Programming Lesson 4: Control flow

Learning goals

• Use branching and switch statements to choose actions.
• Use loops to repeat work and avoid off-by-one errors.
• Understand break, continue, and readable condition design.

Branching: if / else / else if

Use if to run code when a condition is true. Use else if to check additional conditions in order, and else as a fallback.

const score = 85;
if (score >= 90) {
  console.log('A');
} else if (score >= 75) {
  console.log('B');
} else if (score >= 60) {
  console.log('C');
} else {
  console.log('F');
}

switch

switch is useful when comparing the same value against many possible constants — it can be clearer than long else-if chains.

switch (color) {
  case 'red':
    console.log('Stop');
    break;
  case 'green':
    console.log('Go');
    break;
  default:
    console.log('Unknown');
}

Off-by-one errors

Common when looping ranges. Prefer clear boundaries and test small inputs. Use inclusive/exclusive naming: for (let i = 0; i < n; i++) iterates n times from 0..n-1.

Designing conditions

Favor readability: name boolean expressions and avoid deeply nested conditions. Example:

// less readable
if (a && (b || c) && !d) { doWork(); }

// more readable
const canProceed = a && (b || c) && !d;
if (canProceed) { doWork(); }

Examples

Grading logic

function grade(score) {
  if (score >= 90) return 'A';
  if (score >= 80) return 'B';
  if (score >= 70) return 'C';
  return 'F';
}

Repeat N times

function repeat(n, fn) {
  for (let i = 0; i < n; i++) fn(i);
}

repeat(3, i => console.log(i)); // 0,1,2

Iterate an array (light)

const items = [10,20,30];
for (const item of items) {
  console.log(item);
}

Practice tasks

1. Sum numbers from 1 to N (expected: correct sum for given N).
2. Implement FizzBuzz for 1..N (optional classic).
3. Find the first item in a list greater than 100 and return its index (or -1 if none).

Common mistakes & debugging

• Off-by-one in loop bounds — test with N=0,1,2.
• Modifying loop variables inside loops leading to unexpected behavior.
• Overusing break/continue making logic harder to follow.
• Nested conditions without named booleans; extract and name expressions for clarity.