Control Structures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 8:Control Structures Lectures # 15

  2. Chapter 8 Topics • Introduction • Compound Statements • Selection Statements • Iterative Statements • Unconditional Branching • Conclusions Chapter 8: Control Structures 2

  3. Levels of Control Flow • Within expressions • (governed by associativity and precedence rules Ch 7.). • Among program units • ( highest level we will see in Ch 9 and Ch 13). • Among program statements • (in this lecture). Chapter 8: Control Structures 3

  4. Control Statements, Control Structures • Control Statements: • Statements that provide capabilities such as: • Selection among several alternative control flow paths. • Repeated execution of collections of statements. • Control Structure: • A control statement and the collection of statements whose execution it controls. • Categories include: • Compound statements. • Selection statements. • Iterative statements. • Unconditional branching. Chapter 8: Control Structures 4

  5. Compound Statements • Allow a collection of statements to be abstracted to a single statement. • A block is a compound statement which includes data declarations. • Some languages have specially delimited compound statements: • begin … end : in Pascal. • { … } : in C, C++, C#. • Some control constructs have built-in delimiters such as: repeat…until Chapter 8: Control Structures 5

  6. Selection Statements • A selection statement provides the means of choosing between two or more paths of execution. • Two general categories: • Two-way selectors. • Multiple-way selectors. Chapter 8: Control Structures 6

  7. Two-Way Selection Statements • General form: if control_expression then clause else clause • ALGOL 60 and some other languages: if (boolean_expr) then statement (then clause) else statement (else clause) • The statements could be single or compound. Chapter 8: Control Structures 7

  8. Nesting Selectors • Java example: if (sum == 0) if (count == 0) result = 0; else result = 1; • Whichifgets theelse ? • Java's static semantics rule:elsematches with the nearestif. Chapter 8: Control Structures 8

  9. Nesting Selectors (cont.) • To force an alternative semantics, compound statements may be used: if (sum == 0) { if (count == 0) result = 0;} else result = 1; • The above solution is used in C, C++, and C#. • Perl requires that all then and else clauses to be compound. Chapter 8: Control Structures 9

  10. Multiple-Way Selection Statements • Allow the selection of one of any number of statements or statement groups. • Modern multiple selectors: • C’sswitchstatement: switch (expression) { case const_expr_1: stmt_1; … case const_expr_n: stmt_n; [default: stmt_n+1] } Chapter 8: Control Structures 10

  11. Multiple-Way Selection: Examples • Early multiple selectors: • FORTRAN arithmetic IF (a three-way selector): IF (arithmetic expression) N1, N2, N3 • Ex:IF (expression) 10, 20, 30 10 …… GO TO 40 20 …… GO TO 40 30 …… 40 • Segments require GOTOs. • The arithmetic IF can be entered through any of its statements from anywhere in the program. Chapter 8: Control Structures 11

  12. Multiple-Way Selection: Examples • The Ada’scasestatement: case expression is when choice list => stmt_sequence; … when choice list => stmt_sequence; [when others => stmt_sequence;] end case; • More reliable than C’sswitch: once a stmt_sequence execution is completed, control is passed to the first statement after the case statement. Chapter 8: Control Structures 12

  13. Multiple-Way Selection Using if • Multiple Selectors can appear as direct extensions to two-way selectors, using else-if clauses, for example in Ada: if ... then ... elsif ... then ... elsif ... then ... else ... end if Chapter 8: Control Structures 13

  14. Iterative Statements • The repeated execution of a statement or compound statement is accomplished either by iteration or recursion. • Iterative statements categories: • Counter-controlled loops. • Logically-controlled loops. • User-located loop control mechanisms. • Data structure based-iteration. Chapter 8: Control Structures 14

  15. Iterative Statements: Counter-Controlled Loops • A counting iterative statement has a loop variable, and a means of specifying the initial and terminal, and stepsize values. Chapter 8: Control Structures 15

