IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 6

Microphone

Questions

Are there any questions before I begin?

Announcement

The UMB IT Club and Boston Linux User Group will hold a Linux InstallFest on Wednesday, September 24th, from 4 to 8 in the University Hall Room 3-3540.

If you have a machine on which you would like to install Linux and would like some help in doing this, bring it the InstallFest.

Volunteers from the Boston Linux User Group with be on hand to help with the installation.

They will also help you install the Window Subsystem for Linux (WSL) on your machine or install Linux as a dual boot.

You can also bring your questions about Linux or Unix to the InstallFest.

The Boston Linux and Unix User Group counts among its members some of the most knowledgeable Linux and Unix people in the Boston area.

Quiz 1

I have posted the answers to Quiz 1 here.

Homework 3

I have posted homework 3 here.

It is due this coming Sunday at 11:59 PM.

Let's take a look at it.

Resources

The Resources page has links that you might find useful.

There is a link to it on the class web page.

Tips and Examples

Converting Decimal to Integer

In this week's homework you will use a formula to convert values
The formula will create a decimal number
But the assignment says the result must be an integer
You might be tempted to use the int conversion function
But int converts a decimal to an integer by throwing away the decimal part
This is OK when the decimal part is less than .5
```
>>> int(5.3)
5
```
But not when the decimal part is greater than .5
```
>>> int(5.9)
5
```
To get the correct result you need to use another function called round
```
>>> round(5.3)
5
>>> round(5.9)
6
```

Review

Performing Calculations

Operators are special characters that act upon one or more values and perform an action
Usually that action creates a new value
The values that the operators work on are called operands

Python has operators for basic arithmetic

Character	Operation	Description
+	Addition	Adds two numbers
−	Subtraction	Subtracts one number from another
*	Multiplication	Multiplies one number by another
/	Division	Divides one number by another and gives the result as a decimal number
//	Integer division	Divides one number by another and gives the result as an integer
%	Remainder	Divides one number by another and gives the remainder
**	Exponent	Raises a number to a power

All these operators can be used with variables as well as numbers

>>> a = 2
>>> b = 4
>>> a + b
6
>>> a - b
-2
>>> a * b
8
>>> a / b
0.5

Decimal and Integer Division

Python has two division operators
- /
- //
The first works like ordinary division
```
>>> 4 / 2
2.0
>>> 4 / 5
0.8
```
Notice that the result is always a decimal ...
even when the result is a whole number
// division always results in an integer
When the result of the division is positive, any fraction is thrown away
```
>>> 4 // 2
2
>>> 5 // 2
2
```
When the result of the division is negative, the result is rounded down to the next lower integer
```
>>> -4 // 2
-2
>>> -5 // 2
-3
```

Exponent Operator

** is the exponent operator
The number before this operator is raised to the power of the second number
```
>>> 3 ** 2
9
>>> 3 ** 3
27
>>> 3 ** 4
81
```

Remainder Operator

When we perform long division and divide one number by another ...
we get two results
- A quotient
- A remainder
If we divide 17 by 5 we get a quotient of 3 and a remainder of 2
Integer division, //, gives us the quotient
```
>>> 17 // 5
3
```
We can get the remainder with %
```
>>> 17 % 5
2
```
The remainder operator is sometimes called the modulus operator
You can use the remainder operator to determine whether a number is odd or even
If, when we divide by 2, the remainder is 0, the number is even
If, when we divide by 2, the remainder is 1, the number is odd

Operator Precedence

When more than one type of operator is used in a calculation the interpreter must decide which to use first
The rules that are used to make this decision are called rules of operator precedence
The operator with the higher precedence is used first

The precedence for Python's arithmetic operators is

**	Exponentiation
* / // %	Multiplication, division and remainder
+ -	Addition and subtraction

Grouping with Parentheses

When we write
```
2 + 3 * 5
```
Python will multiply 3 times 5 before it adds 2
But what if we wanted a different order?
What if we wanted 2 to be added to 3 ...
and the result multiplied by 5?
We can do this if we put the addition inside parentheses
```
>>> 2 + 3 * 5
17
>>> (2 + 3) * 5
25
```
Whenever you are in doubt about the order of precedence
you can use parentheses to make your intention clear

Mixed-Type Expressions and Data Type Conversion

If you multiply two integers, you get an integer
```
>>> 3 * 5
15
```
In calculations where the values are all integers ...
the result will always be an integer ...
except when division is used
```
>>>  6 / 2
3.0
```
If you multiply two decimals, you get a decimal
```
>>> 3.0 * 5.0
15.0
```
In calculations where all values are decimals ...
the result will always be a decimal
But what if you multiply an integer by a decimal?
In Python, you will get a decimal
```
>>> 3 * 5.0
15.0
```
This is is always true with calculations using both integers and decimals
An expression that uses operands of different data types
is called a mixed-type expression

