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Grade 4 · lesson 2 of 4

Watch the Order of Updates

Trace two variables and explain how assignment order changes the result.

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Supplied materials

  • Paper and pencil; an adult may read aloud. No device, account or purchase is needed.

Learn the idea

A variable name refers to its current value. An assignment such as b = a copies a’s value at that moment; it does not make b follow every later change to a. Execute statements from top to bottom and write both variables after each statement, even when only one changes.

Paper program: set a = 2; set b = 0; set b = a; set a = a + 3; show a and b. The copy happens before the addition. We will compare it with a modified program that adds to a before copying a into b. The statements use the same numbers but their order changes b.

  1. Draw columns statement, a and b; use a dash before a variable is set.
  2. Trace a = 2 and b = 0.
  3. Copy a into b, then add 3 to a and record both values.
  4. Swap the last two updates and trace again from the start.
Worked example

See the method in action

Original rows after both variables are set: (a, b) = (2, 0) → (2, 2) → (5, 2). Reordered updates give (2, 0) → (5, 0) → (5, 5). The original shows 5 and 2.

Your turn

Check your reasoning

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Start a = 4 and b = 0; then b = a; then a = a + 1. What pair is shown?

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See the answer and explanation

a = 5, b = 4

b copies the current 4 before a increases to 5. It remains 4 afterward.

Take it further

In the original program, change the start to a = 6. Expected original output: a = 9, b = 6. If you add first and copy second, both are 9.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.