Tonight’s JavaScript Topics • Conditional Statements: if and switch • The Array Object • Looping Statements: for, while and do-while Conditional Statements in JavaScript • A conditional statement is a statement that runs a command or command block only when certain conditions are met. The if Statement in JavaScript • To test a single condition, use: if (condition) { commands } • To test between two conditions, use: if (condition) { commands if condition is true } else { commands if otherwise } The if Statement in JavaScript (cont.) • To test multiple conditions, use: if (condition 1) { first command block } else if (condition 2) { second command block } else if (condition 3) { third command block } else { default command block } The Switch Statement in JavaScript • To test for different values of an expression, use: switch (expression) { case label1: commands1 break; case label2: commands2 break; case label3: commands3 break; ... default: default commands } • The break statement terminates any program loop or conditional. A Complete Example • Let’s modify last week’s dice program to play a simplified version of craps: – if the dice roll is 2, 3 or 12, the user loses (craps) – if the dice roll is 7 or 11, the user wins – any other number and the user continues rolling • We’ll use if statements to do the job. As an outside-class exercise, try changing the program to use a switch statement. The Array Object in JavaScript • An array is a collection of data values organized under a single name – Each individual data value is identified by an index • To create an array: var array = new Array(length); • To populate an element of the array: array[i] = value; • To create and populate an array: var array = new Array(values); • To get the size of the array, use the property array.length Some Useful Array Methods A Program Example Using an Array • Let’s write a JavaScript program that does the following: – displays a button on the web page – whenever the user presses the button, a random fortune will be displayed to the user. – the list of “random” fortunes will be stored in an array Looping in JavaScript: The for Loop • A program loop is a set of commands executed repeatedly until a stopping condition has been met • for loops use a counter variable to track the number of times a block of commands is run • To create a for loop, use: for (start at; continue until; update by){ commands to repeat } A for Loop Example Another for Loop Example • for loops are often used to cycle through the different values contained within an array: Loops in JavaScript: The while Loop • A while loop runs as long as a specific condition is true • To create a while loop, use: while(continue until condition){ commands to repeat } • Example: var sum = 0; while(sum < 10) { sum = sum + Math.random(); } Loops in JavaScript: The do-while Loop • A do-while loop runs as long as a specific condition is true but the condition is at the end • To create a do-while loop, use: do { commands to repeat while(continue until condition); • A do-while loop is always executed at least one time which isn’t necessarily true of while loops. • Watch out for infinite loops! A Final Example: Rock, Paper, Scissors 1. For player 1, generate a random # between 1 and 3. 1= rock, 2=paper, 3-scissors. 2. For player 2, generate a random # between 1 and 3. 1= rock, 2=paper, 3-scissors. 3. Figure out which player wins by the rules: - paper covers rock (paper wins) - scissors cuts paper (scissors win) - rock breaks scissors (rock wins) - both players have the same item (tie) 4. Display an alert message like: “Player 1 has rock. Player 2 has paper. Player 2 wins.” 5. Keep playing if they tie until one player wins.