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Grade 7 · lesson 2 of 4

Decode a Run-Length Record

Expand each count-symbol pair in order, keeping separate runs separate.

About 18 minutes, plus any extended practice

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Supplied materials

  • Paper and pencil; answers may also be spoken or typed.
  • All cases, labels, numbers and rules are supplied on this page. No real account, private information or purchase is needed.

Learn the idea

Run-length encoding stores a repeated value with its run count. Our format 3A2B means three A symbols followed by two B symbols. It is lossless for these records because decoding restores every value in order. Compression efficiency depends on the input and representation overhead.

Expand each count-symbol pair in order, keeping separate runs separate. Do not sort runs by symbol or add all counts to one symbol.

Use this supplied case: Record 3A2B. Decode the stated run-length record. The task is to connect the given information to the computing idea. Write a short evidence trail so another learner can reproduce your choice. The simplified case describes the stated model, not every real system.

  1. Represent the supplied worked case with a labeled drawing, table or short trace.
  2. Expand each count-symbol pair in order, keeping separate runs separate.
  3. Explain why the worked answer fits and why one alternative does not.
  4. Try the new check and practice cases; revisit the stated rule if your evidence does not match.
Worked example

See the method in action

A fresh run-length record 4C1D has two ordered runs. Expand 4C into CCCC and 1D into D; joining them gives CCCCD. In a second worked record 1C4D, C followed by DDDD gives CDDDD. A count applies only to its paired symbol. Keep both count and run order: sorting or merging separated runs would apply a different rule and might not recover the supplied sequence.

Your turn

Check your reasoning

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Helpful tips
  • Expand each count-symbol pair in order, keeping separate runs separate.
  • Do not sort runs by symbol or add all counts to one symbol.
Record 2A3B. Decode the stated run-length record.

Feedback appears here after you check. This is practice, not a graded school assessment.

See the answer and explanation

AABBB

The supplied case states: Record 2A3B. Decode the stated run-length record. Under the stated distinction, this identifies AABBB. Run-length encoding stores a repeated value with its run count. Our format 3A2B means three A symbols followed by two B symbols. It is lossless for these records because decoding restores every value in order. Compression efficiency depends on the input and representation overhead. Expand each count-symbol pair in order, keeping separate runs separate.

Take it further

New run-length record: 2C3D1C. Expected: CC followed by DDD followed by C, giving CCDDDC. Preserve both C runs instead of sorting symbols or merging their locations.

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More practice: try new cases

Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.

More practice: try new cases
1. Record 1A4B. Decode the stated run-length record.
Need a hint?
  • Expand each count-symbol pair in order, keeping separate runs separate.
  • Do not sort runs by symbol or add all counts to one symbol.
2. Record 4A1B. Decode the stated run-length record.
Need a hint?
  • Expand each count-symbol pair in order, keeping separate runs separate.
  • Do not sort runs by symbol or add all counts to one symbol.
3. Record 3 A followed by 2 B. Decode the stated run-length record.
Need a hint?
  • Expand each count-symbol pair in order, keeping separate runs separate.
  • Do not sort runs by symbol or add all counts to one symbol.

Your choices are temporary. Feedback appears after you check all the answers.

Nothing here is saved to an account or sent to a teacher. Keep your own notes or download them before leaving. The score describes these questions; it does not certify subject mastery.

Check the subject sources

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.