All language subtitles for 6. [Dart] Conditionals - IFELSE

All right.
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So for those you guys who are unfamiliar with Dart, in this lesson, I want to quickly cover Dart conditionals.
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And more specifically, the IF and ELSE. Now an IF statement basically just checks to see if a condition
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is true. And if the condition is true, then it's going to carry out the instructions inside a set of curly
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braces.
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So in this case, our IF statement says that if the track is clear, the double equals means assess to see
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if the left hand side is equal to the right hand side,
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but a single equals is an assignment.
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So it means make the left hand side equal the right hand side,
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and that's a subtle difference to be aware of when you're coding.
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So in this case we're saying in English it means if the track is clear, then go.
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If we go through the statement, then this is where the condition is,
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and this is what the instruction is.
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So when the computer evaluates the statement and in this case the track is indeed clear, then it will
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just go.
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But if when we evaluate this it's actually not true,
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the track is not clear, in this case there's a giant rock there,
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then it won't carry out the instructions inside the curly braces.
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Instead it will skip all of this and go onto the next line of code
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that it needs to execute.
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So in this case, the train goes nowhere. In the last lesson,
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we saw that in addition to just using a simple IF to trigger some statement when the conditions are met,
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we could also use IF and ELSE.
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So let's say that we're saying if the track is clear, then the train should go straight but else i.e.
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if the track is not clear, then it should turn right instead.
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In this case this is what the code would look like. the condition that we're testing for is if the track
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is clear.
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And in that case, it should carry out this piece of instruction,
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everything in between this set of curly braces. So go straight.
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But if this condition was not true then it skips ahead to the else statement and it carries out what's
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inside this set of curly braces instead, which happens to be turn right. Now more commonly, you'll see IF
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and ELSE statements structured more like this, where we have our condition and then we have a set of curly
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braces and then we have all else
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and what to do if that condition is false. And this is what we did in our last lesson as well.
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Now if you live with a programmer then it's really dangerous and you have to be very careful how you word
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things.
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For example if you told your husband or wife who's a programmer, 'Please go to the supermarket and buy
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one bottle of milk, if they have eggs buy 6.'
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You might think what you're saying is buy one bottle of milk and if there's eggs by six eggs.
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But what they might hear is if haveEggs = true, then buy 6 bottles of milk,
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otherwise buy one bottle of milk.
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So a shout out to everybody who lives with programmers.
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Now in order to demonstrate how this code actually works, we're going to build a love calculator.
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So it's one of those throwbacks from the 80s where you test your compatibility with whoever you fancy.
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Now we're going to keep our love calculator really simple.
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We're going to make a function that's called loveCalculator.
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And it's simply just going to generate a random number between 1 and 100.
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So we know how to do that already
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from our previous lessons. We have to first import the Dart math library.
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Remember it's math, and not maths.
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And once we've imported it, then we can use the random number generator. So we can create a new variable
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called loveScore and it's going to be equal to the random number generator .nextInt because we're
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generating whole numbers.
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And we're going to make it 100.
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So they generate all the numbers from 0 to 99. And then we add 1 to change our range to
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1 to 100.
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So now that we can generate our random number loveScore, and I apologize to everybody who really believes
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in love calculators.
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But in this case it's going to be a little bit random.
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So now that we've generated loveScore, let's print it out and let's see what we get.
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So if we call our function up here in the main function and we call our love calculator, and then we
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run our code, we should see the result in the console.
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So the first time we got 29%, we got 18%, keeps on getting lower, 22%.
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So today's not a day for love it seems it's, oh! There we go.
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We got a 96%. We finally got a high score. So you can see there are a whole bunch of random numbers between
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1 and 100.
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Now if we wanted to give the user an interpretation of their love score based on this number, then
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we could use an IF statement. So we could say that if the love score is greater then let's say 70, then
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that means that they, you know, maybe they really like each other. Maybe they... you love each other like
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Kanye loves Kanye.
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And if they got a score that's lower than 70, then it will just say
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you'd like each other.
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All right.
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So now, depending on our love score, we should get something different printed in our console.
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And if we run, it we can see that the first time we got 30, so we get the text we like each...
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you like each other.
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Not sure how I missed so many letters.
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And if we run it again, we get 9,
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so you like each other. And eventually hopefully we should get... oh wow!
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We actually got 100.
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Jackpot!
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So you love each other like Kanye loves Kanye. So you can see that depending on the love score that
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we got out of the random generator, we're able to print different text into the console.
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And this is all because of our IF statement.
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Now what if we wanted to have more than just one bracket because at the moment we can only have a different
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piece of interpretation for above 70 or below 70?
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What if we wanted it to be more nuance?
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What if we wanted 70 to 100 and 50 to 70 and 0 to 50?
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How would we do that?
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Well when we're using IF statements,
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we also have a whole bunch of operators that we can use.
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For example we already saw that we can use to double equals to check if the left hand side is equal to
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the right hand side.
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But we can also check if it's not equal to by using an exclamation mark, an equal sign. Or if the left
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hand side is greater than the right hand side, lesser than, greater or equal to, or lesser or equal to.