Breaking Long Statements into Multiple Lines

Statements are the basic unit of Python programs
A statement can be as long as you like
But if a statement is longer than the width of your window ...
you won't be able to read it all without moving things around
This makes it hard to see what is going on ...
and can make fixing your code hard
It's best to have all you statements fit inside your window
You can break a long Python statement into two or more lines ...
by hitting backslash, \, then Enter or Return ...

and continuing the statement on a new line

print("You will need to invest", present_value, "dollars at", \
    rate, "interest for", years, "years")

Attendance

New Material

More Expressions inside Expressions

In the last class I talked about using a function call ...

as an argument to another function call

number = int(input("Please enter an integer: "))

The argument to int (in red) is itself a function call
But using an expression inside another expression ...
happens in other places too
It happens with calculations using operators
Look at the following expression
```
rate * balance
```
* is an operator
But what about rate and balance?
They are variables
But variables are expressions themselves
The general format for a calculation with operators is
```
EXPRESSSION  OPERATOR  EXPRESSION
```
Whenever we use a calculation, each of the operands are themselves expressions
This means we can write something like this
```
5 + round(value)
```
Or this
```
value_1 + round(value_2)
```
Or even this
```
round(value_1) + round(value_2)
```
Expression appear everywhere in Python statements

Whitespace Characters

Not all characters appear as marks on a page
The Python statement
```
print("Hello")
```
prints 5 characters
But the statement
```
print("Hello world")
```
prints 11 characters, one of them a Space
Space is a perfectly good character
Even though it does not put a mark on the page
Spaces are very important
Canyoureadthisline?
Not easily
Spaces mark the end of one word and the beginning of the next
Spaces are a relatively recent invention
They first appeared in the Middle Ages
The Space is a member of a special group of characters whitespace
These characters do not put a mark on the page
But make the printed characters easier to read
The other two whitespace characters are Tab and newline
The newline character moves the printed output down one line
You type a newline by hitting Enter (PC) or Return (Mac)

Escape Sequences

How do you add whitespace characters to a string string literal ?
With Space, you just hit the space bar on the keyboard
There is a Tab key on your keyboard
But when you use it inside a text editor ...
different number of spaces are added ...
depending on where you are in the line ...
when you hit tab
Here is what happens in emacs ...
the text editor I use ...
when I type some characters ...
hit the Tab then |
```
x	|
xx	|
xxx	|
xxxx	|
```
Hitting Tab adds spaces ....
to bring you to a tab stop ...
which has a certain position on each line
That is not the same thing as entering a Tab character ...
which is a legitimate Unicode character
And how are we to go down to the next line in a string literal?

Just hitting the Enter or Return key does not work

>>> print("Line 1
  File "<stdin>", line 1
    print("Line 1
                ^
SyntaxError: EOL while scanning string literal)

In Python and most computer languages ...
we need a special way to write these characters inside a string literal
The way we write the newline character is \n

Like this

print("Line 1\nLine 2\nLine 3")
Line 1
Line 2
Line 3

This is an example of an escape sequence
An escape sequence is two characters you type ...
to get one special character in your Python string literal
Escape sequences are used to represent characters ...
that cannot be written any other way
Escape sequences consist of the \ (backslash)

Followed by one of these characters

Escape Sequence	Effect
\n	Causes output to be advanced to the next line
\t	Causes output to skip over to the next horizontal tab position
\'	Causes a single quote mark to be printed
\"	Causes a double quote mark to be printed
\\	Causes a backslash character to be printed

Why do we need the escape sequence \\?
Unix uses a slash, /, to separate directories ...
in a pathname like /home/ghoffman/it116
Windows uses \ for this purpose
So a Windows pathname would be \home\ghoffman\it116

To print a Windows pathname we need to use \\

>>> print("The path is C:\\temp\\data.")
The path is C:\temp\data.

