Veriables

values and types

A value is one of the basic thing a program works with, like a letter or a number.
The values we have seen so far are 1,2, and "hello world!"

These values belong to different types: 2 is an integer, and "hello world!" is a string, so called because is contains a "string" of letters.
You (and the interpreter)can identify strings because they are enclosed in quotation marks.

The print statements also works for integer. We use the python command to start the interpreter.

python
>>> print(4)
4

If you are not sure what type a value has, the interpreter can tell you.

>>> type('Hello,world!')
<class 'str'>
>>> type(17)
<class 'int'>

Not surprisingly,strings belong to the type str and integers belong to the type int.
Less obviously, numbers with a decimal point belong to a type called float, because these numbers are represented in a format called floating point.

>>> type(3.2)
<class 'float'>

What about values like "17" and "3.2"? They look like numbers, but they are in quotation marks like strings.

>>> type('17')
<class 'str'>
>>> type('3.2')
<class 'str'>

They're strings.

When you type a large integer, you might be tempted to use commas between groups of three digits, as in 1,000,000. This is not a legal integer in Python, but it is legal:

>>> print(1,000,000)
1 0 0

Well, that's not what we expected at all! Python interprets 1,000,000 as a comma-separated sequence of integers, which it prints with spaces between.

This is the first example we have seen of a semantic error: the code runs without producing an error message, but it doesn't do the "right" thing.

Variables

One of the most powerful feature of a programming language is the ability to manipulate variable.
A variable is a name refers to a value.

An assignment statement creates new variables and gives them values:

>>> message = 'And now for something completely different'
>>> n = 17
>>> pi = 3.1415926535897931

This example makes three assighments. The first assighs a string a new varialbe named message; the second assighs the integer 17 to n; the third assighs the(approximate)value of π to pi.

To display the vlaue of a variable, you can use a print statement:

>>> print(n)
17
>>> print(pi)
3.141592653589793

The type of a variable is the type of the value it refers to.

>>> type(message)
<class 'str'>
>>> type(n)
<class 'int'>
>>> type(pi)
<class 'float'>

Variable names and keywords

Programmers generally choose names of their variables that are meaningful and document what the variable is used for.

variable names can be arbitrarily long. They can contain both letters and numbers, but they cannot start with a number.
It is legal to use uppercase letters, but it is a good idea to begin variable names with a lowercase letter(you'll see why later).

The underscore charater(_) can appear in a name. It is often used in names with multiple words, such as my_name or airspeed_of_unladen_swallow.
Variable names can start with an underscore charater, but we generally aviod doing this unless we are writing library code for others to use.

If you give a variable an illegal name, you get a syntax error:

>>> 76trombones = 'big parade'
SyntaxError: invalid syntax
>>> more@ = 1000000
SyntaxError: invalid syntax
>>> class = 'Advanced Theoretical Zymurgy'
SyntaxError: invalid syntax

76trombones is illegal because it begings with a number.
more@ is illegal because it contains an illegal charater, @.
But what's wrong with class?

It turns out that class is one of Python's keywords.
The interpreter uses keywords to recognize the structure of the program, and they cannot be used as variable names.

Python reserves 35 keywords:

False      await      else       import     pass
None       break      except     in         raise
True       class      finally    is         return
and        continue   for        lambda     try
as         def        from       nonlocal   while
assert     del        global     not        with
async      elif       if         or         yield

You might want to keep this list handy. If the interpreter complains about one of your variable names and you don't know why, see if it is on this list.

Statements

A statement is a unit of code that the Python interpreter can execute.
We have seen two kinds of staements: print being an expression assignment.

When you type a statement in interactive mode, the interpreter executes it and displays the result, if there is one.

A script usually contains a sequence of statement. If there is more than one statement, the rusults appear one at a time as the statements execute.

For example, the script

print(1)
x = 2
print(x)

produces the output

1
2

The assignment statement produces no output.

Operators and operands

Operators are special symbols that represent computations like addition and multiplication. The values the operator is apllied to are called operands.

The operators + - * /, and ** perform addition, subtraction, multiplication, division, and exponential， as in the following example:

20+32
hour-1
hour*60+minute
minute/60
5**2
(5+9)*(15-7)

There has been a change in the division operator between Python 2 and Python 3. In Python3, the result of this division is a floating point result:

>>> minute = 59
>>> minute/60
0.9833333333333333

The division operator in Python 2 would divide two integers and truncate the result to an integer:

>>> minute = 59
>>> minute/60
0

To obtain the same answer in Python 3 use floored(// integer) division.

>>> minute = 59
>>> minute//60
0

In python 3 integer division functions much more as you would expect if you entered the expression on a calculation.

Expressions

An expression is a combination of values, variable, and operators.
A value all by itself is considered an expression, and so is a variable, so the following are all legal expression(assuming that the variable x has been assigned a value)

17
x
x + 17

If you type an expression in interactive mode, the interpreter evaluates it and displays the result:

>>> 1+1
2

But in a script, an expression all by itself doesn't do anything! This is common source of confusion for beginners.

