Flow of Control

1. Flow of Control Kernighan/Ritchie: Kelley/Pohl: Chapter 3 Chapter 4

2. Control Flow Statements • Normal program flow executes statements one after the other • Special statements allow the programmer to change the flow of control • Selection statements: if-else and switch • Iteration statements: while, do and for • Direct jump: goto

3. Lecture Overview • Boolean expressions • The if statement • The switch statement • Iteration statements –for and while • The break and continue statements • The goto statement

4. Boolean Expressions • Each of these statements (except goto) depends on a condition • Conditions normally contain one or moreof the following operators: • Relational operators: >, >=, <, <= • Equality operators: ==, != • Logical operators: &&, ||, !

5. Boolean Expressions • Since there is no Boolean type in C, Boolean expressions evaluate to 0 or to 1 • Similarly, the condition in a control flow statement is simply tested for its numeric value – either zero or non-zero • Therefore, there is no restriction on the type of the expression in the condition

6. Boolean Expressions – Examples

7. Operator Precedence • When an expression contains several operators, the compiler needs to knowthe order in which to evaluate them • The order of precedence for each operator is defined in a table (see book) • Parentheses must be used when: • A different order of evaluation is desired • Operators have the same level of precedence

8. Operator Precedence • In general, the order in which operators are evaluated, from high to low, is: • Access and Grouping: (), [], ., -> • Unary: ++, --, !, & (address), * (dereference) • Arithmetic: +, -, * (multiplication), /, % • Relational and Equality: >, >=, <, <=, ==, != • Logical: &&, || • Assignment: =, +=, -=, *=, /=, etc.

9. Operator Precedence – Examples

10. Operator Precedence – Examples int a = 5; float b = 3; int isSmaller; isSmaller = a < b; printf ("isSmaller : %d\n", isSmaller); printf ("a > b : %d\n", a > b); printf ("a != b + 2 : %d\n", a != b + 2); printf ("a = b == 3 : %d\n", a = b == 3); isSmaller : 0 a > b : 1 a != b + 2 : 0 a = b == 3 : 1

11. Short-Circuited Operators • The processing of logical AND and logical OR is “short-circuited” • If the left operand is sufficient to determine the result, the right operand is not evaluated • This feature should be used with care if (count != 0 && total/count > MAX) printf ("Ratio exceeds maximum.\n");

12. Lecture Overview • Boolean expressions • The if statement • The switch statement • Iteration statements –for and while • The break and continue statements • The goto statement

13. The if Statement • The if statement has the following syntax: • The else clause may be dropped • Each statement may also be a compound statement (a block) if (condition) statement1; else statement2;

14. Nested if Statements • The statement executed as a result of an if statement or else clause could be another if statement • These are called nested if statements • An else clause is matched to the last unmatched if

15. The Dangling else Problem • Consider the following code segments – which one is properly formatted? if (a == 1) if (b == 2) printf ("***\n"); else printf ("###\n"); if (a == 1) if (b == 2) printf ("***\n"); else printf ("###\n");

16. The Dangling else Problem • The second version is the correct one (although the compiler will accept both) • To match an else to a different if than the nearest one, we must use braces: if (number >= 0 ) { if ((number % 2) == 0) printf ("number is even."); } else printf ("number is negative.");

17. Multi-branch if • Syntax of the multi-branch if statement: if (condition1) statement1; else if (condition2) statement2; . . . else if (condition_n) statement_n; else default_statement;

18. Multi-branch if • The multi-branch if statement is just a list of nested if statements • However, since it is so commonly used, it is treated as if it were a new kind of selection statement • Thus, it is formatted in a way that conveys its meaning more clearly

19. Multi-branch if– Example int balance; printf ("Please enter your balance: "); scanf ("%d", &balance); if (balance > 0) printf ("Positive balance.\n"); else if (balance < 0 && balance >= -100) printf ("Negative balance.\n"); else if (balance < -100) printf ("Balance exceeds limit.\n"); else printf ("Zero balance.\n"); Please enter your balance: 45 Positive balance.

