1. CS100J Lecture 11 Previous Lecture This Lecture
Scope of names and the lifetime of variables
blocks and local variables
methods and parameters
classes, class variables, and instance variables
This Lecture
Java Constructs
arrays and indexing
increment and decrement
for-statements
Reading:
Lewis and Loftus, Sections 3.8 and
Savitch, Sections 6.1 – 6.3
CS 100
Lecture 11

2. Motivating Example
Given a list of 0 or more grades (between 0 and 100) followed by -1, print a histogram of the grades, i.e., a table of grades and their frequencies.
grade frequency
0 0
1 0
2 1
… …
99 18
100 13
Is there a program pattern that applies?
What must be done for each grade?
What must be done before reading any grades?
What must be done after reading all grades?
How many separate counters are needed?
CS 100
Lecture 11

3. A Relevant Pattern where int grade; // the grade being processed.
/* “Process” grades until (but not including) a stopping signal of -1. */ a
grade = in.readInt();
while (grade != -1 ) { b } g
where
a : /* Initialize 101 counters to 0. */
b : /* Increment counter for grade. */
g : /* Print histogram. */
CS 100
Lecture 11

4. A Tedious Solution int grade; // the grade being processed.
// For i in , freqi is # of grades of i.
int freq0, freq1, . . ., freq100;
/* “Process” grades until (but not including) a stopping signal of -1. */
/* Initialize 101 counters to 0. */
freq0 = 0; freq1 = 0; . . .; freq100 = 0;
grade = in.readInt();
while (grade != -1 ) {
/* Increment counter for grade. */
if ( grade == 0 ) freq0 = freq0 + 1; else
if ( grade == 1 ) freq1 = freq1 + 1; else
. . .
if ( grade == 100 ) freq100 = freq ; }
/* Print histogram. */
System.out.println(”Grade Frequency”);
System.out.println( 0 + ” ” + freq0 );
System.out.println( 1 + ” ” + freq1 );
System.out.println( ” ” + freq100 );
CS 100
Lecture 11

5. An Elegant Solution int grade; // the grade being processed.
// For i in , freq[i] is # of grades of i.
int[] freq = new int[101];
/* “Process” grades until (but not including) a stopping signal of -1. */
/* Initialize 101 counters to 0. */
grade = 0;
while ( grade<=100 ) {
freq[grade] = 0;
grade = grade + 1; }
grade = in.readInt();
while (grade != -1 ) {
/* Increment counter for grade. */
freq[grade] = freq[grade] + 1;
/* Print histogram. */
System.out.println(”Grade Frequency”);
while ( grade <= 100 ){
System.out.println(
grade + ” ” + freq[grade] );
CS 100
Lecture 11

6. Trace . . . Input Data 100 99 100 2 . . . -1 grade 0 1 2 99 100 freq
freq
. . .
CS 100
Lecture 11

7. Array Declaration and Construction
An array is an object that consists of a collection of variables (its elements) all with the same type, e.g., array-of-int objects.
The declaration:
int[] identifier ;
declares identifier, a variable that can contain a
reference to an array-of-int object.
The constructor invocation:
new int[ length ]
constructs an array-of-int object that consists of
length int variables with subscripts 0 through
length-1
Thus, the initialized declaration
int[] identifier = new int[ length ];
creates the following structure:
identifier
length-1
. . .
CS 100
Lecture 11

8. The Index Operator [] Examples: The phrase identifier [ expression ]
provides indexed access to an element of the
array referred to by identifier, namely, access
to the element with subscript equal to the value of
expression.
Examples:
freq[1+2+3] = 17; // set freq[6] to 17.
grade = 6;
freq [grade] = 0; // set freq[6] to 0.
// Increment freq[6] by 1.
freq [grade] = freq[grade] + 1;
CS 100
Lecture 11

9. Subscripts
The phrase identifier[expression] is like identifier.field-name except that the “field name” is an integer computed by evaluating the expression.
Subscripts can be specified by arbitrary expressions. The value of the subscript is obtained by evaluating the expression.
The value of the subscript must be in the range 0 through one less than the number of elements in the array.
Do not confuse the subscript of an array element with the value contained in the array element.
CS 100
Lecture 11

10. Some Shorthand Increment and Decrement Operations variable ++
is equivalent to
variable = variable + 1
Similarly,
variable --
variable = variable - 1
Example:
freq[grade]++ ;
CS 100
Lecture 11

11. More Shorthand is shorthand for The statement
for (initialization; condition; increment)
statement
is shorthand for
initialization;
while ( condition ) {
statement ;
increment ; }
Example:
for (grade=0; grade<=100; grade++)
freq[grade] = 0;
CS 100
Lecture 11

12. An Elegant Solution, revisited
int grade; // the grade being processed.
// For i in , freq[i] is # of grades of i.
int[] freq = new int[101];
/* “Process” grades until (but not including) a stopping signal of -1. */
/* Initialize 101 counters to 0. */
for ( grade = 0; grade<=100; grade++ )
freq[grade] = 0;
grade = in.readInt();
while (grade != -1 ) {
/* Increment counter for grade. */
freq[grade]++; }
/* Print histogram. */
System.out.println(”Grade Frequency”);
System.out.println(
grade + ” ” + freq[grade] );
CS 100
Lecture 11