Variables, Expressions, and Statements

Chapter Overview

• Chapter 2 of Python for Everybody, focusing on the fundamental concepts of variables, expressions, and statements in Python.

Constants

• Definition: Fixed values used in programming that do not change.

  • Types of Constants:

  • Numeric Constants: Regular numerical values (e.g., integers, floats).

  • String Constants: Enclosed in single (') or double (") quotes.

• Examples:

  • python print(123) # Output: 123 print(98.6) # Output: 98.6 print('Hello world') # Output: Hello world

Reserved Words

• Definition: Words that cannot be used as variable names due to their reserved status in Python syntax.

• List of Reserved Words:

  • 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

Variables

• Definition: A named location in memory where data can be stored and retrieved.

• Characteristics:

  • Programmers can choose the names of variables at their discretion.

  • Variable contents can be altered in subsequent statements.

• Example of Assignment:

  • python x = 12.2 y = 14 x = 100 # Changing x's value

Python Variable Name Rules

• Naming Conventions:

  • Must start with a letter or underscore (_).

  • Can consist of letters, numbers, and underscores.

  • Is case sensitive (e.g., spam, Spam, SPAM are different).

• Examples of Valid and Invalid Names:

  • Good: spam, eggs, spam23, _speed

  • Bad: 23spam, #sign, var.12

Mnemonic Variable Names

• Concept: Using meaningful names as a memory aid to remember the value represented.

  • Well-chosen names may sound appealing, potentially causing confusion if they resemble reserved keywords.

  • Refer to Wikipedia on Mnemonics for further information.

Code Examples and Explanation

• Sample Code Snippet:

  • python x1q3z9ocd = 35.0 x1q3z9afd = 12.50 x1q3p9afd = x1q3z9ocd * x1q3z9afd print(x1q3p9afd)

  • This code assigns and multiplies two values, storing the result in x1q3p9afd, and prints the result.

• Similar examples are provided with varied variable names to demonstrate that the underlying operations remain the same.

Assignment Statements

• Definition: Used to assign a value to a variable with the equals sign (=).

• Syntax: Comprises an expression on the right-hand side and a variable on the left-hand side.

• Examples:

  • python x = 3.9 * x * (1 - x)

  • After evaluation, the result is stored in the variable x.

Numeric Expressions

• Operations: All basic mathematical operations done in Python using specific symbols:

  • +: Addition

  • -: Subtraction

  • *: Multiplication

  • /: Division

  • **: Exponentiation (power)

  • %: Remainder (modulus)

• Examples:

  • python xx = 2 xx = xx + 2 # Output: 4

• The use of these symbols is essential for performing calculations in Python.

Order of Evaluation

• Importance: Determines the sequence in which operations are performed based on operator precedence.

  • Higher precedence operations are calculated before lower precedence ones.

• Operator Precedence Rules:

  1. Parentheses

  2. Exponentiation (**)

  3. Multiplication (*), Division (/), Remainder (%)

  4. Addition (+), Subtraction (-)

  5. Left to Right

• Using parentheses helps clarify complex expressions and prioritize certain calculations.

Type in Python

• Definition: The classification of values (e.g., integer, float, string).

  • Python differentiates between types, allowing operations specific to each.

• Example:

  • python ddd = 1 + 4 # Integer addition eee = 'hello ' + 'there' # String concatenation

Type Matters

• Behavior: Certain operations depend on types. For instance, concatenating a string and an integer without conversion will result in an error.

• Checking Types: Use the type() function to determine the type of a variable.

  • Example of an error due to type mismatch:

  • python eee = 'hello ' + 'there' eee = eee + 1 # Raises TypeError

Different Types of Numbers

• Main Types:

  • Integers: Whole numbers (e.g., -14, -2, 0)

  • Floating Point Numbers: Numbers with decimals (e.g., -2.5, 98.6)

• Examples of Type Checking:

  • python xx = 1 temp = 98.6 type(xx) # Output: <class 'int'> type(temp) # Output: <class 'float'>

Type Conversions

• Concept: When mixing integers and floats, integers are implicitly converted to floats.

• Control Conversion: Use built-in functions int() and float() to manually convert types as needed.

  • Example of floating point conversion:

  • python print(float(99) + 100) # Output: 199.0

Integer Division

• Definition: Division where the result is always a float in Python 3.x (unlike Python 2.x where it behaved differently).

• Examples:

  • python print(10 / 2) # Output: 5.0 print(9 / 2) # Output: 4.5

String Conversions

• Converting between strings and numerical types requires care. If the string is non-numeric, an error will occur:

  • python sval = '123' ival = int(sval) # Converts '123' to integer print(ival + 1) # Output: 124 nsv = 'hello' niv = int(nsv) # Raises ValueError

User Input

• Getting Input: Use the input() function to pause the program and read data from users.

  • Example:

  • python nam = input('Who are you? ') print('Welcome', nam) # Outputs: Welcome [User's Name]

Converting User Input

• User input is always read as a string; conversions are necessary for numerical data.

  • Example:

  • python inp = input('Europe floor? ') usf = int(inp) + 1 print('US floor', usf) # Outputs US floor based on user input

Comments in Python

• Usage: Anything after a # is ignored by the interpreter. Comments are useful for:

  • Describing code functionality.

  • Documenting authorship, or other important information.

  • Temporarily disabling code lines during testing or debugging.

Example of Code with Comments

• ```python

Get the name of the file and open it

name = input('Enter file:')
handle = open(name, 'r')

Count word frequency

counts = dict()
for line in handle:
words = line.split()
for word in words:
counts[word] = counts.get(word, 0) + 1

Find the most common word

bigcount = None
bigword = None
for word, count in counts.items():
if bigcount is None or count > bigcount:
bigword = word
bigcount = count

All done

print(bigword, bigcount)
```

Summary

• Key topics covered in this chapter include:

  • Type of variables

  • Reserved words in Python

  • Principles of variable naming (including mnemonic names)

  • Operators and operator precedence in expressions

  • Integer division vs. floating-point results

  • Type conversion between numerics and strings.

  • User input handling and converting inputs.

  • Importance of comments for code documentation and clarity.

Exercise

• Prompt users for hours and rate per hour to compute gross pay:

  • Example Input:

  • Enter Hours: 35

  • Enter Rate: 2.75

  • Example Output:

  • Pay: 96.25