Control Statements: for, while, if statement

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En provenance du cours de Stanford University

Machine Learning

79596 notes

Stanford University

Machine Learning

79596 notes

Machine learning is the science of getting computers to act without being explicitly programmed. In the past decade, machine learning has given us self-driving cars, practical speech recognition, effective web search, and a vastly improved understanding of the human genome. Machine learning is so pervasive today that you probably use it dozens of times a day without knowing it. Many researchers also think it is the best way to make progress towards human-level AI. In this class, you will learn about the most effective machine learning techniques, and gain practice implementing them and getting them to work for yourself. More importantly, you'll learn about not only the theoretical underpinnings of learning, but also gain the practical know-how needed to quickly and powerfully apply these techniques to new problems. Finally, you'll learn about some of Silicon Valley's best practices in innovation as it pertains to machine learning and AI. This course provides a broad introduction to machine learning, datamining, and statistical pattern recognition. Topics include: (i) Supervised learning (parametric/non-parametric algorithms, support vector machines, kernels, neural networks). (ii) Unsupervised learning (clustering, dimensionality reduction, recommender systems, deep learning). (iii) Best practices in machine learning (bias/variance theory; innovation process in machine learning and AI). The course will also draw from numerous case studies and applications, so that you'll also learn how to apply learning algorithms to building smart robots (perception, control), text understanding (web search, anti-spam), computer vision, medical informatics, audio, database mining, and other areas.

À partir de la leçon

Octave/Matlab Tutorial

This course includes programming assignments designed to help you understand how to implement the learning algorithms in practice. To complete the programming assignments, you will need to use Octave or MATLAB. This module introduces Octave/Matlab and shows you how to submit an assignment.

Basic Operations13:59

Moving Data Around16:07

Computing on Data13:14

Plotting Data9:38

Control Statements: for, while, if statement12:55

Vectorization13:48

Rencontrer les enseignants

Andrew Ng
Co-founder, Coursera; Adjunct Professor, Stanford University; formerly head of Baidu AI Group/Google Brain

In this video, I'd like to

tell you how to write

control statements for your

Octave programs, so things

like "for", "while" and "if" statements

and also how to define and use functions.

Here's my Octave window. Let

me first show you how to use a "for" loop.

I'm going to start by setting v

to be a 10 by

1 vector 0.

Now, here's I write

a "for" loop for I equals 1 to 10.

That's for I equals Y colon 10.

And let's see, I'm

going to set V of I

equals two to the

power of I, and finally

end.

The white space does not matter,

so I am putting the spaces

just to make it look nicely indented,

but you know spacing doesn't matter.

But if I do this, then the

result is that V gets

set to, you know, two to

the power one, two to the power two, and so on.

So this is syntax for I

equals one colon 10 that

makes I loop through the

values one through 10.

And by the way, you can also do

this by setting your

indices equals one to

10, and so the

indices in the array from one to 10.

You can also write for I equals indices.

And this is actually the same as if I equals one to 10.

You can do, you know, display

I and this would do the same thing.

So, that is a "for" loop,

if you are familiar with "break"

and "continue", there's "break" and

"continue" statements, you can

also use those inside loops

in octave, but first

let me show you how a while loop works.

So, here's my vector

V. Let's write the while loop.

I equals 1, while I

is less than or equal to

5, let's set

V I equals one hundred

and increment I by

one, end.

So this says what?

I starts off equal to

one and then I'm going

to set V I equals one

hundred and increment I by

one until I is, you know, greater than five.

And as a result of that,

whereas previously V was this powers of two vector.

I've now taken the first

five elements of my vector

and overwritten them with this value one hundred.

So that's a syntax for a while loop.

Let's do another example.

Y equals one while

true and here

I wanted to show you how to use a break statement.

Let's say V I equals 999

and I equals i+1

if i equals 6 break and

end.

And this is also our first

use of an if statement, so

I hope the logic of this makes sense.

Since I equals one and, you know, increment loop.

While repeatedly set V I equals 1

and increment i by 1,

and then when 1 i

gets up to 6, do a

break which breaks here although

the while do and so, the

effective is should be to take

the first five elements of this

vector V and set them to 999.

And yes, indeed, we're taking

V and overwritten the first five elements with 999.

So, this is the

syntax for "if" statements, and

for "while" statement, and notice the end.

We have two ends here.

This ends here ends the if statement

and the second end here ends the while statement.

Now let me show you the more general syntax for

how to use an if-else statement.

So, let's see, V 1

is equal to 999, let's

type V1 equals to 2 for this example.

So, let me type

if V 1 equals 1 display the value as one.

Here's how you write an else

statement, or rather here's an

else if: V 1 equals

2. This is, if in case that's true in our example, display

the value as 2, else

display, the value is not one or two.

Okay, so that's a if-else

if-else statement it ends.

And of course, here we've just

set v 1 equals 2, so hopefully, yup,

displays that the value is 2.

And finally, I don't

think I talked about this earlier, but

if you ever need to exit Octave,

you can type the exit command and

you hit enter that will cause Octave

to quit or the 'q'--quits

command also works.

Finally, let's talk about

functions and how to define

them and how to use them.

Here's my desktop, and I

have predefined a file

or pre-saved on my desktop a file called "squarethisnumber.m".

This is how you define functions in Octave.

You create a file called, you know,

with your function name and then ending in .m,

and when Octave finds

this file, it knows that this

where it should look for the definition of the function "squarethisnumber.m".

