Given the following definition of 'sqr' and 'res1'

How many times was 'sqr' called to compute 'res1'?

Note: 'even' is an inbuilt function which returns true if the argument is an even number

Question

Given the following definition of 'sqr' and 'res1'

How many times was 'sqr' called to compute 'res1'?

Note: 'even' is an inbuilt function which returns true if the argument is an even number

BlackTom AI · Accepted Answer

We’re given a lazy evaluation expression: take 4 (map sqr (filter even [1 ..])) and asked how many times sqr is called to compute res1.

Option 1: 8
Reasoning: If you think in terms of how many input numbers are inspected, you may assume the filter needs 8 inputs to produce 4 evens. However, the question asks how many times sqr is invoked, not how many inputs are checked by the filter. The map sqr is applied to elements of the filtered list, and take 4 needs only the first 4 results after mapping. That means sqr is applied once for each of the first four even numbers (2, 4, 6, 8). So sqr is not called 8 times.

Option 2: 20
Reasoning: This would imply many more elements are processed, perhaps by extending the input range or by misunderstanding the laziness of take together with map and filter. In a lazy evaluation, only as many elements as needed to produce the requested 4 outputs are evaluated, so 20 is not necessary here.

Option 3: 2
Reasoning: This would be too few. To produce four results after applying sqr, at least four applications of sqr are required (one per result). A count as low as 2 would only yield two squared values, not four.

Option 4: 4
Reasoning: This matches the number of outputs requested by take 4. Since the evaluation is lazy, we generate the first four elements of the filtered stream (which are 2, 4, 6, 8), apply sqr to each to form (4, 16, 36, 64), and take those four results. Therefore, sqr must be invoked once for each of the first four evens, totaling four calls.

CPT_S-355-PULLM-1-LEC Scheduled Quiz 1

Given the following definition of 'sqr' and 'res1' How many times was 'sqr' called to compute 'res1'? Note: 'even' is an inbuilt function which returns true if the argument is an even number

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