CS113 Lab 9: Recursion excursion

Due Friday, May 7, before midnight

Goals

  • Practice writing recursive functions

Recursive functions

In the file, Recursion.java, write each of the following functions recursively. You are given basecode containing functions stubs along with a main for testing. You can download the basecode here or use the file in Dropbox.

You must not modify the function parameters and return types for the given functions!

  1. even numbers

    Function reven(int n) should recursively sum the even numbers starting at 2 and ending at n. For example, reven(10) should return 2+4+6+8+10 = 30.

  2. reverse a string

    Function rrevStr(String word) should return the reverse of a given string. For example: the reverse of "hello" is "olleh".

  3. blastoff

    Function rblastoff(int n)`should count down (print one number per line) from the given start number to 1, then print "blastoff!". For example, `rblastoff(10) should count down from 10 to 1, then print "blastoff!".

  4. get integer divisible by 3

    Function (rgetMod()) asks the user to enter an integer that is divisible by 3. Assume the user will enter an integer. If the given integer is not divisible by 3 (Hint: use % to check), the function should recursively ask the user for another integer. This function should return the valid integer. Below is console output from running rgetMod.

    Enter a number: 4
Not divisible by 3!
Enter a number: 6
    rgetMod should return the user’s number!

  5. xerox

    Function (rxerox(String phrase,int n)) should generate and return a String containing n copies of the item. For example, rxerox("paper", 4) should return "paperpaperpaperpaper", and rxerox("2", 10) should return "2222222222".

  6. raise to power

    Function rraise(int n, int m) returns n raised to the mth power. For example, calling rraise(6,2) returns 36.

  7. is palindrome?

    Function risPalindrome(String text) returns true if given text is a palindrome (same forwards and backwards) and false otherwise. For example, risPalindrome("HANNAH") should return true.

  8. count in String

    Function rcount(char x,String S) returns how many times x is in the String S. For example, rcount("a", "aardvark") should return 3.

Testing your functions

You are given test code in main. Functions that return a value are tested using the test function. This function throws an exception if the function returns an incorrect value. For example, suppose there is a bug in our function rrevStr, running gives this error

java.lang.Exception: Test Failed: yolleh != olleh
        at Recursion.test(Recursion.java:118)
        at Recursion.main(Recursion.java:133)

The above error shows us that our function returned "yolleh" but should have returned the value "olleh". We can also see that the function we need to fix is called on line 133.

We encourage you to add your own tests!

What to hand-in

  1. The program, Recursion.java

  2. Make sure your programs have a header containing your name, date, and description.

  3. No writeup is necessary for this assignment

How to hand-in

  1. Copy your files to your dropbox, into the folder called A09.