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

New Material

• More Expressions inside Expressions
• Whitespace Characters
• Escape Sequences
• Concatenation Operator
• Concatenating Strings with Numbers
• 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

Studying

• Attending Class

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.

Questions

Are there any questions before I begin?

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 the remainder is 0, the number is even
• If 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 decimala
• 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

  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 language
• 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

• A Windows pathname uses the backslash character \ after the name of each directory
• 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 vales 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
  >>> cheer = "Let's go " + team
  >>> print(cheer)
  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 values on both sides of the + must be strings
• If they are not, 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

• The concatenation operator 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

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

• The print function normally adds the newline character ...
• 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
• This is done using a technique similar to what we did above
• Here the argument name is 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 takes another approach

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.

These are the sorts of things you look up when you need them.

Studying

• Attending Class

Class Exercise

Class Quiz