Variables

Back in our temp.rb file, our wait method always waits three seconds. 0:00 What if we wanted to be able to specify how long it should wait for? 0:05 Puts and print both take text that they want displayed. 0:11 Sleep takes a number of seconds to sleep for, but how are they doing that? 0:14 Before we can answer that we need to explain variables. 0:19 Variables aren't a complex concept but they're fundamental to programming. 0:22 You probably remember working with variables in your Algebra class. 0:27 They were often denoted with with an X or a Y and 0:30 were used to hold a variety of possible values. 0:33 That is, variables are names for values that can vary. 0:36 In this equation Y = X + 5, if we set X equal to 1, then Y would equal 6. 0:40 If we set X equal to 10, then Y would equal 15, and so on. 0:47 Variables in a programming language are like that, but 0:52 they can hold other things besides numbers. 0:55 They can hold texts or a calendar date, or 0:57 any other piece of data that can be stored in your computer's memory. 0:59 Let's create a new Ruby source file so 1:03 we can play around with variables a little bit. 1:05 I'll click on the file menu, choose New File, and 1:07 I'll type the name of variables.rb. 1:10 When you assign a value to a variable, 1:14 you're giving that value a name that you can refer to it by. 1:16 You assign a value to a variable with single equal sign. 1:20 So, let's create a new variable named number and 1:23 assign a value to it with =, and we'll give it a value of 4. 1:27 Then we'll create another variable named greeting, and 1:31 we'll assign a value to that with equals. 1:34 And this one, we're going to assign a string value to. 1:36 So we open our string with a quotation mark. 1:39 We'll make the string say hello. 1:42 And we'll end the string with another quotation mark. 1:44 We can then use that variable anywhere we might use the original piece of data. 1:47 So let's try making a call to puts to print a value out. 1:51 And instead of passing puts the number 4, I'm going to pass it the variable number. 1:55 Let's try saving that and running it. 2:03 Down here in the console I will type ruby, 2:05 space, and then our new source code file name, variables.rb. 2:11 And you can see that it prints out the number 4. 2:16 It prints out the value that the variable number holds. 2:18 Let's try doing the same with the greeting variable. 2:23 We'll call it the puts method, and 2:25 as an argument we'll pass it the variable greeting. 2:27 Save that and try running it again. 2:30 And first it will print the value of number followed by the value of greeting. 2:33 We can even use variables within other expressions. 2:36 Let's make another call to puts, and 2:40 this time we'll pass it the result of adding 2 to the value in number. 2:43 Save that, try running it. 2:47 And you see that first it prints the original value of number four, 2:53 then the value of greeting, hello, and 2:56 then it prints the value resulting from adding 2 to the value in number. 2:58 You get 6 as a result. 3:04 It's worth noting that math operations like this don't affect the value 3:06 that's in the variable, unless you assign that new value back to the variable. 3:10 So if we were to call puts again here and 3:15 pass number to it and try running that again. 3:18 You'll see that the original value held in number is 4, 3:22 we add 2 to it and print that out here and get the result 6. 3:26 But you can see that the value contained within the number variable hasn't changed, 3:30 it's still 4 and that's what gets printed last. 3:34 Let's do one more operation with the value and 3:39 number we'll subtract, 12 minus number. 3:43 Try saving that. 3:46 Run it. 3:47 And you can see that it finally prints the value 8 because number still holds 4, 3:50 and 12 minus 4 is 8. 3:56 If we change the value that a variable holds, 3:59 the remainder of the program will use that new value instead of the old one. 4:02 So let's try changing the value and number to 6 here at the top of the program, and 4:06 we'll try changing the string in the greeting variable from hello to hi. 4:10 Try running that. 4:14 And you can see the results reflected in the output here, 4:17 it prints out the value in number which is now 6 instead of four. 4:20 It prints out the value of your greeting which is now hi, instead of hello. 4:24 Number plus 2 is now 8 because 6 plus 2 is 8. 4:28 And 12 minus number is 6 because number still holds 6. 4:32 We can even replace the value a variable holds in the middle of a program. 4:38 So here after number plus 2, let's try assigning a new value to number. 4:42 We'll say number equals 10. 4:46 Try running that. 4:48 And you'll see that the program starts 4:50 printing 6, because that's the value that number holds at first. 4:55 And here it prints 8, because 6 plus 2 is 8. 5:00 But here we changed the value to 10. 5:03 And so, 12 minus 10 gives us a result of 2. 5:06 We were able to change the value that our number variable held 5:09 in the middle of our program. 5:13