Computer Science: The Third Presentation

1. Computer Science: The Third Presentation For Loops, Multiple Inputs, Miscellaneous

2. Review • Last time we learned that “=“ is an assignment operator used to assign values to variables • It’s important that we declare the variable’s type when we declare a variable • Also learned about primitive data types- Java reserved data types • Ones you need to know: int, double, boolean

3. While Statements • We learned about while statements, syntax: while ([something holds true) { //do something } • We have to remember to move the while loop toward termination • Eventually the [something holds true] must not be true

4. If Statement • We also learned if statement, syntax: if ([something is true]) { //do something } • We usually follow this with an “else” statement: if this is true do this, or else do this

5. Major Things in Programs You Wrote • Things we need to remember: • Not okay: while (condition); statement1; • From the book: It is important not to put a semicolon after while(condition). With a semicolon, the loop would have no body, only an empty statement; statement1 would be left completely out of the loop. • When intializing variables, be sure to include the variable type • Eg “count = 1;” is not valid • “double count = 1;” is

6. A Reminder • The powerpoints are not supposed to be a replacement for the book • They are supposed to provide clarification on beginning concepts • So read the book • John Shumway, Block 1 and 2

7. First, some additional syntax • In java, we are often adding, subtracting, etc variables • There is a shorthand way for writing • sum = sum +2; • sum +=2; • The two ways are identical • We could also use “sum -= 2;” or “sum *= 2”

8. Incrementing • We are also often adding one to our variables- think back to our while loop for sum of squares, where we added one to count at the end of the loop • Instead of writing out “count = count + 1;” or even “count +=1;” there is an even simpler way • “count++;” • Could also use “count--;”

9. For Loops (copied from the book) • Think back to while loops • Here’s a typical while loop • int n = 1; int sum = 0; while (n < 10) { sum += n; n++; } • Can identify three elements of this while loop

10. Parts to a While Loop • int n = 1; • int sum = 0; • while (n < 10) • { • sum = sum + n; • n++; • } • Initialization: we must initialize the variable to be used in the condition before the loop is entered • int n = 1; • Testing: condition is tested for truthfulness before it executes another pass through the loop • Is n < 10? • Change: our condition must at some point be false, otherwise – INFINITE LOOP • n++;

11. “For” Loops • What if we could combine the three elements from a while statement into one cohesive statement? • We can • for (initialization; test; change) { //do something }

12. How would we approach our sum of squares method while using a for loop? • private static int sumOfSquares (int n) { int sum = 0; for (int j = 1; j <= n; j++) { sum +=j; } return sum; } • Our “counter” variable is j, we set it equal to one • Our test: is j less than or equal to n? • After each execution of the for loop, add one to j

13. This is one of those things you just have to remember, you’ll see it very, very often on the AP exam • for (initialization; test; change) • You’re going to have to painstakingly traverse complicated for loops- look forward to it

14. Assignment • Write a method that, using for loops and what you’ve learned so far takes n as an input and returns the nth Fibonacci number • Hints: for n=1, and n= 2 the method should return 1 without any calculation. How could you do this? • With a somewhat complicated program like this, it helps to write out/think about how you would do this before putting it in abstract computer code

15. Simple Piece of New Info • A method can take multiple inputs • For example, say we wanted to write a method that takes two numbers a and b and multiplies them together • private static int multiplyNumbers (int a, int b) { return a*b; } • Notice there is no need to create a new variable

16. Assignment • Write a method that takes integers a, b, and c as inputs and returns the area of a triangle with a, b and c as side lengths • Hints: • Can you test if a, b, and c are valid triangle lengths and return “0” if they are not? • If we have an arithmetic series: a + (a+d) + (a+2d) +…+(a + (n-1)d) write a program that takes integers, a (beginning term), d (difference), and n (number of terms) and returns the sum of the series

17. Another random piece of Syntax • There is another big command out there besides adding, subtracting, dividing, and multiplying • That’s taking the mod of a number • If you don’t know what a mod is, wikipedia or google are excellent sources • Basically saying “ 7 mod 5 is congruent to 2” is equivalent to saying the remainder when we divide 7 by 5 is 2

18. We write this as • 7 Ξ 2 (mod 5) • More mod practice: • 6 Ξ 1 (mod 5) • 17 Ξ 8 (mod 9) • 78 Ξ 8 (mod 10) • 176532 Ξ 2 (mod 10) • Notice that taking a number mod 10 gives us the last digit of that number • If you’re still confused, read a math book

19. Java Syntax • Suppose we want to take the mod of a number in Java…for example, we wish to know the remainder when 5 is divided by 3 • We want to set the integer variable j equal to this • int j = 5%3; • Really the only time I’ve used this is to take numbers mod 10, mod 100, or mod 1000 to get the last 1, 2, or 3 digits respectively

20. What’s Ahead • Next time, we will consider arrays and ArrayLists, Strings, and what’s happening with the rest of the programs outside the method • Eclipse • Then write a bunch of programs to learn how to write a good program • At some point we will consider the hard part of Java- classes • Apologies for the short presentation this week