Computer Science 15-111 (Sections A & B), Spring 2007

Class Notes: 19-Jan-2007

Course Web Site, Syllabus, etc: See http://www.kosbie.net/cmu/spring-07/15-111/.

Reading:

On Monday and Wednesday, we covered:

Chapter 1+2: Introduction and Program Fundamentals (Number Rep’s)

Appendix A.1: Integer Binary Representation

Appendix B.1: Operator Precedence Table (portions thereof)

This lecture, we are covering:

Chapter 3: Statements and Control Flow

Appendix B.2: The Standard Java Math Functions

Quiz:

Our first quiz, covering Chapters 1-3 and Appendices A-B, is on Wednesday 24-Jan (note: we are moving quickly as this is mostly material you should have seen already).

Topic Outline:

1. From last lecture

a. The importance of reading the book!

b. Scanner example (Section 2.6)

i. import, assert and AssertionException

ii. Problems with sample code (comments, exceptions, UI)

c. A little more on printf
System.out.printf(“<%6.2f>”,13.579) à < 13.58> Convenient!

d. ASCII: Just know: ‘ ‘ (space,32), ‘0’ (48), ‘A’ (65), and ‘a’ (97)

e. Casting: Covered in Section 2.10.2

i. int x = 3.9; // not allowed! Will not compile!

ii. int x = (int) 3.9; // ok! We are casting the double to an int.

iii. double d = (int) 3.9; // ok! Assignment (=) widens int back to a double!

f. Increment / Decrement operators: Covered thoroughly in Section 2.12.

g. Base conversions

2. Blocks, Nested Blocks, and Scope

{

int x = 3;

System.out.println(x); // this is legal!

}

System.out.println(x); // but this is not!

3. The Null Statement

4. Relational and Equality Operators

a. <, <=, ==, >=, >, !=

// good, concise, clear! boolean flag = (x != y);

// bad boolean flag; if (x != y) flag = true; else flag = false;

5. Logical Operators

a. && (and), || (or), and ! (not)

b. Short-circuit evaluation!

Very handy use: if ((index < array.length) && (array[index] == foo)) …

c. Precedence:

i. (true || true && false || false)
Is this: ((true || true) && (false || false)) == (true && false) == false
Or: (true || (true && false) || false) == (true || false || false) == true ?

ii. Best answer: avoid this with parentheses!
(true || (true && false) || false)
(a || (b && c) || d)

6. Conditional Statements

a. if ( BooleanExpr ) Statement

i. Single statement body:
if (x < 10) x += 3; // this is usually ok style
Or:
if (x < 10)
x += 3; // this is ok, too

ii. Block (multi-statement) body:

// correct usage if (x < y) { // use braces!!! x += 3; y = y / 2; }

// incorrect usage if (x < y) // no braces!!! x += 3; y = y / 2; // always runs!

b. if ( BooleanExpr ) Statement else Statement

i. Beware the Dangling Else! Solution: Braces!!!

// correct usage // “else” binds to outer “if” if (x < y) { if (y < z) y++; } else x++;

// incorrect usage // “else” binds to inner “if” if (x < y) if (y < z) y++; else x++;

ii. Avoid redundant logic, especially in final boolean expression:

// correct usage if (x < y) y++; else x++;

// incorrect usage // redundant logic in “else-if” if (x < y) y++; else if (x >= y) x++;

7. Looping Statements

a. while ( BooleanExpr) Statement

b. do Statement while ( BooleanExpr )

c. for ( ForInit ; BooleanExpr ; UpdateExpr ) Statement
The same as:
ForInit;
while ( BooleanExpr ) {
Statement
UpdateExpr
}

8. Adding the integers from 1 to 10:
public static void main(String[] args) {

int sum, counter;

// for loop:

sum = 0;

for (counter = 1; counter <= 10; counter++)

sum += counter;

System.out.println(sum);

// while loop:

sum = 0;

counter = 1;

while (counter <= 10) {

sum += counter;

counter++;

}

System.out.println(sum);

// do-while loop:

sum = 0;

counter = 1;

do {

sum += counter;

counter++;

} while (counter < 11);

System.out.println(sum);

}

9. Break and Continue statements (see examples in Section 3.8)

10. Switch statement (see examples in Section 3.9)

11. DeMorgan’s Laws: !(a || b) == (!a && !b), !(a && b && c) == (!a || !b || !c)

12. Math functions:
abs, min, max, exp, log, sqrt, pow, ceil, floor, rint, round, round, random,
sin, cos, tan, asin, acos, atan, atan2