20. The Conditional Operator • The following programming construct is quite common: • The conditional operator providesa short-cut for the above: if (id1 > id2) larger = num1; else larger = num2; larger = (id1 > id2)? num1: num2;

21. Lecture Overview • Boolean expressions • The if statement • The switch statement • Iteration statements –for and while • The break and continue statements • The goto statement

22. The switch Statement • The syntax of the switch statement: switch (expression) { case value1: statement-list1 case value2: statement-list2 case value3: statement-list3 case ... default: }

23. The switch Statement – Example • An example of a switch statement: switch (letter) { case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break; default: errorCount++; }

24. Intentional Fall-through • Normally, a break is used as the last statement in each case's block • If not, operation falls-through to the first statement of the next case • Missing breaks can result in hard to find bugs • If fall-through is used intentionally, it must be properly documented

25. switch Expression Type • The type of the expression in a switch statement must be integral (or char ) • The value in the label of each case must be a constant • The implicit Boolean condition in a switch statement is equality, thus no other relational checks can be performed

26. Lecture Overview • Boolean expressions • The if statement • The switch statement • Iteration statements –for and while • The break and continue statements • The goto statement

27. The while Statement • The syntax of the while statement: • The syntax of the do-while statement: • statement is executed at least once while (condition) statement; do statement; while (condition)

28. The while Statement – Example while ((c = getchar()) != EOF) { if (c == ' ') blank_cnt++; else if (c >= '0' && c <= '9') digit_cnt++; else if (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z') letter_cnt++; else if (c == '\n') nl_cnt++; else other_cnt++; }

29. The Empty Statement • Wherever a statement may be used, the empty statement is also allowed • Example – skip all initial blank characters in the input stream: while ((c = getchar()) == ' ') ; /* empty statement */

30. The for Statement • The syntax of the for statement: • A for loop is functionally equivalent to the following while loop structure: for (initialization; condition; increment) statement; initialization; while (condition) { statement; increment; }

31. The for Statement • The initialization section cannot be used to declare a variable, only to set its value • Any or all of the three parts may be omitted, for example (infinite loop): printf ("This can take a while...\n"); for ( ; ; ) printf ("*");

32. The Comma Operator • The comma operator has the lowest precedence of all the operators in C: • expr1 is evaluated first, and then expr2 • The value returned by this expression is the value returned by expr2 expr1, expr2

33. The Comma Operator – Examples • Commonly used for multiple initializations: • What will be printed by this code segment? for (sum = 0, i = 1; i <= n; i++) sum += i; int a = 5, b = 3, c; a++, c = (b++, a + b); printf ("%d\n", c); 10

34. Lecture Overview • Boolean expressions • The if statement • The switch statement • Iteration statements –for and while • The break and continue statements • The goto statement

35. The break Statement • The break statement causes an exit from the innermost enclosing loop or switch • Example: while (1) { scanf ("%f", &x); if (x < 0.0) break; /* exit loop if x is negative */ printf ("%f\n", sqrt(x)); } /* break jumps to here */

36. The continue Statement • The continue statement causes the current iteration of a loop to stop, and the next one to begin immediately • The continue statement may only occur inside for , while , and do-while loops for (i = 0; i < TOTAL; i++) { c = getchar(); if (c >= '0' && c <= '9') continue ; ... /* process non-digit characters */

37. break and continue– Example #include <stdio.h> int main() { int i, j; for (i = 0; i < 10; i++) { for (j = 0; j < 10; j++) { printf ("%3d ", i * 10 + j); } printf ("\n"); } return 0; }

38. break and continue– Example • The output of the previous program: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

39. break and continue– Example • We change the main part of the program: ... for (i = 0; i < 10; i++) { if (i % 3 == 0) continue; for (j = 0; j < 10; j++) { if (j == 8) break; printf ("%3d ", i * 10 + j); } printf ("\n"); } ...

40. break and continue– Example • The output of the modified program: • The lines beginning with 0, 30, 60 and 90 are skipped (i is a multiple of 3) • All lines are truncated when j reaches 8 10 11 12 13 14 15 16 17 20 21 22 23 24 25 26 27 40 41 42 43 44 45 46 47 50 51 52 53 54 55 56 57 70 71 72 73 74 75 76 77 80 81 82 83 84 85 86 87

41. Lecture Overview • Boolean expressions • The if statement • The switch statement • Iteration statements –for and while • The break and continue statements • The goto statement

42. The goto Statement • A goto statement causes an unconditional jump to a labeled statement somewhere in the current function • In order to perform a goto, some special label must be defined • A label is an identifier, with a ':' after it – control will transfer to the statement directly following the label

43. The goto Statement • The first rule to remember about goto: goto is bad! • Using it may undermine all of the useful structure provided by other control flow mechanisms (such as for and while) • The only possible exception: goto is sometimes used for critical error handling

44. The goto Statement – Example • Handling errors using goto: ... while (scanf ("%f", &x) == 1) { if (x < 0.0) goto negative_alert; printf ("%f %f\n", sqrt (x), sqrt (2 * x)); } ... negative_alert: printf ("Negative value encountered! \n"); exit (EXIT_FAILURE); ...

45. The goto Statement From xkcd.com