We can use the Tab escape character \t to space out text on the screen

>>> print("Monday\tTuesday\tWednesday\tThursday\tFriday")
Monday  Tuesday Wednesday   Thursday    Friday

So that values in a table align properly

$ cat table.py
# prints a small table

print("Max\tMin")
print("-----------")
print("5\t1")
print("12\t2")
print("100\t0")

$ python3 table.py 
Max Min
-----------
5   1
12  2
100 0

The escape sequences for single and double quotes, \' and \" ...
are useful when writing a string literal with many quotes
```
>>> print("He said \"It\'s me!\")
He said "It's me!"
```

Concatenation Operator

In the last class we introduced the arithmetic operators
These operators work on values that are integers or decimal numbers
Different operators work on different data types
That's because an operation that makes sense in one data type ...
may not make sense in another
Dividing two numbers make sense
But not dividing two strings
The most common operation performed on strings ...
is to take two strings and join them
This is called concatenation

The concatenation operator in Python is +

>>> team = "Red Sox"
>>> print("Let's go", team)
Let's go Red Sox
>>> print("Let's go " + team)
Let's go Red Sox

The arguments of the first print statement are a string literal and a variable
The second print statement has one argument
That argument is the concatenation of a string literal and a variable

Concatenating Strings with Numbers

When you give the print function a number ...
it has to convert the number into a string in order to print it
```
>>> value = 5
>>> print("The value is", value)
The value is 5
```
So printing a string and a number is no problem ...
when they are separate arguments
But you cannot concatenate a string and a number
The + means addition when the first value is an integer or a float ...
but it means cocatenation when the first value is a string
The values on both sides of a + must both be numbers ...
or be strings
You can't mix strings and numbers with +

If you do, you will get an error

print("The value is" + value)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly

Concatenation only works with strings

So this works

>>> balance = "1000"
>>> print("The balance is " + balance)
The balance is 1000

because balance is a string

But this does not

>>> balance = 1000
>>> print("The balance is " + balance)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly

In order to use a number in concatenation ...
you have to convert it into a string

You do this with the str conversion function

print("The balance is " + str(balance))
The balance is 1000

Optional Material - Not Needed for Exams or Assignments

Using `print` without Going to a New Line

The print function normally prints its output on one line ...

and moves down to the next

$ cat print_04.py 
# Prints three lines

print("Line 1")
print("Line 2")
print("Line 3")

$ python3 print_04.py 
Line 1
Line 2
Line 3

It does this by adding the newline character, \n...
to the string you give it as an argument
But you can tell print not to add this newline character at the end
Or you can tell it to print some other character instead of newline
To do this, you have to give print a special kind of argument

Normally an argument to a function is an expression like

>>>  print(5)
5
>>>  a = 6
>>>  print(a)
6
>>>  print(a * 5)
30

But in certain situations you can give an argument in a different way
This different form looks like an assignment statement
I will explain this in more detail in a later class
For now here is what you need to know
To have print add a space instead of newline
You add the following argument inside the parenetheses
```
end=" "
```

If we run the following script

$ cat print_02.py 
# Prints a single line

print("Line 1", end=" ")
print("Line 2", end=" ")
print("Line 3")

we get

$ python3 print_02.py 
Line 1 Line 2 Line 3

You can also tell print to add nothing at the end
You do this by setting end to the empty string ""

The empty string is the string with no characters

$ cat print_03.py 
# Prints a single line

print("Line 1", end="")
print("Line 2", end="")
print("Line 3")

$ python3 print_03.py 
Line 1Line 2Line 3

The empty string is a perfectly good string
It just contains no characters
It's like 0 for strings

You can use more than one character as the value of end

$ cat print_04.py 
# Prints a single line

print("Line 1", end="---")
print("Line 2", end="---")
print("Line 3")

$ python3 print_04.py 
Line 1---Line 2---Line 3

`print` Item Separator

If you give print more that one argument to print ...
it will add a Space between each value
```
>>> print(1,2,3,4,5)
1 2 3 4 5
```
You can change the character automatically added ...
between each printed argument ...
by doing something similar
Instead of using end ...
we use sep
So we can change the space between values to a comma like this
```
>>> print(1, 2, 3, 4, 5, sep=",") 
1,2,3,4,5
```
We can also use more than one character as a separator
```
>>> print(1, 2, 3, sep=", ")
1, 2, 3
```

You can also use both sep and end in the same print statement

$ cat print_05.py 
# Prints a single line

print(1, 2, sep=", ", end="---")
print(3, 4, sep=", ", end="---")
print(5)

$ python3 print_05.py 
1, 2---3, 4---5

I am showing you these features of print because they can be useful
But I will never ask you a question about the end or sep arguments on an exam

F-strings

Often we want to print a string that contains the value of variables
For example, a script might want to print a greeting ...
with the name of a person and their username ...

using the variables name and username

>>> name = "John Smith"
>>> username = "jsmith"

What if I wanted to print the following string
```
Hello, John Smith! Your username is jsmith
```

