COP2500 Lab 5

COP 2500 Spring 2012

Lab #5

Deliverables: To complete this assignment you must --
1.) Write the HTML and JavaScript page described below.
2.) Email your results to your lab instructor.

Motivation: How do we use lots of variables?

Introduction: We've seen how to sum over several variables:

var x = 5, y = 10, z = 15;
var sum = 0;
sum = sum + x;
sum = sum + y;
sum = sum + z;

Let's extend this to an arbitrary number, say 20, of variables with arrays. This would be tedious to input by hand with the method seen before.

var size = 20;
// Declare the array of 20 numbers
var numbers[size];

An array contains lots of variables, in this case, 20.

...

You can access each variable (element) in the array with a special syntax:

numbers[0] = 5;

This gets the 0th variable in the array (the first one) and assigns it to 5, just like we set x = 5 earlier. Setting the rest:

numbers[1] = 10;
numbers[2] = 15;

What about the rest of the 20 variables? Let's assign them to random integers.
As a review, a random decimal can be created with Math.random(), then scaled to some range, say 100 by multiplying: Math.random() * 100, then rounded to an integer with Math.round(): Math.round(Math.random() * 100).

numbers[3] = Math.round(Math.random() * 100);
numbers[4] = Math.round(Math.random() * 100);
...
numbers[19] = Math.round(Math.random() * 100);

It's still tedious to initialize 20 numbers, but it's easier to keep track of with our numbers array. Here's a very nice way of repeating code, the for loop, which has the general syntax:

for(initialize; condition; increment){
	statement
}

Which gets performed in the following order:
1: initialize
2: condition: if true, perform statement, otherwise skip statement and increment and continue executing like normal.
3: statement
4: increment, repeat from 2

var i, numbers[20];
for(i = 0; i < 20; i++)
	numbers[i] = Math.round(Math.random() * 100);

Let's trace how this code is executed:
1. i and numbers declared in memory.
2. i = 0
3. Check if i < 20. (0 < 20 yes), do statement.
4. numbers[i (which is 0)] is set to a random integer from 0-100.
5. Syntax: i++ is the same as i += 1 is the same as i = i + 1, so i = 0 + 1 = 1.
6. Check if i < 20. (1 < 20, yes), do statement.
7. numbers[1] = random integer from 0 to 100.
...

After examining the trace, we know this will create an array of 20 random 0-100 ranged integers.
Review loops and arrays lecture notes for further explanation. Let's return to the original example with
Problem 1: summing over a list of numbers.
Algorithm:
1. Create an array called list of size 10.
2. Initialize all elements of the list to random integers between 0 and 100 using a for loop.
3. Sum each element in the list with a for loop. You will need a new variable, sum, to store your total.
4. Output the sum with document.write or alert.

Section	Code Outline	Comments
INPUT:	var list[10];	This represents a list of ten numbers.
OTHER VARIABLES:	var sum = 0; var i;
INITIALIZATION:	FOR i = 0 to 9 list[i] = random integer between 0 and 100;	This fills the array with ten random numbers from 0 - 100.
COMPUTATION:	FOR i = 0 to 9 sum = sum + list[i];	This loop simply adds each list element to the sum one by one.
OUTPUT:	Display sum using document.write() or something similar.	Remember to format your results in a sensible way.

Problem 2: Comparing numbers.
Algorithm:
1. Create two arrays called list1 and list2 of size 10.
2. Initialize all elements of the lists to random integers between 0 and 100 using a for loop.
3. Compare each element in list1 to its corresponding element in list2 with a for loop.
4. Output the comparision with document.write.

Section Code Outline Comments

INPUT: var list1[10], list2[10]; This represents two lists
of ten numbers each.

OTHER VARIABLES: var i; We only need the loop
variable.

INITIALIZATION: FOR i = 0 to 9
list1[i] = random number between 0 - 100;
list2[i] = random number between 0 - 100;
i = 0 to 10 not inclusive of 10.

COMPUTATION: FOR i = 0 to 9
IF ( list1[i] > list2[i])
Output list1[i] + "is greater than" + list2[i] with document.write
ELSE
Output list1[i] + "is less than" + list2[i] with document.write

This simply compares two
corresponding numbers
from each list and prints out
which one is bigger.

OUTPUT: No output required since we
have printed out the results
in the loop body.

Open a browser and verify that the file displays properly. In order to obtain full credit for the lab you need to email your source code to your grader.