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Grade 6 · lesson 4 of 4

Measure a Collection with len

Count the elements directly inside the outer sequence.

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

The Python len function reports how many items a sequence contains. For a list, each element is one item, even when an element is itself a list. Counting outer items differs from adding their numeric values or counting all nested contents. These supplied expressions make the nesting visible.

Count the elements directly inside the outer sequence. Do not replace list length with the sum of its values.

Use this supplied case: Paper Python 3 expression: len([2,5,9]). What value results? 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. Count the elements directly inside the outer sequence.
  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

The list [2,5,9] has three outer elements, so its length is 3, rather than their sum 16. The distinct nested list [[8],[4,6,2],[1,5]] also has three outer elements: the first, second and third inner lists. The six numbers inside those lists do not change the outer length to six. Counting outer elements differs from recursive counting. An empty collection has no outer elements; identify which collection is being measured.

Your turn

Check your reasoning

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Helpful tips
  • Count the elements directly inside the outer sequence.
  • Do not replace list length with the sum of its values.
Paper Python 3 expression: len([8,1]). What value results?

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See the answer and explanation

2

The outer sequence is [8, 1]. It contains 2 directly enclosed elements, so len returns 2. Nested contents, if any, do not increase the outer count. Count the elements directly inside the outer sequence.

Take it further

For values=[[1, 2, 3], [], [4]], expected len(values)=3 outer items. Nested contents do not increase the outer list length.

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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. Paper Python 3 expression: len([[1,2],[3,4]]). What value results?
Need a hint?
  • Count the elements directly inside the outer sequence.
  • Do not replace list length with the sum of its values.
2. Paper Python 3 expression: len([0,0,0,0]). What value results?
Need a hint?
  • Count the elements directly inside the outer sequence.
  • Do not replace list length with the sum of its values.
3. Paper Python 3 expression: len([]). What value results?
Need a hint?
  • Count the elements directly inside the outer sequence.
  • Do not replace list length with the sum of its values.

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

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