Practice with if-then-else. COMP101 Lecture 7 Spring 2018

Transcription

1 Practice with if-then-else COMP101 Lecture 7 Spring 2018

2 Announcements PS01 - Due Friday at 11:59pm 3 parts, each in increasing difficulty. You will submit individually for each. Tutoring - Tomorrow from 2-6pm in SN115 Come review problems you missed on quiz or other concepts. Quiz Will cover commonly missed questions on previous quiz (review yours!) Will have if-then-else questions similar to those we do in PollEv today.

3 Fill in the blank with c's type. Choose c's initial value. let a: string = "1"; let b: number = 2; let c: = a + b;

4 What are the printed values of a, b, c? let a: number = 32; let b: number = a; a = 16; let c: boolean = a === b; print(a); print(b); print(c);

5 if-then-else Statements General form of an if-then-else statement: if (<"test" - boolean expression>) { <then block runs when test is true> else { <else block runs when test is false> Works the same as an if-then statement, however, when the test expression evaluates to false the statements within the else block will run. Once either block completes, the computer resumes at the next line following the else block.

6 if-then-else Statements Notice, like the if-then statement, the then block runs only when the test condition is true Unlike the if-then statement, the else block runs only when the test condition is false After either the then-block or else-block complete, they both continue to the same next step if test then else true false

7 What is printed when 55 is entered for x? let x: number = await promptnumber("x"); if (x < 55) { print("u"); if (x >= 10) { print("n"); if (x > 50) { print("c");

8 What values could be entered for x and y such that the letters "a" and "d" would be printed out? let x: number = await promptnumber("enter x:"); let y: number = await promptnumber("enter y:"); if (x!== y) { print("a"); else { print("b"); if (x > y) { print("c"); else { print("d");

9 What value of x would cause both "BEAT DOOK" and "BISCUITS" to print out? let x: number = await promptnumber("x"); if (x < 55) { print("go UNC"); else { print("beat DOOK"); if (x >= 55) { print("jump AROUND"); else { print("biscuits");

10 What is printed when 8 is entered for x? What is printed when 10 is entered for x? let x: number = await promptnumber("x"); if (x < 10) { x = x * 2; else { x = x / 2; if (x > 10) { x = x + 2; else { x = x - 2; print(x);

11 What is printed when 11 is entered for x? let x: number = await promptnumber("x"); let y: number = 0; if (x > 10) { y = 10; else { if (y === 10) { y = 20; else { y = 30; print(y);

12 Hands-on: Calling the random Function Open 00-random-app.ts 1. At TODO 1: Declare a number variable named choice. Syntax reminder: let <name>: <type>; 2. At TODO 2: Assign to choice the result of calling the random function as shown below: choice = random(0, 100); 3. At TODO 3: Print the value of the choice variable. 4. Save. Try refreshing your browser a few times. Try changing 0 and 100 to different numbers. Check-in on PollEv.com/compunc when your program is printing random numbers.

13 export let main = async () => { ; // TODO 1: Declare a number variable named choice let choice: number; // TODO 2: Initialize choice to be assigned random(0, 100) choice = random(0, 100); // TODO 3: Print the value of the choice variable print(choice); // TODO 4: Try refreshing your browser a few times

14 Generating Random Numbers The introcs Library has a special function for generating random numbers called... random Before using random, we must import it into our program like print: import { print, random from "introcs"; The random function generates a random number, so we can use it anywhere we can use a number: let choice: number = random(1, 10); "Let choice be a number variable that is assigned the result of calling the random function with the values 1 and 10." The two numbers we "give" to the random function specify the bounds of the random number generated (a number between 1 and 10).

15 Magic 8 Ball

16 Hands-on: Magic 8-Ball Open: 01-magic-8-ball-app.ts Write an if-then-else statement (syntax below) at TODO #1 that will: 1. test if the choice variable is equal to zero and then print "Very doubtful" if so 2. else, print "Yes definitely" if-then-else statement syntax: if (<test>) { // then block else { // else block Check-in on pollev.com/compunc when your program prints one of these messages

17 print("welcome to the Magic 8 Ball"); let question: string = await promptstring("what is your question?"); let choice: number = random(0, 1); // TODO #1: Implement an if-else if (choice === 0) { print("very doubtful"); else { print("yes definitely");

18 Nesting if-then-else statements within if-then-else statements The then and else blocks may contain one or more statements but isn't if-then a statement? Yes! You can write further if-then statements inside of then or else blocks and the same rules apply.

19 Pattern: Nesting if-then in an else Pattern It is commonly useful to nest additional if-then-else statements inside of subsequent else-blocks Why? It allows us to choose one branch from many possible options. "If choice is 0 then do X, otherwise if choice is 1 then do Y, otherwise do Z." if (choice === 0) { print("very doubtful"); else { if (choice === 1) { print("yes, definitely"); else { print("ask again later");

20 Hands-on: Improving the Magic 8-Ball Still in: 01-magic-8-ball-app.ts 1. Change the choice variable to be initialized to a random value between 0 and 2 instead of 0 and Add a nested if-then-else statement inside of the current else block. The nested if-then-else should look like the code to the right. Notice the indentation! Check-in when all three answers are being printed. if (choice === 0) { print("very doubtful"); else { if (choice === 1) { print("yes, definitely"); else { print("concentrate and ask again");