Python Selections: Conditions and Decisions

If you need a random floating-point number between $0.0$ and $1.0$ (excluding $1.0$), use random.random(). You can scale this to different ranges using math. To generate a float with $n$ digits before the decimal point, you can use the formula:

$$\text{number} = \text{random.random()} \cdot 10^n$$

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Selection Control: if, if-else, and elif

Selection control is implemented through several types of statements, ranging from simple checks to complex branching.

One-Way if Statements

A one-way if statement executes a block of code only if a condition is True. If the condition is False, the program simply moves to the next line of code outside the block.

Two-Way if-else Statements

When you want to perform one action if a condition is True and a different action if it is False, you use a two-way if-else statement.

Nested if and Multi-Way if-elif-else

For complex logic involving multiple possibilities, you can "nest" one if statement inside another. However, deep nesting can make code hard to read. The preferred way to handle multiple alternatives is the if-elif-else structure.

For example, to assign a letter grade based on a score:

if score >= 90.0:
    grade = 'A'
elif score >= 80.0:
    grade = 'B'
elif score >= 70.0:
    grade = 'C'
else:
    grade = 'F'

In this structure, Python tests each condition in order. A condition is only tested if all preceding conditions were False.

Logical Operators and Precedence

You can combine multiple Boolean expressions using logical operators: and, or, and not.

• and: True only if both expressions are True.
• or: True if at least one expression is True.
• not: Reverses the Boolean value (True becomes False and vice versa).

When these operators are mixed in a single expression, Python follows specific rules of precedence and associativity to determine the order of evaluation.

Exam Tip: The order of precedence for Boolean operators is:

1. not (highest)
2. and
3. or (lowest).

Conditional Expressions

For very simple decisions, Python offers a concise "one-liner" known as a conditional expression. This is useful when you want to assign a value to a variable based on a condition in a single line.

The syntax is: expression1 if boolean-expression else expression2

If the boolean-expression is True, the result is expression1; otherwise, it is expression2. For example, to find the maximum of two numbers:

max_val = number1 if number1 > number2 else number2

Summary of Debugging

Even with these tools, logic errors (bugs) are common. While syntax errors are caught by the computer, logic errors require debugging—the process of finding and correcting mistakes. Effective methods include hand-tracing code, inserting print statements to check variable values, or using a professional debugger utility within an IDE like PyCharm.