Exercise 1: Type the following statements in the Python interpreter to see what they do:

5
x = 5
x + 1

Order of operations

When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.
For mathematical operators, Python follows mathematical convention.
The acronym PEMDA is a useful way to remenber the rules:

• Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluate first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even if it doesn't change the result.

• Exponential has the next highest precedence, so 2**1+1 is 3, not 4, 3*1**3 is 3, not 27.

• Multiplication and division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So 2*3-1 is 5, not 4, and 6+4/2 is 8, not 5.

• Operators with the same precedence are evaluated from left to right. So the expression 5-3-1 is 1, not 3, because the 5-3 happens first and then 1 is subtracted from 2.

When in doubt, always put parentheses in your expressions to make sure the computations are performed in the order you intend.

Modulus operator

The modulus operator works on integers and yields the remainder when the first operand is divided by the second. In Pyton, the modulus operator is a percent sign(%). The syntax is the same as for other operators:

>>> quotient = 7 // 3
>>> print(quotient)
2
>>> remainder = 7 % 3
>>> print(remainder)
1

So 7 divided by 3 is 2 with 1 left over.

The modulus operator turns out t be surprisingly useful. For example, you can check whether one number is divisible by another: if x % y is zero, then x is divisible by y.

You can also extract the right-most digit or digits from a number.
For example, x % 10 yields the right-most digit of x (in base 10).
Similarly, x % 100 yields the last two digits.

String operations

The + operator works with strings, but it is not addition in the mathematical sense.
Instead it performs concatenation, which means joining the string by linking them end to end.
For example:

>>> first = 10
>>> second = 15
>>> print(first+second)
25
>>> first = '100'
>>> second = '150'
>>> print(first + second)
100150

The * operator also works with strings by multiplying the content of a string by an integer.
For example:

>>> first = 'Test '
>>> second = 3
>>> print(first * second)
Test Test Test

Asking the user for input

Sometimes we would like to take a value for a variable from the user via their keyboard.
Python provides a built-in function called input that gets input from the keyboard.
When this function is called, the program stops and waits for the user to type something.
When the user presses Return or Enter, the program resumes and input returns what user typed as a string.

>>> inp = input()
Some silly stuff
>>> print(inp)
Some silly stuff

Befor getting input from the user, it is a good idea to print a prompt telling the user what to input.
You can pass a string to input to be displayed to the user before pausing for input:

>>> name = input('What is your name?\n')
What is your name?
Chuck
>>> print(name)
Chuck

The senquence \n at the end of the prompt represents a newline, which is a special charater that cause a line break.
That's why the user's input appears below the prompt.

If you expect the user to type an integer, you can try to convert the return value to int using the int() function:

>>> prompt = 'What...is the airspeed velocity of an unladen swallow?\n'
>>> speed = input(prompt)
What...is the airspeed velocity of an unladen swallow?
17
>>> int(speed)
17
>>> int(speed) + 5
22

But if the user types something other than a string of digits, you get an error:

>>> speed = input(prompt)
What...is the airspeed velocity of an unladen swallow?
What do you mean, an African or a European swallow?
>>> int(speed)
ValueError: invalid literal for int() with
base 10: 'What do you mean, an African or a European swallow?'

We will see how to handle this kind of error later.

Comments

As programs get bigger and more complicated, they get more difficult to read.
Formal language are dense, and it is often difficult to look at a piece of code and figure out what it is doing, or why.

For this reason, it is a good idea to add notes to your programs to explain in natrual language what the program is dong. Thest notes are called comments, and in python they start with # symbol:

# compute the percentage of the hour that has elapsed
percentage = (minute * 100) / 60

In this case, the comment appears on a line by itself. You can also put commands at the end of a line:

percentage = (minute * 100) / 60     # percentage of an hour

Everything from the # to the end of the line is ignored; it has no effect on the program.

Comments are most useful when they document non-abvious features of the code.
It is reasonable to assume that the reader can figure out what the code does; it is much more useful to explain why.

This comments is redundant with the code and useless:

v = 5   # assign 5 to v

This comment contains useful information that is not in the code:

v = 5   # velocity in meters/second

Good variable names can reduce the need for comments, but long names can make complex expressions hart to read, so there is a trade-off.

Choosing mnemonic variable names

As long as you follow the simple rules of variable naming, and avoid reserved words, you have a lot of choice when you name your variables. In the beginning, this choice can confusing both when you read a program and when you write your own programs. the following three programs are identical in terms of what they accomplish, but they different when you read them and try to understand them.

a = 35.0
b = 12.50
c = a * b
print(c)

hours = 35.0
rate = 12.50
pay = hours * rate
print(pay)

x1q3z9ahd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ahd * x1q3z9afd
print(x1q3p9afd)

The Python interpreter sees all three of these programs as exactly the same but humans see and understand these programs quite differently. Humans will most quickly understand the intent of the second program because the programer has chosen variable names that reflect their intent regarding what data will be stored in each variable.

We call these wisely chosen variable names "mnemonic variable names". The word mnemonic means "memory aid".
We choose mnemonic variable names to help us remember why we created the variable in the first place.