I could try

>>> print("Hello ", name, "! Your username is ", username)
Hello  John Smith ! Your username is  jsmith

But the ! should not have a space before it

I could use concatenation

>>> print("Hello, " + name + "! Your username is", username)
Hello, John Smith! Your username is jsmith

But that is a lot of work and is ugly
However, since Python 3.6 I have another option
I can use F-strings, formated strings
An F-string begins with an f ...
followed by a quoted string ...
with the variables inside { } ...

like this

>>> print(f"Hello, {name}! Your username is {username}")
Hello, John Smith! Your username is jsmith

This is much easier to type ...
and much less ugly
But the real power of F-strings lies in what they can do with numbers

Printing Numbers

Printing numbers properly is often a challenge
If we print 1 divided by 3 we get a repeating decimal
```
>>> print(1/3)
0.3333333333333333
```
This decimal goes on forever ...
but the Python interpreter does not have a unlimited memory
So there is a limit to the number of digits after the decimal point ...
a float value can hold
When we print a money value, we only want two decimal places

But Python will always give us as many decimal points as it can

>>> bill = 54.35
>>> print(bill, "split three ways is", bill/3)
54.35 split three ways is 18.116666666666667

Of course we can always use the round function

>>> print(bill, "split three ways is", round(bill/3, 2))
54.35 split three ways is 18.12

A similar problem occurs when we want to print really big or really small numbers

>>> print("The age of the Universe is 13800000000 years")
The age of the Universe is 13800000000 years
>>> print("The size of an electron is .0000000000000282 meters")
The size of an electron is .0000000000000282 meters

This is why scientists use scientific notation
This notation represents a value with two numbers
The first number is a value and the second number is a power of ten
So we could represent the age of the Universe as 13.8 x 10⁹ years
And the size of an electron as 2.82 x 10^-15 meter
Languages like C, and Java use a special function, printf, in these situations
The name printf stands for "print formated"
But Python uses F-strings

Format Specifiers in F-strings

The { } inside F-strings can hold more than the name of a variable
They can hold a string called a format specifier ...
that tells Python how to display the value of the variable
The format is
```
{VARIABLE_NAME:FORMAT_SPECIFIER}
```
The format specifier is a string that tells Python how the number is to be formated
It can have many different parts
Most of which are optional
The part that isn't optional is the part that specifies the data type of the value
Most of the time format specifiers are used with a float
The type specifier for a float is f
You can also use the format specifier to determine the number of digits ...
after the decimal point
To do this, the format specifier uses a dot, . ...
followed by an integer ...
followed by f
The number of digits after the decimal point is called the precision

The format string asking for 2 digits of precision when printing a float is .2f

>>> value = 12345.6789
>>> print(f"The value is {value}")
The value is 12345.6789
>>> print(f"The value is {value:.2f}")
The value is 12345.68

To get three digits after the decimal point we would use .3f

>>> print(f"The value is {value:.3f}")
The value is 12345.679

Notice that the format function automatically rounds the last digit ...
so we do not have to use the round function

Using Format Specifiers with Scientific Notation

You can use the format specifier to print a number in scientific notation

All you have to do is use e instead of f in the format specifier

>>> universe_age = 13800000000
>>> print(f"The age of the universe is {universe_age:e} years")
The age of the universe is 1.380000e+10 years
>>> 
>>> print(f"The size of an electron is {electron_size:e} meters")
The size of an electron is 2.820000e-14 meters

We can specify the precision the same way we did before

>>> print(f"The age of the universe is {universe_age:.2e} years")
The age of the universe is 1.38e+10 years
>>> print(f"The size of an electron is {electron_size:.2e} meters")
The size of an electron is 2.82e-14 meters

Using Format Specifiers to Put Commas in Big Numbers

Normally, when we write big numbers we use commas
They separate thousand from millions, from billions and so on
At least this is how we do it in English
Many other languages such as German and French use spaces
Spanish and Italian use periods (.)
We can use format specifiers to add commas to numbers
To do this add a comma before the period in the format specifier

Like this

>>> value = 12345.67890
>>> print(f"The value is {value:,.2f}")
The value is 12,345.68

We never need the precision part when writing the format specifier for an integer

But we have to specify the data type with d

>>> universe_age = 13800000000			
>>> print(f"The age of the Universe is {universe_age:,d} years")
The age of the Universe is 13,800,000,000 years

Specifying Minimum Length for a Number

When you print a table of numbers ...
you want the right-hand sides of the numbers to line up

