Original subtitles

0 Welcome back. 1

Let's now make coding even more fun 2

and actually take decisions with our code 3

to make it look a lot more real. 4

So let's say it that we want to write a program 5

which checks whether a person 6

is allowed to start taking a driver's license or not. 7

And if she is, then it will print that to the console. 8

And if not, it will print how many years are left 9

until the person can start taking the driver's license. 10

So let's start with an age and let's set it to 19. 11

So we have this age and we already know 12

how to check if this age is at least 18, 13

which is the legal required age to start a driving license 14

at least here in Europe. 15

So let's actually create a variable with that. 16

And I think we actually already did that. 17

Let's call this one 'is old enough'. 18

So again, with a variable name 19

that describes exactly what we want here. 20

So remember to check if the age is at least 18, 21

we need it to be greater or equal than 18. 22

And so this will then include 18. 23

So if the age is 18, the result of this operator here 24

or of this operation will then be true. 25

Only if the age is 17 or 16 or anything below, 26

it will turn out to be false. 27

Okay. 28

So we have a Boolean value now, 29

and we can use this 30

to take decisions using an 'if statement' 31

and the 'if statement' works like this. 32

All we have to do is write 'if' then open parenthesis 33

and then in here goes a condition that is evaluated. 34

And if this condition turns out to be true, 35

then this block will be executed. 36

So basically whatever code that we write 37

into this block, which is denoted by 38

these curly braces will be executed 39

whenever the condition that's here is true. 40

So in this case, let's put the condition, 41

and with condition I basically mean something 42

that is a Boolean value. 43

So let's use, 'or is old enough' variable here 44

because we already know that this will always be a Boolean. 45

So again, whenever this value here is true 46

then the code that's in this block here will be executed. 47

And if it's false, well, then it's not going to be executed. 48

So let's log to the console. 49

Sarah can start driving license 50

and let's just add a fun emoji here 51

and on the Mac, that's Command + Control + Space 52

and on Windows 10, the shortcut is Windows + . 53

and on Windows 10, the shortcut is Windows + . 54

but, adding the emoji here is not really important. 55

So let's give it a safe and try this out now. 56

And indeed we get, Sarah can start her driving license 57

and that's because the age is 19. 58

And so it's above 18. 59

So this is true. 60

And so if 'is old enough' is true 61

then this block of code here will be executed 62

which in this case is just one line 63

but it could of course be multiple lines. 64

So now let's put it to something else. 65

So 15. 66

And so now this is false. 67

And so then this code block here should not be executed. 68

So give it a safe, let's try. 69

And indeed we get no output this time. 70

And so that means that it worked. 71

So let's put it back to 19. 72

And in practice, 73

we always write this condition directly here. 74

So let's do that. 75

So what we want is just this part. 76

So let's get rid of this. 77

And so this is a lot more common. 78

Let's actually getting rid of this. 79

And so we basically do this operation here. 80

This will then return true or false. 81

And depending on that, 82

this code block will then be executed. 83

And actually let's put it back to 15 84

because now we can do something else 85

which is to actually add an 'else' block. 86

And we do that simply 87

by writing else right after this first, 'if' block, 88

and then we add another block. 89

So what do you think this 'else' block is for? 90

Well, this 'else' block will basically be executed 91

whenever this condition here is false. 92

So right now the age is below 18. 93

And so this is false. 94

And so as we already know, 95

this block here will not be executed. 96

Instead, the 'else' block will be executed. 97

So whatever code we have here. 98

So let's write some code in here. 99

And as I sad in the beginning, 100

we will calculate how many years are left 101

for Sarah to start taking her license. 102

So let's calculate that, 'years left'. 103

And so that's simply, 18 minus the age. 104

Right? 105

And then we can log in a nice string into the console, 106

which contains this variable. 107

And as we learned in the last lecture 108

the best way of doing that is to use a template literal. 109

So back ticks. 110

And then we say, Sarah is too young'. 111

'Wait another' 112

and then we insert or placeholder here basically. 113

So years left 114

and then years. 115

Okay. 116

So the result of this should be 18 minus 15, which is 3. 117

And so we should see 'Wait another 3 years". 118

So let's checked that. 119

And indeed, that's exactly what we got. 120

And so again, since this condition here 121

so this operation basically turned out to be false 122

then this block of code was executed. 123

Great. 124

Just keep in mind that this 'else' block 125

is actually optional, right? 126

So in the beginning we didn't have this 'else' block 127

and it still did work. 128

So when there is no 'else' block, 129

