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AP® Computer Science A · Unit 1 · Topic 1.5

Casting and Range of Variables: Checklist (Computer Science A 1.5)

A self-check list for casting with (int) and (double), rounding doubles, the range of int, integer overflow and round-off error, linked to the guide and practice questions.

Work through the list without notes. If you cannot do a statement, follow its link, then come back. Guide = study guide; Practice = practice questions.

Understanding

  • I can explain that casting a double to an int truncates the digits after the decimal point. (Guide: “Casting a double to an int truncates”; Practice Question 1)
  • I can explain when an int is widened to a double automatically. (Guide: “Automatic widening”)
  • I can say what Integer.MAX_VALUE and Integer.MIN_VALUE hold and why an int has a limited range. (Guide: “The range of int”; Practice Question 4)
  • I can explain what integer overflow is and that it throws no exception. (Guide: “Integer overflow”; Practice Questions 4 and 8)
  • I can explain what a round-off error is and how using int values avoids it. (Guide: “Round-off error in double”; Practice Question 5)

Skills

  • I can evaluate expressions that contain (int) and (double), including truncation of negative values. (Guide: Worked example 2; Practice Questions 1 and 10)
  • I can place a cast so that a division keeps its fractional part. (Guide: Worked example 1; Practice Questions 2, 7 and 9)
  • I can round a non-negative or a negative double to the nearest int using a cast. (Guide: “Rounding to the nearest integer”; Practice Questions 3 and 7)
  • I can write an average or percentage expression that is correct for int variables. (Practice Questions 7 and 9)

Reasoning

  • I can explain why (double) (a / b) and (double) a / b differ. (Guide: “Where the cast goes matters”; Practice Questions 2 and 6)
  • I can recognise code where an int expression can overflow and describe what the result will be like. (Guide: Worked example 2; Practice Question 8)
  • I can explain why double avg = sum / count; can lose a fraction even though avg is a double. (Practice Question 9)

All ticked? Move on to the Topic 1.6 study guide, Compound Assignment Operators, or return to the course roadmap.

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