Python: Assignment and Print Statements

• Python can be used interactively through a command line interface accessed by typing "python".
• Python interprets commands that are typed and waits for instructions.

Assignment Statements

• Assignment statements in Python use the equal sign (=) to assign a value to a variable
• Python takes a piece of computer memory, names it with the variable, and stores the value in that memory location.
• x = 1 means the value 1 is stored in memory and named x.

Print Statements

• print(x) retrieves the value stored in the memory location x and displays it.
• Expressions like x = x + 1 will take the current value of x, add 1 to it, and then store the new value back into x.
• Errors in the typed statements will lead to syntax errors.

Python Vocabulary

• Reserved words have specific meanings in Python and cannot be used as variable names.
• Examples include import, assert, if, for, pass, and while.
• Reserved words like walk are understood by dogs in the same way that Python recognizes reserved words.

Python Sentences

• Sentences are lines of code in a Python program
• Paragraphs are constructed from a series of these lines.

Operators, Functions, and Parameters

• Operators such as + and = are used to perform operations
• print is a function that takes a parameter (e.g., x) to output its value.

Python Programs

• Interactive Python is useful for learning, but for larger programs, code is typically written in a text file.
• Python files typically have a .py extension.
• Text editors like Atom can syntax highlight .py files, which helps in understanding the code and identifying syntax errors.
• Scripts are stored sets of instructions in text files that Python can execute.

Basic Patterns

• Sequential execution involves performing one step after another in order.
• Conditional execution involves executing a block of code only if a certain condition is true
• Repetition involves repeating a series of steps multiple times.
• The fourth pattern, store and retrieve, is introduced in Chapter 4.

Sequential Steps

• Code executes line by line in the order it appears in the script.
• Example:
  • x = 2
  • print(x) (output: 2)
  • x = x + 2
  • print(x) (output: 4)

Conditional Steps

• Conditional execution is controlled by if statements.
• If the condition in the if statement is true, the indented block of code is executed; otherwise, it is skipped.
• Example:
  • x = 5
  • if x < 10:
print('Smaller')
  • if x > 20:
print('Bigger')
  • print('Finish')
• Output: Smaller, Finish since the first if condition is true and the second is false.

Repeated Steps (Loops)

• Loops are used to repeat a block of code multiple times.
• Two main types of loops: while and for.
• A while loop continues executing as long as a certain condition is true.
• Example:
  • n = 5
  • while n > 0:
print(n)
n = n - 1
  • print('Blastoff')
• Loops must have a mechanism to terminate to avoid infinite loops.
• Iteration variables are used to control the number of times a loop executes

Combining Patterns

• Programs often combine sequential, conditional, and repeated steps.
• Shows an example word-counting program that uses all three patterns.
• The program reads a file, creates a histogram of word frequencies, and finds the most frequent word.

Program Structure

• Top part of the example program reads a filename from the user and opens the file.
• Middle section creates a histogram, counting word frequencies.
• Final part goes through the histogram to locate the word that occurs most often

Course Progression

• Concepts will become clearer around Chapters 6 and 7.
• Chapter 1 provides an overview of the course and introduces basic vocabulary related to hardware and programming concepts.