assertion: A statement that is either true or false.

Transcription

1 Logical assertions assertion: A statement that is either true or false. Examples: Java was created in The sky is purple. 23 is a prime number. 10 is greater than 20. x divided by 2 equals 7. (depends on the value of x) Provable Assertion an assertion that can be proven to be true at a particular point in program execution Help simplify code Understand code better 1

2 Program Verification A field of computer science that involves reasoning about the formal properties of programs to prove the correctness of a program if (test) { //test is always true here else { //test is always false here while(test) { //test is always true here //test is always false here CSC116: Intro to Java Programming Sarah Heckman 2

3 Assertions in code We can make assertions about our code and ask whether they are true at various points in the code. Valid answers are ALWAYS, NEVER, or SOMETIMES. System.out.print("Type a nonnegative number: "); double number = console.nextdouble(); // Point A: is number < 0.0 here? (SOMETIMES) while (number < 0.0) { // Point B: is number < 0.0 here? (ALWAYS) System.out.print("Negative; try again: "); number = console.nextdouble(); // Point C: is number < 0.0 here? (SOMETIMES) // Point D: is number < 0.0 here? (NEVER) Copyright 2006 by Pearson Education 3

4 Reasoning about assertions Right after a variable is initialized, its value is known: int x = 3; // is x > 0? ALWAYS In general you know nothing about parameters' values: public static void mystery(int a, int b) { // is a == 10? SOMETIMES But inside an if, while, etc., you may know something: public static void mystery(int a, int b) { if (a < 0) { // is a == 10? NEVER...

5 "Sometimes" Things that cause a variable's value to be unknown (often leads to "sometimes" answers): reading from a Scanner reading a number from a Random object a parameter's initial value to a method If you can reach a part of the program both with the answer being "yes" and the answer being "no", then the correct answer is "sometimes". If you're unsure, "Sometimes" is a good guess.

6 Assertion example 1 public static int mystery(scanner console) { int prev = 0; int count = 0; int next = console.nextint(); // Point A while (next!= 0) { // Point B if (next == prev) { // Point C count++; prev = next; next = console.nextint(); // Point D // Point E return count; Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES. Point A Point B Point C Point D Point E next == 0 prev == 0 next == prev Copyright 2006 by Pearson Education 6

7 Assertion example 1 public static int mystery(scanner console) { int prev = 0; int count = 0; int next = console.nextint(); // ASSERT: prev == 0 while (next!= 0) { // ASSERT: next!= 0 if (next == prev) { // ASSERT: next == prev count++; prev = next; next = console.nextint(); // ASSERT: next == 0 return count; Point A Point B Point C next == 0 prev == 0 next == prev Point D Point E Copyright 2006 by Pearson Education 7

8 Using Assertions Use System.out.println statements to print variable values at certain program points Can use System.out.println statements to debug your program based on expected output at various program points Java assert statement Appendix C assert <boolean test>; Computer checks that assertions are correct Failures lead to exceptions Testing assertions are expensive CSC116: Intro to Java Programming Sarah Heckman 8

9 Helpful Method /** * Continuously tests user input until an integer is entered. * Reads in bad input and prints out invalid message with reprompt when * non-integer is entered. * console console Scanner object entered integer */ public static int getint(scanner console) { while (!console.hasnextint()) { // ASSERT: user did not enter an integer console.next(); System.out.print("Not an integer. Try again: "); // ASSERT: user entered an integer return console.nextint(); 9

10 Lab Exercise not in class DiceGame.java assignment: Your DiceGame class must contains a method called dicesum that: prompts the use for a desired sum, then repeatedly simulates the rolling of 2 six-sided dice until their sum is the desired sum. You must perform checks to make sure the user enters an integer between 2 and 12 and reprompt if not, you must also reprompt for nonint values. Your code should include assertion comments for assertions that are always true (provable) in the format: //ASSERT: true statement (eg. sum == desiredsum) Here is a sample dialogue with the user: Desired dice sum: 9 4 and 3 = 7 3 and 5 = 8 5 and 6 = 11 5 and 6 = 11 1 and 5 = 6 6 and 3 = 9

11 Assertions in-class exercise public static void mystery(int x, int y) { int z = 0; // Point A while (x >= y) { // Point B x -= y; // Point C z++; // Point D Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES. // Point E System.out.println( z + " " + x); Point A Point B Point C x < y x == y z == 0 Point D Point E Copyright 2006 by Pearson Education 11