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And then we can combine these conditions together, in an IF statement, using these comparatives.
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So for example we can have two && for AND, and two || for OR, and a single exclamation mark
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for NOT. So let me show you how this works. If we have are IF statement for if loveScore is greater than
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70, then do this. We can also have another IF statement down here. Let's say in this one we want to check
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if the love score is greater than say 50 and the love score is less than 70, then we print 'You like each
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other'. But if the love score is less than 50 then we print you don't like each other. All right.
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So now we get an error over here that says too many positional arguments.
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One expected, but it found five.
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What is this all about?
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Well remember what we said about the single quotes.
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It thinks that this is what we want to print and it doesn't know why there's all of this code at the
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end.
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So whenever we're using single quotes inside our strings, we always have to use the backslash to escape
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it.
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Tell it, 'Don't look at this as code.
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This is just something that we use in English grammar and not a part of the code.'
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Now let's run our code and you can see that if we get 71, which is greater than 70, then we get
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you love each other like Kanye loves Kanye.
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But if we get a different number, say 8 which is less than 50, then it tells us you don't like each
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other.
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So now, we're able to use these operators and combine different conditions to check for multiple things.
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So we can check if love score is greater than 50,
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and the love score is less than 70. Or we could check if the love score is greater than 50 or the love
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score is less than 70. Although it won't be very helpful in our case because that means this is going
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to be true when this is also going to be true.
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So when it's 22, because it's less than 70 and this is an OR, then this will trigger and 22 is also less
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than 50.
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So this will also trigger.
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Now while this works, while we can use these ways of combining different things to make our statements
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more specific, there's a much better way of doing this.
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We can start to build up a ladder of IF and ELSE statements.
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So this is how it would work.
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We can check to see if track one, this one, is clear.
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Then we're going to get our train to go on to track one.
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This is our first choice. But if that was false,
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so if there was a rock there, then we can go to the next statement. And we're saying that ELSEIF track
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2 is clear, then we should go down track2. But what if track2 also has a rock on it?
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I don't know who's maintaining these railways by the way why there are rocks everywhere.
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If track2 is also blocked then we can have another
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Else If statement to check if track3 is clear, then that's going to be the one we'll take.
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Now our code looks a bit like this.
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So we can say if the love score is greater than 70, then do A.
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But if that's not true, then we get on to our second choice.
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So else if love scores greater than 30, then do B.
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And finally if that's also false, then do C.
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So if we get a Love Score of 50, then it's obviously not greater than 70,
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so it skips A. But then it checks the next statement.
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It checks to see if the love score is greater than 30, which it is.
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So then it will go ahead and do B.
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However if the love score was 75, then the first statement is already true.
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So in this case it will simply go and do A.
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And notice that if this was not an ELSE IF, but it was just an if, then this would also be true,
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right?
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75 is also greater than 30.
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But this doesn't happen because this is an ELSE IF. So when we have an IF statement followed by an ELSE IF
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followed by other ELSE IFs or ELSE, it means that once it finds one of the conditions to be true then
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it will skip all of the other ones.
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But if all of these were IF statements then it will check them one by one.
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So if we had if love score was greater than 70 and if love score was greater than 30, then it will go
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and evaluate the first condition.
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So it's greater than 70.
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It'll do A.
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And then it sees that oh this is the end of the IF statement, and we've got another IF statement.
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Well this one's also true.
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75 is also greater than 30.
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So I'm gonna go and do B instead.
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Now there's cases where you might want your code to work like this but there's also cases where you
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might want to use an ELSE IF instead.
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So heading back to our code, we can actually change it so we don't even need to compare and have more
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than one condition. Instead of checking to see if the love score is greater than 50 and less than 70,
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I can change this to and ELSE IF instead. And I can simply write
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else if the love score is greater than 50. and the final one doesn't even need an if, it could simply just
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be an else.
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So now what happens is that when I click run and we generate another new random number and we get a
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score of 35, the first thing that happens is it checks to see if the love score is greater than
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70. 35 is not greater than 70, so it skips this and goes to the next one.
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Now it checks to see if 35 is greater than 50. which is also not true.
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So it skips this block.
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And finally because this is an else block, when everything above is not true, then it will simply carry
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out the instruction inside here, which is to print 'You don't like each other.'
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Now if we try again and this time we get 94,
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then once it checks the first IF statement, it's already true and it carries out this function, then it
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skips all of the other ones.
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It doesn't also go and check to see if it's greater than 50.
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But say if we had changed this to another if, then if I click to run and we get a large number like seventy
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five, then this is true,
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so it does this. But this is also true,
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so it does that.
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So this is the difference between an IF and ELSE IF. And you can actually build up your ELSE IF Statements
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to have lots of them.
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So you can have another one for example, if love score is greater than, I don't know, 30. Maybe you want a different
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message for 30 to 50
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in this case. And you have a different message for 50 to 70,
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and here you have a different message for 70 to 100.
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So have a play around with this code to fully appreciate the difference between a whole bunch of IF
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statements and also a ladder of ELSE IF statements and how these types of conditional statements work
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in Dart. Once you're done, we're going to head back to making our Quizzler app.
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So I'll see you on the next lesson.