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9.2: Exercise 7

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  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    In the repository for this book, you’ll find the source files for this exercise:

    • contains starter code for the first part of the exercise.
    • contains the unit tests for MyLinearMap.

    You’ll also find the Ant build file build.xml.

    Run ant build to compile the source files. Then run ant MyLinearMapTest; several tests should fail, because you have some work to do!

    First, fill in the body of findEntry. This is a helper method that is not part of the Map interface, but once you get it working, you can use it for several methods. Given a target key, it should search through the entries and return the entry that contains the target (as a key, not a value) or null if it’s not there. Notice that I provide an equals method that compares two keys and handles null correctly.

    You can run ant MyLinearMapTest again, but even if your findEntry is correct, the tests won’t pass because put is not complete.

    Fill in put. You should read the documentation of Map.put at so you know what it is supposed to do. You might want to start with a version of put that always adds a new entry and does not modify an existing entry; that way you can test the simple case first. Or if you feel more confident, you can write the whole thing at once.

    Once you’ve got put working, the test for containsKey should pass.
    Read the documentation of Map.get at and then fill in the method. Run the tests again.
    Finally, read the documentation of Map.remove at and fill in the method.
    At this point, all tests should pass. Congratulations!

    This page titled 9.2: Exercise 7 is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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