JavaScript will then simply move on to the next line 130

after the, 'if' statement 131

in case that the condition is false. 132

Great. 133

Now, you know what an 'if', 'else' statement is 134

and how it works. 135

And this is actually one of the most important things 136

in programming. 137

We take decisions with code all the time 138

which is essentially what we did here 139

so that we can execute certain parts of our program 140

based on certain conditions. 141

Okay. 142

Now what we have here is 'if', 'else' statement, 143

which is the official name 144

of this kind of construction here, 145

is called a control structure. 146

So basically this 147

if this structure here is called an 'if', 'else' 148

control structure 149

and it is called a control structure 150

because this structure actually allows us 151

to have more control over the way that our code is executed. 152

For example, in this, 'if', 'else' statement 153

the whole code does not just execute in a linear way. 154

So JavaScript does not execute 155

all of this code here, linearly. 156

Instead we can now control blocks of code 157

that should execute 158

and blocks that should not execute. 159

And so again that gives us a lot more control 160

over how our code works 161

and that's why this is called a control structure. 162

Okay. 163

And there are other control structures 164

that we're gonna go into a bit later. 165

All right. 166

So I hope that you can see how this really takes our code 167

to the next level. 168

Now just to make sure 169

that you really understood this concept 170

let's create another, a very small and simple example here. 171

And this time let's actually create a variable conditionally 172

and not just always use''console.log'. 173

So let's create a birth year variable. 174

I think we don't have it in this lecture yet. 175

That's right. 176

And so now what we want to do 177

is to create a variable called 'century' 178

which will basically contain the century 179

in which this person was born. 180

So in this case, it was the 20th century. 181

But for example, if the person was born 182

in, like, let's say 2015, then it would be the 21st century. 183

So to do that, we can, of course write 184

an 'if', 'else' statement, 185

so we can say if the birth year is below 186

or equal to 2000, 187

then let century equal 20. 188

And so that then means the 20th century. 189

And if not, then let century be 21. 190

So we're assuming that the person is not born 191

in the 19th century. 192

So we're not accounting for something like this, 193

for example. 194

So instead we always have the 20th or the 21st century. 195

Now to actually make this work 196

we need to define the century variable outside 197

of the, 'if' or 'else' blocks. 198

We will go deeply into why that is 199

but for now, what you need to know is 200

that this is because a variable that we define 201

inside of any code block, which is what this year is, 202

so this is a code block, and this is a code block. 203

And any variable that we declare 204

inside of one of these blocks will not be accessible 205

outside of the block. 206

So let me just show that to you very quickly. 207

So if I try to read 'century' now, then I will get a 208

an error here, you see, century is not defined. 209

And so what we need to do is to define 'century' outside 210

and simply leave it empty. 211

And then in here we can then conditionally reassign it. 212

Okay. 213

And so now let's see how this works. 214

And indeed now we get the century 20. 215

So 1998 is below 2000 216

and so this person was born in the 20th century. 217

Now let's say 2012 218

and so that should not be 21st century. 219

And indeed that's exactly the result. 220

So I hope that this example too made sense to you. 221

Now, don't worry about the fact 222

that we had to declare the century variable outside 223

of the blocks, but matters here is that you understand 224

the logic of the 'if', 'else' statement. 225

Okay? 226

And I hope that was clear enough 227

but anyway, let's quickly recap. 228

We can take decisions using code, 229

using the, 'if', 'else' statement. 230

And for that we write 'if', and then we open apprentices 231

and in there we need a condition 232

and the condition is essentially 233

any code that returns a true or a false value. 234

And so this is a perfect example of that 235

because this operator here will return true or false right? 236

And so we can use this operator to create a condition 237

essentially. 238

Then if the condition is true, this block will be executed. 239

So all the code that is in there 240

no matter how many lines of code there are. 241

Now, if the condition turns out to be false 242

then this 'else' block will be executed instead. 243

So JavaScript will then skip this first block 244

and go to the second block 245

which is this one, but the 'else' block 246

is actually optional. 247

So if you don't have the 'else' block 248

then simply no code will be executed. 249

And this in a nutshell 250

is how the, 'if', 'else' statement works 251

and remember that this is really powerful. 252

So make sure that you really understand how this works. 253

And when you do understand, then you can move on 254

to the next video where there is a small challenge 255

waiting for you.