Understand and use sequence, selection, and iteration

Programming: Sequence, Selection, and Iteration

Objective: Understand and use sequence, selection, and iteration to control the flow of a program. 🚀

1. Sequence

Sequence is the most basic control structure. It means that the program executes instructions one after another, just like following a recipe step‑by‑step.

  • Start with a clear order of actions.
  • Each step depends on the previous one.
  • Think of it as a line of dominoes: when the first falls, the rest follow.

Example (pseudocode):

print("Hello, world!")
x = 5
y = x + 2
print(y)

2. Selection (If‑Else)

Selection lets the program choose between two or more paths based on a condition. It’s like deciding which door to open.

  • Use if to test a condition.
  • Use else for the alternative path.
  • Use elif (or else if) for multiple conditions.

Example (pseudocode):

if (x > 0) {
    print("Positive")
} else if (x < 0) {
    print("Negative")
} else {
    print("Zero")
}

Mathematical condition: $x > 0$ means “x is greater than zero”.

3. Iteration (Loops)

Iteration repeats a block of code until a condition is met. Think of it as a merry‑go‑round that keeps spinning until you press stop.

  • While loop: repeats while a condition is true.
  • For loop: repeats a fixed number of times.
  • Always update the loop variable to avoid infinite loops.

Example (pseudocode):

for i from 1 to 5 {
    print(i)
}
while (x < 10) {
    x = x + 1
}

Mathematical notation: $$\sum_{i=1}^{5} i$$ represents the sum of numbers from 1 to 5.

Control Structure Comparison

Structure When to Use Key Syntax
Sequence Simple, linear tasks. Just write statements one after another.
Selection Deciding between alternatives. if (condition) { … } else { … }
Iteration Repeating actions. for or while loops.

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