Arrays (Lists) in Python................

CSE 143
Lecture 1
Arrays (review)
slides created by Marty Stepp
http://www.cs.washington.edu/143/

2
Arrays (7.1)
• array: An object that stores many values of the same type.
– element: One value in an array.
– index: A 0-based integer to access an element from an array.
index 0 1 2 3 4 5 6 7 8 9
value 12 49 -2 26 5 17 -6 84 72 3
element 0 element 4 element 9

3
Array declaration
type[] name = new type[length];
– Example:
int[] numbers = new int[10];
– All elements' values are initially 0.
index 0 1 2 3 4 5 6 7 8 9
value 0 0 0 0 0 0 0 0 0 0

4
Accessing elements
name[index] // access
name[index] = value; // modify
– Example:
numbers[0] = 27;
numbers[3] = -6;
System.out.println(numbers[0]);
if (numbers[3] < 0) {
System.out.println("value 3 is negative");
}
index 0 1 2 3 4 5 6 7 8 9
value 0 0 0 0 0 0 0 0 0 0
index 0 1 2 3 4 5 6 7 8 9
value 27 0 0 -6 0 0 0 0 0 0

5
Out-of-bounds
• Legal indexes: between 0 and the array's length - 1.
– Reading or writing any index outside this range will throw an
ArrayIndexOutOfBoundsException.
• Example:
int[] data = new int[10];
System.out.println(data[0]); // okay
System.out.println(data[9]); // okay
System.out.println(data[-1]); // exception
System.out.println(data[10]); // exception
index 0 1 2 3 4 5 6 7 8 9
value 0 0 0 0 0 0 0 0 0 0

6
The length field
name.length
• An array's length field stores its number of elements.
for (int i = 0; i < numbers.length; i++) {
System.out.print(numbers[i] + " ");
}
// output: 0 2 4 6 8 10 12 14
– It does not use parentheses like a String's .length().

7
Quick initialization
type[] name = {value, value, … value};
– Example:
int[] numbers = {12, 49, -2, 26, 5, 17, -6};
– Useful when you know what the array's elements will be.
– The compiler figures out the size by counting the values.
index 0 1 2 3 4 5 6
value 12 49 -2 26 5 17 -6

8
The Arrays class
• Class Arrays in package java.util has useful static
methods for manipulating arrays:
Method name Description
binarySearch(array, value) returns the index of the given value in a
sorted array (< 0 if not found)
copyOf(array, length) returns a new array with same elements
equals(array1, array2) returns true if the two arrays contain
the same elements in the same order
fill(array, value) sets every element in the array to have
the given value
sort(array) arranges the elements in the array into
ascending order
toString(array) returns a string representing the array,
such as "[10, 30, 17]"

9
Array as parameter
public static type methodName(type[] name) {
– Example:
public static double average(int[] numbers) {
...
}
• Call:
methodName(arrayName);
– Example:
int[] scores = {13, 17, 12, 15, 11};
double avg = average(scores);

10
Array as return
public static type[] methodName(parameters) {
– Example:
public static int[] countDigits(int n) {
int[] counts = new int[10];
...
return counts;
}
• Call:
type[] name = methodName(parameters);
– Example:
int[] tally = countDigits(229231007);
System.out.println(Arrays.toString(tally));

11
Exercise
• Write a method named stutter that accepts an array of
integers as a parameter and returns a new array, twice as long
as the original, with two copies of each original element.
– If the method were called in the following way:
int[] a = {4, 7, -2, 0, 15};
int[] a2 = stutter(a);
System.out.println("a is " + Arrays.toString(a));
System.out.println("a2 is " + Arrays.toString(a2));
– The output produced would be:
a is [4, 7, -2, 0, 15]
a2 is [4, 4, 7, 7, -2, -2, 0, 0, 15, 15]

12
Exercise solutions
public static int[] stutter(int[] a) {
int[] result = new int[a.length * 2];
for (int i = 0; i < a.length; i++) {
result[2 * i] = a[i];
result[2 * i + 1] = a[i];
}
return result;
}
public static int[] stutter(int[] a) {
int[] result = new int[a.length * 2];
for (int i = 0; i < result.length; i++) {
result[i] = a[i / 2];
}
return result;
}

13
Testing code (bonus)
• Q: How can we tell if our stutter method works properly?
– A: We must test it.
• Q: How do we test code?
– A: Call the method several times and print/examine the results.
• Q: Can we test all possible usages of this method?
Q: Can we prove that the stutter code has no bugs?
– A: No; exhaustive testing is impractical/impossible for most code.
– A: No; testing finds bugs but cannot prove the absence of bugs.

14
How to test code
• test case: Running a piece of code once on a given input.
• Q: Which cases should we choose to test?
– equivalence classes of input : Think about kinds of inputs:
• positive vs. negative numbers vs. 0; null (maybe)
• unique values vs. duplicates (consecutive and non-consecutive)
• an empty array; a 1-element array; a many-element array
• Q: What are some properties to look for in testing code?
– boundaries : Hits cases close to a relevant boundary, e.g. the
maximum allowed value, the first/last element in an array, etc.
– code coverage : Hits all paths through code (if/elses, etc.)
– preconditions : What does the method assume? Does the code
ever violate those assumptions?

15
Exercise
• Write a short piece of code that tests the stutter method.
– Decide on a group of test input cases.
– For each test case:
• Print the array's contents before and after stuttering.
• Print whether the test was successful or failed.

16
Exercise solution 1
public static void main(String[] args) {
int[] a1 = {1, 2, 4, 5, 6};
int[] a2 = stutter(a1);
System.out.println(Arrays.toString(a2));
...
}
• Pros:
– simple, short
• Cons:
– must manually check output to see if it is correct
– must copy/paste to create each test case (redundant)

17
Exercise solution 2
public static void main(String[] args) {
test(new int[] {1, 2, 4, 5, 6, 8},
new int[] {1, 1, 2, 2, 4, 4, 5, 5, 6, 6, 8, 8});
test(new int[] {0, 0, 7, 9},
new int[] {0, 0, 0, 0, 7, 7, 9, 9});
test(new int[] {-50, 95, -9876},
new int[] {-50, -50, 95, 95, -9876, -9876});
test(new int[] {42}, new int[] {42, 42});
test(new int[] {}, new int[] {});
}
public static void test(int[] a, int[] expected) {
int[] a2 = stutter(a);
System.out.print(Arrays.toString(a) + " -> " +
Arrays.toString(a2) + " : ");
if (Arrays.equals(a2, expected)) {
System.out.println("Pass");
} else {
System.out.println("FAIL!!!");
}
}

Arrays (Lists) in Python................

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