Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 2: Guess that number game
Lecture: Using loops and conditionals

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0:00 So let's write the test for our code
0:02 to see if the guess that the user entered is correct.
0:05 So what we want to do is want to say if, and then write a conditional comparison statement,
0:10 a boolean statement,
0:12 so we can say if the guess, not the text remember,
0:15 the actual number is less than the number,
0:18 then we want to print something out
0:20 and for defining these if statements
0:22 and all sort of code blocks into Python
0:24 we use a colon to indicate we are about to define a block of code
0:29 that is going to run in some situation
0:31 maybe it's a function, maybe it's a loop,
0:32 in this case, it's a block in our if statement.
0:35 So when it's true the guess is less than a number,
0:38 we are going to print out something like too low,
0:41 the guess is too low, the guess is lower
0:43 than the number the computer came up with.
0:45 So that is one part,
0:47 next, we are going to ask else if it's the case so we'll say elif again,
0:52 if the guess is too high, if it's higher than a number we'll print too high, like so,
0:58 and notice, when I hit enter,
1:00 when I type this block, I will just go back for a second,
1:02 if I type x and hit enter, or complex or whatever,
1:06 PyCharm wants to correct it for us
1:08 and I say test and I hit enter, it goes back to the same line here,
1:12 but because PyCharm knows about the structure with colons,
1:14 if I go and I type something like if x: enter,
1:19 it automatically indents for me,
1:20 so you will see these Python intelligent editors like PyCharm
1:24 make it really easy to write code correctly.
1:26 So we have if, indent, print too low,
1:29 else if "elif" guess is too high, print too high,
1:33 and the last thing I want to do is say else: and we'll print you win.
1:38 So there is a few more things we are going to need to do
1:41 to make this work just the way we'd like
1:42 but let's go and run it and see how it's working.
1:45 Ahh what's a good guess?, 54 it must be 54, too high,
1:49 oh, well the way game is supposed to work
1:51 it's supposed to say too high,
1:53 now you've got another chance.
1:55 So what we actually need to do is go around and around,
1:58 and that means we need to find something called a loop,
2:00 and the type of loop that work really well for this situation,
2:04 in Python is something called a while loop.
2:06 So you say while, and then some sort of condition
2:08 goes here that you want to test,
2:10 and as long as that is true,
2:11 you are going to go around and around and around,
2:13 so we want to say long is the guess is not a winning guess,
2:16 we want to keep going around
2:17 so we'll say guess, you say equals like this, remember,
2:21 and if you say not equals you say exclamation equals !=
2:24 So now we'll say colon,
2:26 we need this whole if statement/else if part to run,
2:30 while this loop is going, and the way that we do that in Python
2:35 is we indent it so I can highlight in PyCharm and hit tab
2:38 and it is going to indent everything four spaces for us.
2:41 And then when we are no longer indented,
2:43 if I print something down here, done like so,
2:46 you can see this is actually out of the loop.
2:49 Now, if I run this something really bad is going to happen,
2:52 we are going to end up in what is called an infinite loop,
2:55 because, we get one piece of input here from the person
2:59 we convert it to a guess let's say it's 52, who knows, whatever,
3:03 and when we go in here
3:04 and unless you guess the winning number,
3:06 it is going to go around and around and around
3:08 either say too low, too low, too low, or too high too high, too high,
3:11 and it's never going to give the user another chance to enter this text.
3:14 So what we need to do is make sure that we ask this question,
3:17 and do the conversion within the loop.
3:20 Ok, so remember intend, so it's part of the loop,
3:22 we'll talk more about that in a moment,
3:24 so when it get the number, and then we are going to convert it and so on.
3:27 Now, you can see PyCharm has this sort of highlighted
3:30 as an error and it says guess cannot be defined after it's used.
3:35 So basically we need to give some kind of value
3:37 for the initial test so we get into this loop at least once
3:41 so we can pick something like negative one
3:43 or something that will never be a winning guess,
3:46 that will make sure that we at least run to this scenario one time,
3:49 now let's try it. 54- too high,
3:52 40- too low,
3:54 45- too high,
3:56 43- do I feel lucky- YES!!!, I was lucky, awesome.
4:00 So, that is how the game works,
4:02 we can do a little bit nicer stuff
4:04 but let's take a moment and pause and look at this concept
4:06 of this indentation and defining code blocks that run by using indentation.
4:11 This is one of our core concepts that we are introducing as part of this app.