While this all sounds great, and it is a very good idea to use mnemonic variable names, mnemonic variable names can get in the way of a beginning programmer's ability to parse and understand code. This is because beginning programmers have not yet memorized the reserved words (there are only 35 of them) and sometimes variable with names that are too descriptive start to look like part of the language and not just well-chosen variable names.

Take a quick look at the following Python sample code which loops through some data. We will cover loop soon, but for now try to puzzle through what this means:

for word in words:
    print(word)

What is happening here? Which of the tokens (for,word,in,etc.) are reserved words and wich are just variable name? Does Python understand at a fundamental level the notion of words? Beginning programmers have trouble separating what parts of the code must be the same as this example and what parts of the code are simply choices made by programmer.

The following code is equivalent to the above code:

for slice in pizza:
    print(slice)

It is easier for the beginner programmer to look at this code and know which parts are reserved words defined by Python and which parts are simply variable names chosen by the programmer. It is pretty clear that Python has no fundamental understanding of pizza and slices and the fact that a pizza consists of a set of one or more slices.

But if our program is truly about reading data and looking for words in the data, pizza and slice are very unmnemonic variable names. Choosing them as variable names distracts from the meaning of the program.

After a pretty short period of time, you will know the most common reserved words and you will start to see the reserved words jumping out at you:

for word in words:
    print(word)

The parts of the code that are defined by Python (for,in,print,and :) are in bold and the programmer-chosen variable(word and words) are not in bold. Many text editors are aware of Python syntax and will color reserved words differently to give you clues to keep your variables and reversed words separate. After a while you will begin to read Python and quickly determine what is a variable and what is a reserved word.

Debugging

At this point, the syntax error you are most likely to make is an illegal variable name, like class and yield, which are keywords, or odd~job and US$, wich contain illegal charaters.

If you put a space in a variable name, Python thinks it is two operands without an operator:

>>> bad name = 5
SyntaxError: invalid syntax

For syntax error, the error messages don't help much. The most common message are SyntaxError: invalid syntax which is not very informative.

The runtime error you are most likely to make is a "use befor def;" that is, trying to use a variable before you have assigned a value. This can happen if you spell a variable name wrong:

>>> principal = 327.68
>>> interest = principle * rate
NameError: name 'principle' is not defined

Variable names are case sensitive, so LaTex is not the name as latex.

At this point, the most likely cause of a semantic error is the order of operations.
For example, to evaluate 1/2π, you might be tempted to write

>>> 1.0 / 2.0 * pi

But the division happens first, so you would get π/2, which is not the same thing!
There is no way for Python to know what you meant to write, so in this case you don't get an error message; you just get the wrong answer.

Glossary

assignment
A statement that assign a value to a variable.

concatenate
To join two operands end to end.

comment
Information in a program that is for other programmer (or anyone reading the souce code) and has no effect on the execution of the program.

evaluate
To simplify an expression by performing the operations in order to yield a single value.

expression
a combination of variables, operators, and values that represents a single result value.

floating point
A type that represents numbers with fractional parts.

integer
A type that represents whole numbers.

keyword
A reserved word that is used by the compiler to parse a program; you cannot use keyword like if, def, and while as variable names.

mnemonic
A memory aid. We often give variables mnemonic names to help us remember what is stored in the variable.

modulus operator
An operator, denoted with a percent sign (%), that works integers and yields the remainder when one number is divided by another.

operands
One of the values on which an operator operates.

operator
A special symbol that represents a simple comutation like addition, multiplication, or string concatenation.

rules of precedence
The set of rules governing the order in which expression involving multiple operators and operands are evaluated.

statement
A section of code that represents a command or action. So far,the statements we have seen are assignments and print expression statement.

string
A type that represents sequences of charaters.

type
A category of values. The types we have seen so far are integer(type int), floating-point numbers(type float), and string(type str)

value
One of the basic units of data, like a number or string, that a program manipulates.

variable
A name that refers to a value.

Exercises

Exercise 2: Write a program that uses input to prompt a user for their name and thehn welcomes them.

Enter your name: Chuck
Hello Chuck

inp = input('Enter your name: ')
print('Hello', inp)

Exercise 3: Write a program to prompt the user for hours and rate per hour to compute gross pay.

Enter Hours: 35
Enter Rate: 2.75
Pay: 96.25

hours = float(input('Enter Hours: '))
rate = float(input('Enter Rate: '))
print('Pay: ', hours * rate)

We won't worry about making sure our pay has exactly two digits after the decimal place for now. If you want, you can play with the built-in Python round function to properly round the resulting pay to two decimal places.

Exercise 4: Assume that we execute the following assignment statements:

width = 17
height = 12.0

For each of the following expressions, write the value of the expression and the type (of the value of the expression).

1. width//2 8 int
2. width/2.0 8.5 float
3. height/3 4.0 float
4. 1 + 2 * 5 11 int

Exercise 5: Write a program which prompts the user for a Celsius temperature, convert the temperature to Fahrenheit, and print out the converted temperature.

inp = input('Enter Celsius Temperature: ')
cel = float(inp)
fahr = (cel * 9.0) / 5.0 + 32.0
print(fahr)