  16. Counter-Controlled Loops:Examples • Pascal’sforstatement: for variable := initial (to|downto) final do statement; • Design choices: • Loop variable must be an ordinal (such as int or char) type of usual scope. • After normal termination, loop variable is undefined. • The loop variable cannot be changed in the loop. Chapter 8: Control Structures 16

  17. Counter-Controlled Loops:Examples (cont.) • Ada: for var in [reverse] discrete_range loop ... end loop • A discrete range is a sub-range of an integer or enumeration type. • Scope of the loop variable is the range of the loop. • Loop variable is implicitly undeclared after loop termination. Chapter 8: Control Structures 17

  18. Counter-Controlled Loops:Examples (cont.) • C’sforstatement: for ([expr_1] ; [expr_2] ; [expr_3]) statement; • Everything can be changed in the loop. • The first expression is evaluated once, but the other two are evaluated with each iteration. Chapter 8: Control Structures 18

  19. Counter-Controlled Loops:Examples (cont.) • C++ differs from C in: • The initial expression can include variable definitions (scope is from the definition to the end of the loop body). Chapter 8: Control Structures 19

  20. Summing numbers in C++(Reading) int sum(int start, int finish, int incr){ int total = 0; for (int i = 0; i <= finish; i += incr) { total += i; } return total;} Chapter 8: Control Structures 20

  21. Summing numbers in Scheme(Reading) (define (sum start finish incr) (let ((total 0)) (do ((i start (+ i incr))) ((> i finish) total) (set! total (+ i total))))) Local variable total = 0 Loop increment Initial value for i Return value Test to quit Loop “body” Chapter 8: Control Structures 21

  22. Iterative Statements: Logically-Controlled Loops • Repetition control is based on a Boolean. • General forms: • while (ctrl_expr) do loop body • dO loop body while (ctrl_expr) Chapter 8: Control Structures 22

  23. Logically-Controlled Loops: Examples • Pascal has separate pre-test and post-test logical loop statements (while-do and repeat-until). • C and C++ also have both, but the control expression for the post-test version is treated just like in the pre-test case (while-do and do-while). • Java is like C, except the control expression must be Boolean. • Adahas a pre-test version, but no post-test. • Perlhas two pre-test logical loops, while and until, but no post-test logical loop. Chapter 8: Control Structures 23

  24. Iterative Statements: User-Located Loop Control Mechanisms • Sometimes it is convenient for the programmers to decide a location for loop control (other than top or bottom of the loop). • Examples: • Ada loop – exit [loop_label] [when condition]. • C loops – break and continue. Chapter 8: Control Structures 24

  25. User-Located Loop Control Mechanisms breakand continue • C, C++, and Java: breakstatement. • Unconditional; for any loop or switch; one level only. • Java and C# have a labeledbreakstatement: control transfers to the label. • An alternative: continuestatement; it skips the remainder of this iteration, but does not exit the loop. Chapter 8: Control Structures 25

  26. Iterative Statements: Iteration Based on Data Structures • C#’s foreach statement iterates on the elements of arrays and other collections: Strings[] = strList = {“Bob”, “Carol”, “Ted”}; foreach (Strings name in strList) Console.WriteLine (“Name: {0}”, name); • The notation {0} indicates the position in the string to be displayed. • Perl has foreach which iterates through lists: @names = {“Bob”, “Carol”, “Ted”, “Alice”};…foreach $name (@names) { print $name} Chapter 8: Control Structures 26

  27. Unconditional Branching • Transfers execution control to a specified place in the program. • Well-known mechanism: goto statement. • Label forms: • <<ADA_LABEL>> • Label: (in other languages). • Major concern: Readability. Chapter 8: Control Structures 27

  28. Unconditional Branching (cont.) • Some languages do not support goto statement (e.g., Module-2 and Java). • C# offers goto statement (can be used in switch statements). • Restrictions on branches: • Into a function from another function? • Into a loop or selection statement? Chapter 8: Control Structures 28

  29. Conclusion • Variety of statement-level structures. • Functional and logic programming languages are quite different control structures. Chapter 8: Control Structures 29

More Related