1 Homework / Exam Turn in HW3 today Exam 1 next class –Open Book / Open Notes –Recommended Use of Book / Notes in Exam: Avoids reliance on “rote memorization” of details Use for reference to tables, figures, etc. Look up a specific syntax detail you’ve forgotten Don’t expect to copy code for the answer to a problem!! If you find yourself copying code – STOP! THINK! Start Chapter 4 for class after Exam

2 Statements / Blocks, Section 3.1 An expression becomes a statement when it is followed by a semicolon x = 0; Braces are used to group declarations and statements into a compound statement { x = 0; y = 1; }/* Note: No semicolon after right brace */

3 if statements, Section 3.2 Shortcut for “equal and not equal to 0” tests Can use: if (expression)if (!expression) In place of if (expression != 0)if (expression == 0)

4 if-else Matching, Section 3.2 Else is matched with the “closest previous else-less if” Example (compiler won’t interpret as the code has been indented on the left. See right side):if (a) if (b)if(b)statement;elsestatement;

5 Switch, Section 3.4 Consider cascading else-if sequence: if (i == 1) /* NOTE: Only one will execute */ statement-1; else if (i == 2) statement-2;... else if (i == 49) statement-49; else statement-50; /* Default or "catch-all" */

6 Switch Also have switch statement **LIKE JAVA** switch (i) { /* in special limited situations */ case 1: statement-1; break; case 2: statement-2; break;... case 49: statement-49; break; default: statement-50; }

7 Switch The way the if-else cascade works is to test –Test if i == 1 –If that fails, test if i == 2. If that fails, test if i == –When we test i == 27, we have done 26 prior tests. With the switch statement, we achieve the same effect, but probably faster: –Usually compiled into assembly language as a jump table –Array of "go to" instructions subscripted by the value of i –If i = 27 we "look up" the “go to” at address 27 in table –And execute only that one “go to” –Note the need for break statements. The default action is to cascade down to the code for the next case!!

8 ASCII Hex to Integer int axtoi (char s[ ]) { int i, n, flag; n = 0; flag = 1; for ( i = 0; flag; i++) /* for (i = 0; ; i++) with ending 2 */ switch (s[i]) { case '0': case '1': case '2': case '3': case '4': /* fall... */ case '5': case '6': case '7': case '8': case '9': /* through */ n = 16*n + (s[i] - '0'); break; /* need this so don't fall through */

9 ASCII Hex to Integer (Ending 1) case 'a':case 'b':case 'c':case 'd':case 'e': case 'f': n = 16*n + (s[i] - 'a' + 10); break; case ‘A’: case ‘B’: case ‘C’: case ‘D’: case ‘E’: case ‘F’: n = 16*n + (s[i] - ‘A' + 10); break; default: /* stop “for” loop on non hex digit */ flag = 0; break; } return n; }

10 ASCII Hex to Integer (Ending 2) case 'a': case 'b': case 'c': case 'd': case 'e': case 'f': n = 16*n + (s[i] - 'a' + 10); break; case ‘A’: case ‘B’: case ‘C’: case ‘D’: case ‘E’: case ‘F’: n = 16*n + (s[i] - ‘A' + 10); break; default: /* stop “for” loop on non hex digit */ goto ret; } ret: return n; }

11 Loops –while and for, Section 3.5 This “for” statement” for (expr 1 ; expr 2 ; expr 3 ) statement; Is equivalent to this “while” statement: expr 1 ; while (expr 2 ) { statement; expr 3 ; }

12 For loop Note any part of for loop can be left out. for (init; loop-test; increment) If init or increment expression is left out, just not evaluated (program must initialize and increment by other means) If loop-test is left out, assumes permanently true condition and loops forever. (program must break or goto to end to exit the loop)

13 Comma operator: "," Most often used in for loop statement Might have been useful for reverse () in hw1 for (i = 0, j = strlen(s) - 1; i < j; i++, j--) Pairs of expressions separated by "," are evaluated left-to-right and type/value of expression is type value of result. See K & R, pg 53, Precedence Table

14 Do … While, Section 3.6 The do … while tests at the end of the loop do { statement(s); } while (expression); Executes the statement(s) once even if the “while” loop expression is false upon entry Used much less often than “for” and “while”

15 Break and Continue, Sections 3.7 The break statement works for: –for loop / while loop / do loop and switch. –Brings you to end of loop or switch statement ONE LEVEL ONLY. The continue statement works for: –for loop / while loop / do loop, but not switch! –It causes next iteration of enclosing loop to begin

16 GOTO and Labels, Section 3.8 The goto statement breaks TWO OR MORE levels It “goes to” a statement with a label, e.g. ret: It may be OK to use goto if you always do it in a FORWARD direction. (See axtoi, if use a goto ret; then don't need flag or test in “for” loop.) K&R, pg66: “goto should be used rarely, if at all.” Note: Project coding standards may prohibit “goto”!

17 GOTO and Labels, Section 3.8 Don’t implement if-else this way! if (!condition) goto elselabel; printf("if clause\n"); goto nextlabel; elselabel: printf("else clause\n"); nextlabel: /* next statement in program */

18 Binary Search /* binsearch: find x in v[0] <= v[1] <=.. <= v[n-1] returns subscript of x if found, -1 if not */ int binsearch ( int x, int v[ ], int n) { int low, high, mid; low = 0; high = n - 1;

19 Binary Search while ( low <= high) { mid = (low + high)/2; if (x < v[mid]) high = mid - 1; else if (x > v[mid]) low = mid + 1; else /* found match */ return mid; } return -1; /* no match */ }