Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.
Input/Output and str Formatting
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From the course by University of Toronto
Learn to Program: The Fundamentals
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From the lesson
Strings and Designing Functions
This module introduces strings (a Python data type used to represent text), and a process to follow when creating a function.
Meet the Instructors
Jennifer CampbellAssociate Professor, Teaching Stream
Department of Computer Science
Paul GriesAssociate Professor, Teaching Stream
Department of Computer Science
Hello.
In this lecture, we're going to do some
more work with strings and do some string formatting.
We're also going to write some interactive programs where we display
messages to the user and get input from the user.
Let's get started.
We'll call Python's built in function Print,
passing a single argument to the function call.
In this case we'll pass the string Hello, and Hello
will be displayed to the screen.
Notcie that when hello is displayed, hello
is displayed, it's without the quotation marks.
The quotes are only for Python's internal string
representation, and are not seen by the user.
Let's call print again, passing a mathematical
expression as an argument to the function.
In this case the expression is 3 plus 7 minus 3.
And that expression will first be evaluated and then printed to the screen.
We'll call print a third time passing in
two arguments this time, two strings; hello and there.
And when the strings are displayed, they're
displayed with a space between them by default.
So hello space there is what's printed to the screen.
Now, we will define two functions.
We'll define the functions in a file
called calc.pi that I've already created and saved.
Both functions will calculate the square of a number.
One function will return that result and the other will print it.
We begin the function defiinition with the word def
and the name of this function is square return.
It will take a single parameter.
I'm calling
this num.
And the job of this function is to return num to the power of two.
The second function, its name will be square print.
It also has a single parameter, num, and
it's also going to calculate the square of num.
But rather than returning that value, this function will print it.
In a moment we will call these two functions from the shell.
I'd like the output of the call to print
to look different from the value returned by square print.
To do
this, I'm adding a string to the list of arguments for the print function
call so that it will display the square of num is before the squared number.
The next step is to run the module.
We'll go to the Run menu and choose Run Module.
And now we have access to the two functions in the shell.
We're going to call these two functions and
we will store the results of calling those functions
in a variable.
So the variable answer_return will get
the result of calling the function square_return.
We're going to pass an argument to square_return and I'm picking the value 4.
I could pick any value here.
What I'm using for just as an example function call.
It's now time to run the function.
When this function executes, we want to watch to see whether anything is printed.
And we
can see when we execute it now, that we immediately get
the prompt, so nothing was printed to the screen by square return.
Next we want to check the value of the answer term variable
and we can see that answer return first to the value 16.
So, square return returned the memory address of 16 and
that memory address was stored in the answer return variable.
Now its time
to call the function square print, we'll also pass
the value four as the argument, we could have picked
anything, and we see this time that something is
displayed to the screen, the square of num is 16.
Next, we'll examine the contents of the variable
answer print and this time we don't see anything.
When a function does not have a return statement that
gets executed, Python will return None, and that's what happens
in this case.
Because answer_return refers to a numeric value, we're able to use
it in a mathematical expression, such as multiplying it by five.
However, answer_print does not refer to a numeric value.
Get an error message in this case because it refers to the value None, or just of
type NoneType. We now know how to use
Python's built-in function print to display messages to users.
The next step is to use Python's built-in
function input to get information from the user.
We'll call input passing a single string as an argument to it.
In this case we're going to ask the user what is your name?
When the function executes that string will be
printed to the screen, and we'll wait for
the user to type some input at the prompt.
So notice that the cursor is flashing waiting for us to type something.
I will now type my name as the user of this program, I said
my name is Jen, and when I hit Enter the function is, completes its execution.
The string that I typed, Jen, is returned as a string from the function.
We see that there are single quotes around
it indicating that the value is type string.
We'll call the function input a second time, this time though we will
store the result returned by input in a variable named name.
When we execute the function the user
types their name and that value is returned
as a string, so the memory address of the value is stored in the name variable.
And when we examine the content of name, we
will see that it refers to the string jet.
We will now use input to prompt the user to enter their location.
The string,
what is your location gets printed and I enter my location; Toronto.
That value is returned by the function.
So location refers to the value Toronto and name refers to Jen.
Let's now use the two variables, name and
location, to display a message back to the user.
We're going to call print passing in three arguments.
The first is the
name The second will be a string saying lives in and the third is the location.
So the way we interpret this call to print is
that first we use the value that name refers to.
Joining that with a space between it and the string lives in.
And next, we'll use a space and the value
that location refers to, to print Jen lives in Toronto.
Earlier, I mentioned that name and location both referred to strings.
In fact, all values, the type of all
values returned by input, will be type String.
Let's cull Input again, asking how many cups of coffee I've had today.
When I enter a value
as a user, I enter 2, and this value
is returned by the Input function as a string.
So we can see that num_coffee refers to the string 2, not to the into.
You've already seen that we can use
single quotes and double quotes to represent strings.
And now I want to show you a third string format, which is triple quote strings.
Triple quote strings are able to span multiple lines.
For example, let's print a triple coated string with how on the
first line, are on the second, and you on the third line.
[SOUND]
So that looks like this, one word per line.
I'm going to use the same string.
Only, rather than printing it, I'll store it in a variable.
And when we examine the contents of variable s,
it looks a lot different from what we inputted.
First of all, it doesn't have triple quotes anymore.
It has single quotes.
And secondly, in the places where I had hit the Enter key or the Return
key, we see this backslash n in two places.
That character is called the newline characters, and it's a special
character in Python.
The backslash symbol is the escape character.
And this combination of backslash and n is escape sequence newline.
There
are several other special characters.
For example, let's print the numbers three four and five, separated by tabs.
We use three, and then for the tab symbol, we
use the escape sequence backslash t, four, backslash t, and five.
So three tab four tab five, and that's what it looks like.
In addition to newline and
tab, there are several other escape sequences.
A couple that will be useful for us, are the escape sequence, backslash, backslash.
Which we use, whenever we want to print a
single backslash, or include a single backslash in the string.
That's because the backslash character, is the escape character and when
we use it on it's own, we get an error message.
As it expects,
something follows it. Another useful sequence, is the single
quote sequence, for when you want to include a single quote, within our string.
For example, if I want to say, don\'t, then I use backslash,
quote, for the place where I'd like the single quote to occur and similarly,
when I want to use a quotation marks, double quotation marks
in a string, I can use the escape sequence, backslash, double quotes.
So, imagine that we want to do this, says, He says.
[SOUND]
and I use \ double quotes to indicate that I
would like to use the double quote in the string
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