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What are the Basic Control Structures of Structured Programming?

Published in Programming Fundamentals 3 mins read

The basic control structures of structured programming, found in essentially all programming languages, are sequence, selection, and iteration.

These fundamental structures dictate the flow of execution within a program, enabling developers to create logical and organized code blocks that are easier to read, write, and maintain.

The Three Basic Control Structures

According to the provided information, all languages have the first three categories of control structures: sequence, selection, and iteration.

Let's explore each of these:

Sequence

  • Description: This is the most basic control structure. Instructions are executed one after another, in the order they appear. There is no branching or skipping.
  • Practical Insight: This is the default flow of execution in any program. Every line of code is inherently part of a sequence unless explicitly altered by selection or iteration structures.
  • Example:
    Read input_value
Calculate result = input_value * 2
Print result

    These steps happen strictly in order.

Selection (or Decision)

  • Description: This structure allows the program to make decisions based on conditions. A block of code is executed only if a specific condition is true.
  • Examples:
    • The if structure: Executes a block of code if a condition is true.
    • The if then else structure: This common structure, explicitly mentioned in the reference, executes one block of code if a condition is true and a different block if the condition is false.
    • Switch/Case structures: Used for selecting one of many code blocks to execute based on the value of an expression.
  • Practical Insight: Selection is crucial for handling different scenarios or inputs. It introduces branching logic into the program flow.
  • Code Snippet (Illustrative):
    if (score >= 50) then
    print "Pass"
else
    print "Fail"
end if

Iteration (or Repetition/Looping)

  • Description: This structure allows a block of code to be executed repeatedly until a certain condition is met or for a specified number of times.
  • Examples:
    • The while structure: Explicitly mentioned in the reference, this loop repeats a block of code as long as a condition remains true. The condition is checked before each iteration.
    • for loops: Typically used when the number of iterations is known beforehand.
    • do-while loops: Similar to while, but the condition is checked after the block is executed, guaranteeing at least one iteration.
  • Practical Insight: Iteration is essential for tasks that involve processing lists of items, performing calculations multiple times, or waiting for an event.
  • Code Snippet (Illustrative):
    count = 0
while (count < 5)
    print "Hello"
    count = count + 1
end while

Summary Table

Control Structure Description Common Examples (Reference Mentioned) Purpose
Sequence Execute instructions in order. (Default flow) Ordered execution
Selection Execute code based on a condition. if then else Decision making, conditional execution
Iteration Repeat a block of code based on a condition/count. while Repetitive tasks, processing collections

These three basic structures—sequence, selection, and iteration—form the bedrock of structured programming, allowing for the construction of complex algorithms from simple, well-defined control flows.

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