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Programming Essentials in Python Section 4 ​

Symbols Used in Flowcharts ​

Flowcharts use various symbols to visually represent the structure of algorithms and processes. Each symbol has a specific meaning, allowing viewers to quickly understand the flow and logic of a program.

1. Oval ​

  • Function: Used to indicate the START or END of the program.

2. Parallelogram ​

  • Function: Represents input and output operations. Statements like INPUT, READ, and PRINT are typically enclosed within this shape.

3. Rectangle ​

  • Function: Used to indicate processing operations, such as storing values or performing arithmetic calculations.

4. Diamond ​

  • Function: Indicates a decision-making step, known as the decision box. This shape is used to test conditions, ask questions, and direct the flow based on responses.

5. Flow Lines ​

  • Function: Arrows indicate the direction of flow in a flowchart. Every line must have an arrow to specify the process sequence.

6. Circle ​

  • Function: Known as on-page connectors, circles help join different parts of a flowchart on the same page. They also assist in shaping complex flowcharts by connecting sections.

Flowchart Example for Decision-Making ​

Decision Structures in Flowcharts ​

There are two common ways to represent decisions in a flowchart:

  1. Binary Questions: The condition is framed as a question with two possible outcomes:

    • Yes
    • No

  2. Binary Statements: The condition is expressed as a statement with two possible outcomes:

    • True
    • False

Decision-Making in Python ​

In Python, the if statement is used to execute code based on a condition.

1. If Statement ​

The if statement executes a block of code if the condition evaluates to True.

Syntax:

python
# Check a condition
if condition:
  statement1_here  # Executes if condition is True

Example:

python
# Check if a number is positive
num = 5
if num > 0:
  print("The number is positive")

2. If-Else Statement ​

The if-else structure provides an alternative set of actions if the condition is False.

Syntax:

python
if condition:
  statement1_here  # Executes if condition is True
else:
  statement2_here  # Executes if condition is False

Example:

python
# Check if a number is positive or negative
num = -3
if num > 0:
  print("The number is positive")
else:
  print("The number is negative")

3. Nested If Statements ​

Nested if statements allow for more complex decision structures by placing if statements inside other if statements.

Example:

python
# Find the largest of three numbers
a = 10
b = 15
c = 7

if a > b:
  if a > c:
    print("a is the largest")
  else:
    print("c is the largest")
else:
  if b > c:
    print("b is the largest")
  else:
    print("c is the largest")

Algorithm to Find the Largest of Three Numbers ​

  1. Start
  2. Input three numbers: a, b, and c
  3. Check if a is greater than b
    • If yes, then check if a is also greater than c
      • If yes, a is the largest
      • Otherwise, c is the largest
    • If no, check if b is greater than c
      • If yes, b is the largest
      • Otherwise, c is the largest
  4. End

Flowchart Example Using Mermaid ​