Sometimes you can do this with tabs

$ cat print_07.py 
# uses tabs to print a table

print("Mon\tTue\tWed\tThr\tFri")
print("15\t24\t89\t31\t86")
print("21\t79\t74\t23\t79")

$ python3 print_07.py 
Mon Tue Wed Thr Fri
15  24  89  31  86
21  79  74  23  79

But this often does not work

$ cat print_08.py 
# uses tabs to print a table

print("Mon\tTue\tWed\tThr\tFri")
print("13452.12\t24\t234523.4589\t31\t86")
print("2324.1\t79\t74\t22343.453\t79")

$ python3 print_08.py 
Mon Tue Wed Thr Fri
13452.12    24  234523.4589 31  86
2324.1  79  74  22343.453   79

What we really want is some way to specify the minimum width of a value

We can do this by putting a number in front of the . in the format specifier

$ cat print_09.py 
# # prints a table of values using f-strings 

print(f"{24:10d} {234523.4589:10.2f} {31:10d} {86:10d}")
print(f"{2324:10d} {79:10d} {74:10d} {22343.453:10.2f} {79:10d}"

$ python3 print_09.py 
  13452.12         24  234523.46         31         86
      2324         79         74   22343.45         79

By writing "10" before the .
We are telling Python to make sure each number is 10 characters wide
Notice that I did not use variable names inside { }
I used number literals instead
I also mixed decimals and integers

Alignment with the `format` Function

I left off the column labels in the previous script ...
because I did not want to complicate things
But we can use format specifiers to set the minimum width of strings ...
as well as numbers

When we work with strings we use the type specifier s

$ cat print_10.py 
# prints a table of values using format specifiers

print(f"{'Mon':10s} {'Tue':10s} {'Wed':10s} {'Thr':10s} {'Fri':10s}")
print(f"{13452.12:10.2f} {24:10d} {234523.4589:10.2f} {31:10d} {86:10d}")
print(f"{2324:10d} {79:10d} {74:10d} {22343.453:10.2f} {79:10d}")

$ python3 print_10.py 
Mon        Tue        Wed        Thr        Fri       
  13452.12         24  234523.46         31         86
      2324         79         74   22343.45         79

The output does not look good because the numbers are right aligned
But the strings are left aligned
We can force all the strings to be right aligned

If we place a > at the beginning of the format specifier

$ cat print_11.py 
# prints a table of values using format specifiers

print(f"{'Mon':>10s} {'Tue':>10s} {'Wed':>10s} {'Thr':>10s} {'Fri':>10s}")
print(f"{13452.12:10.2f} {24:10d} {234523.4589:10.2f} {31:10d} {86:10d}")
print(f"{2324:10d} {79:10d} {74:10d} {22343.453:10.2f} {79:10d}"

$ python3 print_11.py 
       Mon        Tue        Wed        Thr        Fri
  13452.12         24  234523.46         31         86
      2324         79         74   22343.45         79

Formating Percentages

The format specifiers can turn decimals into a percentage

All you have to do is change the type specifier from f to %

>>> print(f"3 out of 10 is {3/10:.0%}")
3 out of 10 is 30%

Notice that the value between the { } is not a variable ...
or a literal ...
but a calculation using operators

Order of F-string Designators

The different characters that appear after the : inside the { } are called designators

And they have appear in a certain order

[alignment] [width] [,] [.precision] [type]

Don't Worry about F-strings

I will not ask you to use them on a quiz or exam.

I do not expect you to memorize all the different components of a format specifier.

Tips and Examples

Review

New Material

Studying

Questions

Announcement

Quiz 1

Homework 3

Resources

Tips and Examples

Converting Decimal to Integer

Review

Performing Calculations

Decimal and Integer Division

Exponent Operator

Remainder Operator

Operator Precedence

Grouping with Parentheses

Mixed-Type Expressions and Data Type Conversion

Breaking Long Statements into Multiple Lines

Attendance

New Material

More Expressions inside Expressions

Whitespace Characters

Escape Sequences

Concatenation Operator

Concatenating Strings with Numbers

Optional Material - Not Needed for Exams or Assignments

Using print without Going to a New Line

print Item Separator

F-strings

Printing Numbers

Format Specifiers in F-strings

Using Format Specifiers with Scientific Notation

Using Format Specifiers to Put Commas in Big Numbers

Specifying Minimum Length for a Number

Alignment with the format Function

Formating Percentages

Order of F-string Designators

Don't Worry about F-strings

Studying

Class Exercise

Class Quiz

Using `print` without Going to a New Line

`print` Item Separator

Alignment with the `format` Function