Homework

1. Homework • Any Questions?

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. Else-if, Section 3.3 • 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" */

5. Switch, Section 3.4 • 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; }

6. 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 == 3. . . . • 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!!

7. Loops –while and for, Section 3.5 • This “for” statement” for (expr1; expr2; expr3) statement; • Is equivalent to this “while” statement: expr1; while (expr2) { statement; expr3; }

8. 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)

9. 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

10. 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”

11. 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

12. 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, pg 66: “goto should be used rarely, if at all.” • Note: Project coding standards may prohibit “goto”!

13. 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;

14. 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 */ }

15. Functions / Program Structure • C makes functions efficient and easy to use • Program != a few big functions • Program = = many small functions • Functions may reside in separate source files • Source files compiled and loaded separately • An Introduction to MAKEFILE is coming (A process that K&R does not go into)

16. Function Prototypes • Return type, function name, and ( ) int foo ( ); float foo ( ); • Argument List List of Types and optionally Variable Names int foo (int *, int, float); int foo (int array[], int i, float j); Output arguments usually listed first

17. Function Prototypes • Special case for null argument lists • Always code function prototype as: int foo (void); • Keeps compiler parameter checking enabled • Demonstration of bad example: int foo (); x = foo (i, j, k); /* compiler won’t catch! */

18. Function Declarations • Same as function prototype, except: • Must have variable names in argument list • Followed by { function statements; } not ; • Example: int foo (int array[], int i, float j) { function statements; }