Let's open up this file.

Notice that I'm using the

Microsoft program Wordpad to open up this file.

I just want to encourage you, if

your using Microsoft Windows, to

use Wordpad rather than

Notepad to open up these

files, if you have a

different text editor that's fine

too, but notepad sometimes messes up the spacing.

If you only have Notepad, that should

work too, that could work

too, but if you

have Wordpad as well, I

would rather use that or some

other text editor, if you have a different text editor for editing your functions.

So, here's how you define the function in Octave.

Let me just zoom in a little bit.

And this file has just three lines in it.

The first line says function Y equals square root

number of X, this tells

Octave that I'm gonna return

the value Y, I'm gonna

return one value and that

the value is going to

be saved in the variable Y

and moreover, it tells Octave

that this function has one argument,

which is the argument X,

and the way the function

body is defined, if Y equals X squared.

So, let's try to call

this function "square", this number

5, and this actually

isn't going to work, and

Octave says square this number it's undefined.

That's because Octave doesn't know where to find this file.

So as usual, let's use PWD,

or not in my directory,

so let's see this c:\users\ang\desktop.

That's where my desktop is.

Oops, a little typo there.

Users ANG desktop

and if I now type square

root number 5, it returns the

answer 25.

As kind of an advanced feature, this

is only for those of you

that know what the term search path means.

But so if you

want to modify the Octave

search path and you

could, you just think of

this next part as advanced

or optional material.

Only for those who are either

familiar with the concepts of

search paths and permit languages,

but you can use the

term addpath, safety colon,

slash users/ANG/desktop to

add that directory to the

Octave search path so that

even if you know, go to

some other directory I can

still, Octave still knows

to look in the users ANG

desktop directory for functions

so that even though I'm in

a different directory now, it still

knows where to find the square this number function.

Okay?

But if you're not familiar

with the concept of search path, don't worry

about it.

Just make sure as you use

the CD command to go to

the directory of your function before

you run it and that actually works just fine.

One concept that Octave has

that many other programming

languages don't is that it

can also let you define

functions that return multiple values or multiple arguments.

So here's an example of that.

Define the function called square

and cube this number X

and what this says is this

function returns 2 values, y1 and y2.

When I set down, this

follows, y1 is squared, y2 is execute.

And what this does is this really returns 2 numbers.

So, some of you depending

on what programming language you use,

if you're familiar with, you know, CC++ your offer.

Often, we think of the function as return in just one value.

But just so the syntax in Octave

that should return multiple values.

Now back in the Octave window. If

I type, you know, a, b equals

square and cube this

number 5 then

a is now equal to

25 and b is equal to

the cube of 5 equal to 125.

So, this is often

convenient if you needed to define

a function that returns multiple values.

Finally, I'm going to show

you just one more sophisticated example of a function.

Let's say I have a data set

that looks like this, with data points at 1, 1, 2, 2, 3, 3.

And what I'd like

to do is to define an

octave function to compute the cost

function J of theta for different values of theta.

First let's put the data into octave.

So I set my design

matrix to be 1,1,1,2,1,3.

So, this is my design

matrix x with x0, the

first column being the said

term and the second term being

you know, my the x-values of my three training examples.

And let me set

y to be 1-2-3 as

follows, which were the y axis values.

So let's say theta

is equal to 0 semicolon 1.

Here at my desktop, I've

predefined does cost function

j and if I

bring up the definition of that function it looks as follows.

So function j equals cost function

j equals x y

theta, some commons, specifying

the inputs and then

vary few steps set m

to be the number trading examples

thus the number of rows in x.

Compute the predictions, predictions equals

x times theta and so

this is a common that's wrapped

around, so this is probably the preceding comment line.

Computer script errors by, you know, taking

the difference between your predictions and

the y values and taking the

element of y squaring and then

finally computing the cost

function J. And Octave knows

that J is a value I

want to return because J appeared here in the function definition.

Feel free by the way to pause

this video if you want

to look at this function

definition for longer and

kind of make sure that you understand the different steps.

But when I run it in

Octave, I run j equals

cost function j x y theta.

It computes. Oops, made a typo there.

It should have been capital X. It

computes J equals 0 because

if my data set was,

you know, 123, 123 then setting, theta 0

equals 0, theta 1 equals

1, this gives me exactly the

45-degree line that fits my data set perfectly.

Whereas in contrast if I set

theta equals say 0, 0,

then this hypothesis is

predicting zeroes on everything

the same, theta 0 equals 0,

theta 1 equals 0 and

I compute the cost function

then it's 2.333 and that's

actually equal to 1 squared,

which is my squared error on

the first example, plus 2 squared,

plus 3 squared and then

divided by 2m, which is

2 times number of training examples,

which is indeed 2.33 and

so, that sanity checks that

this function here is, you

know, computing the correct cost

function and these are the couple examples

we tried out on our

simple training example.

And so that sanity tracks

that the cost function J,

as defined here, that it

is indeed, you know, seeming to compute

the correct cost function, at least

on our simple training set

that we had here with X

and Y being this

simple training example that we solved.

So, now you know how

to right control statements like for loops,

while loops and if statements

in octave as well as how to define and use functions.

In the next video, I'm

going to just very quickly

step you through the logistics

of working on and

submitting problem sets for

this class and how to use our submission system.

And finally, after that, in

the final octave tutorial video,

I wanna tell you about vectorization, which

is an idea for how to

make your octave programs run much fast.

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