Switch and While CSD 340 Blum 1 From

Switch and While CSD 340 (Blum) 1

From if to switch CSD 340 (Blum) 2

Concentric tables: alternating colors (I) CSD 340 (Blum) 3

Concentric tables: alternating colors (II) CSD 340 (Blum) 4

Concentric tables: alternating colors (III) CSD 340 (Blum) 5

if(i % 2 == 0) { Color= "FFCC 99"; } else { Color="99 CCFF"; } • The alternating colors resulted from dividing the counter i by 2 and asking if the remainder is 0. (It can only be 0 or 1. ) If the remainder is 0, the Color variable is assigned one value, otherwise it is assigned another. CSD 340 (Blum) 6

Three Colors CSD 340 (Blum) 7

Three Color Code (version 1) CSD 340 (Blum) 8

Switching to a better statement • In a situation in which you find yourself asking almost the same question over and over – – Is the expression equal to Value 1? Is the expression equal to Value 2? Is the expression equal to Value 3? Etc. • There is a special statement to replace the sequence of ifs It is called a switch. All of the various conditions are called cases. • See page 84 in Beginning Java. Script (Paul Wilton) CSD 340 (Blum) 9

Same result with switch CSD 340 (Blum) 10

Three Color Code (version 1) CSD 340 (Blum) 11

switch(i % 3) Expression upon which cases { are based case 0: Beginning of code to be Color = "FFCC 99"; executed if expression had break; the value of 0. case 1: If you are done executing Color = "99 CCFF"; code associated with the break; condition of the expression case 2: having a value of 0, then Color = "FFCCCC"; leave the switch structure. break; } CSD 340 (Blum) 12

Give me a break! • A switch has a strange behavior. If the expression is equal to 1, the program executes the case 1 code. However, it will continue to execute the code for the following cases unless you explicitly put in that break code that tells it to exit the switch structure. CSD 340 (Blum) 13

Missing break. CSD 340 (Blum) 14

It loses the bluish color (case 1). CSD 340 (Blum) 15

switch(sum) The craps game { is a situation in case 2: which one might alert(“Snake eyes”); find this “fall break; through” case 3: behavior useful. alert(“Sorry you lose. ”); There are break; several cases case 4: case 5: that yield the case 6: same effect. alert(“The point is ”+sum+ “ Roll again. ”); break; … }CSD 340 (Blum) 16

If versus Switch • The more “cases” there are, the more convenient it is to use a “switch”. • But use “if” when the condition is an inequality such as less than or greater than. • Also use “if” when the condition involves comparing an expression to a variable as opposed to a set value. – E. g. you cannot use a switch statement in the second roll of craps because of of the cases involves the sum equaling the point which is a variable determined by the first roll. CSD 340 (Blum) 17

Ripe for switch: Six cases CSD 340 (Blum) 18

Six cases (code) CSD 340 (Blum) 19

While loop • A variation on the for-loop repetitive structure is the while loop. • The for loop has a built in counting element and is more suitable in situations in which one knows when starting the loop how many iterations there will be. • The while loop is more suitable in situations in which one does not know the number of iterations at the start of the loop. CSD 340 (Blum) 20

While loop example • If you were testing your craps program, you might have noticed that you never seem to roll a snake eyes when you want to. • You might ask the question: How many rolls does it take to roll snake eyes for the first time? – It’s possible you could roll it right off, but it’s possible it could take hundreds of rolls. CSD 340 (Blum) 21

Waiting for Snake Eyes (Code I) CSD 340 (Blum) 22

Waiting for Snake Eyes (Code II) CSD 340 (Blum) 23

Waiting for Snake Eyes (Browser I) CSD 340 (Blum) 24

Waiting for Snake Eyes (Browser II) CSD 340 (Blum) 25

while(sum!=2) { dice. Throw 1 = (Math. floor(Math. random() * 6) + 1); dice. Throw 2 = (Math. floor(Math. random() * 6) + 1); sum=dice. Throw 1+dice. Throw 2; //alert(sum); Times. Rolled++; } • This says that so long as the sum variable is not equal to 2 that the program will continually simulate the roll of two dice, add their results and increment a counter. • The variable sum can be started off at any value other than 2 so that we get into the loop in the first place. • The variable Times. Rolled starts off at zero. It counts the number of times we rolled the dice and before the loop we rolled them zero times. • Note that any counting must be done inside the while loop’s curly brackets. CSD 340 (Blum) 26

Reference • Beginning Java. Script (Paul Wilton) CSD 340 (Blum) 27