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Из курса от партнера Vanderbilt University

Introduction to Programming with MATLAB

2791 оценки

Vanderbilt University

Introduction to Programming with MATLAB

2791 оценки

This course teaches computer programming to those with little to no previous experience. It uses the programming system and language called MATLAB to do so because it is easy to learn, versatile and very useful for engineers and other professionals. MATLAB is a special-purpose language that is an excellent choice for writing moderate-size programs that solve problems involving the manipulation of numbers. The design of the language makes it possible to write a powerful program in a few lines. The problems may be relatively complex, while the MATLAB programs that solve them are relatively simple: relative, that is, to the equivalent program written in a general-purpose language, such as C++ or Java. As a result, MATLAB is being used in a wide variety of domains from the natural sciences, through all disciplines of engineering, to finance, and beyond, and it is heavily used in industry. Hence, a solid background in MATLAB is an indispensable skill in today’s job market. Nevertheless, this course is not a MATLAB tutorial. It is an introductory programming course that uses MATLAB to illustrate general concepts in computer science and programming. Students who successfully complete this course will become familiar with general concepts in computer science, gain an understanding of the general concepts of programming, and obtain a solid foundation in the use of MATLAB. Students taking the course will get a MATLAB Online license free of charge for the duration of the course. The students are encouraged to consult the eBook that this course is based on. More information about these resources can be found on the Resources menu on the right.

Из урока

File Input/Output

Files are named areas in permanent memory for storing data that can be used as input or output to MATLAB and to other programs. We will be introduced to MATLAB’s most important methods for reading and writing files. We will learn how to create, read from, and write into MAT-files, Excel files, text files, and binary files. We will learn how to navigate among folders with MATLAB commands.

File Input/Output15:00

Excel Files9:12

Text Files12:17

Binary Files38:54

Познакомьтесь с преподавателями

Akos Ledeczi
Professor of Computer Engineering / Senior Research Scientist
Electrical Engineering & Computer Science / Institute for Software Integrated Systems
Mike Fitzpatrick
Professor Emeritus of Computer Science, Computer Engineering, Electrical Engineering, Neurosurgery, and Radiology
Electrical Engineering & Computer Science, Neurological Surgery, Radiology & Radiological Sciences

[MUSIC]

Microsoft Excel is a widely used tool for analyzing data.

That's why so many programs support the reading and writing of Excel files.

And MATLAB does too, with two built-in functions called xlsread and xlswrite.

Here we see a spreadsheet open in Excel.

It contains monthly weather data for Nashville, Tennessee.

As you can see, it contains not only numerical data, but also various labels

for month names and weather data, such as temperature and precipitation.

Assuming this data is saved in a file called Nashville_climate.xlsx,

we can easily load it into MATLAB.

We do that by calling the xlsread function with three

output arguments, num, txt, and raw.

The only input argument here is a string.

It's the name of the file.

You may be wondering, why are there three different outputs?

Well, MATLAB's nice enough to separate the numbers from the text for us.

Numbers in the Excel file are collected and returned in the first output argument.

Text is returned in the second output argument, and finally,

the third argument contains everything.

Depending on what you need, you can use any or all of these outputs.

Let's look at the first argument, num.

Num is a matrix of doubles, no surprise there.

But an interesting aspect of num is it's size,

which is smaller than the size of the spreadsheet.

What MATLAB does here is find the smallest rectangle of cells in the spreadsheet that

contains all the numbers.

That rectangle may be only a subset of all the cells in the spreadsheet,

as it is here in this example.

In this case it makes up 14 rows and 3 columns, so that's the size of num.

But there's a hitch with this plan.

MATLAB picks up all the cells from this rectangular area, and

some of those cells may not contain numerical data,

like this two by three non-numerical array in the middle of our example.

It contains three empty cells In three cells with text,

none of which is numerical data.

For these cells, MATLAB assigns a special value in the output argument,

NaN, which means not a number.

So in this example, it puts a two by three array of NaN, or nans,

in the middle of the output argument.

Text is returned in a similar fashion.

This time, the smallest rectangular area

is selected that contains all the text cells.

The type of this second output argument,

TXT, is a cell array instead of an array of characters.

That's the only way to return text.

Because there can be different numbers of characters in the cell, so

it's not possible to return it as a rectangular character array.

Now numbers turn into empty strings.

But what about those strange looking entries,

1 by 30 char and 1 by 40 char at the top left?

With eight corresponding strings,

they're simply too long to print conveniently in the command window.

So MATLAB just prints the type, the dimensions plus the element type.

But don't worry, all those data are there in those cells.

Finally, if we need everything from the spreadsheet in one array,

we get that in the third argument.

It's a cell array, that's the union of the previous two arrays.

All the data are here in one cell array, containing doubles and strings.

And this time,

it's the smallest rectangle that contains everything in the spreadsheet.

Okay. Let's try this out in MATLAB.

Well, it's easy to implement pretty much anything in PowerPoint.

But et me show you that it's really this easy to handle Excel files in MATLAB.

First, let's make sure we have the data we need in this directory.

What I want is this Nashville climate data.

There it is right there.

It's an Excel file with the ending xlsx here.

So first thing I want to do is just read the numbers in the file.

To do that, we'll give just one output argument instead of all three.

And there's our function.

Gee, that's a lot to type, and I'm lazy.

So what I'm going to do is just copy.

And paste.

There. There's all our data.

How cool is that?

A single command, and

we have all the numeric data in the Excel spreadsheet loaded into a MATLAB variable.

If we want both numerical and textual data, we can do this.

There are the numbers, there are the strings.

With these empty strings, where there are no strings.

So, that's how you get both of them.

If you're not interested in the numerical data at all and

you just want the textual data, you can do that, too.

There's a new syntax we haven't mentioned before,

that allows you to skip an output argument.

Looks like this.

Instead of temps here, I put a tilde and a comma.

You got to have the comma if you're going to use a tilde.

And so this time,

we didn't get the numerical data, just the second set of data.

And this works with any function that has multiple output arguments.

And if all we care about is the entire file, that last argument, we can do this.

Two tildes.

We don't want the numerical, we don't want the text,

we want everything and there is everything in that format,

where not a number is an empty cell, and

here is numbers over here and here is strings and so on.

Often, we want to read from a specified area within an Excel workbook.

Here's how to do that.

Here we read the data from just a single cell.

The second argument here specifies the sheet.

You can use the number, just like I did here, one for f, the first sheet,

two for the second, and so on.

Or you can specify the name of the sheet using a string in the second argument.

The third argument is the name of the cell.

And you can specify an area using this syntax.

Here we use Excel's own notation in the third argument for a rectangle of cells.

Note that that colon right here is not MATLAB's colon operator,

it's just a character in the string.

It indicates that T15 is the upper left hand corner of the rectangular

array we want, and E17 is the lower right hand corner.

We can also write data tech cell files using the xlswrite function,

at least on Windows we can.

On a Mac, MATLAB provides a somewhat limited alternative,

xlswrite, writes text files instead of Excel files.

Only numeric arrays can be written.

And each row is written on a separate line,

and the numbers on a row are separated by commas.

This format, which is called the comma separated value format or CSV,

is written and read by many applications including Excel.

Because of this limitation, we're not going to cover xlswrite here.

But the function is just as easy to use as the xlsread function.

Only thing to keep in mind is that you can overwrite the contents of

an existing file.

You won't get any warning.

The good news, however, is that you only modify the cells that you actually write.

The other cells will not be affected.

Try Help xlswrite, or read the book for more details.

[MUSIC]

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