CS100J Lecture 11

1. CS100J Lecture 11 • Previous 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 6.1-6.2 • Savitch, Sections 6.1 – 6.3 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? Lecture 11

3. A Relevant Pattern 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 grade = in.readInt(); } g • where • a : /* Initialize 101 counters to 0. */ • b : /* Increment counter for grade. */ • g : /* Print histogram. */ Lecture 11

4. A Tedious Solution int grade; // the grade being processed. // For i in 0..100, 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 = freq100 + 1; grade = in.readInt(); } /* Print histogram. */ System.out.println(”Grade Frequency”); System.out.println( 0 + ” ” + freq0 ); System.out.println( 1 + ” ” + freq1 ); . . . System.out.println( 100 + ” ” + freq100 ); Lecture 11

5. An Elegant Solution int grade; // the grade being processed. // For i in 0..100, 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; grade = in.readInt(); } /* Print histogram. */ System.out.println(”Grade Frequency”); grade = 0; while ( grade <= 100 ){ System.out.println( grade + ” ” + freq[grade] ); grade = grade + 1; } Lecture 11

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

8. The Index Operator [] • 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; Lecture 11

9. Subscripts • The phrase identifier[expression] is likeidentifier.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. Lecture 11

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

11. More Shorthand • 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; Lecture 11

12. An Elegant Solution, revisited int grade; // the grade being processed. // For i in 0..100, 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]++; grade = in.readInt(); } /* Print histogram. */ System.out.println(”Grade Frequency”); for ( grade = 0; grade<=100; grade++ ) System.out.println( grade + ” ” + freq[grade] ); Lecture 11