ITEC 120 2010fall ibarland

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lect09b
loops that accumulate info

We ended last time with:

  int i = 0;  // Index variable: number of times through the loop
  while (i < 1000000) {
    System.out.println( "i is now " + i );
    i = i+1;
  }

Task (for you to do by hand): What is the result of 0+1+2+3+4+…+9?
When you did this by hand, what information did you keep track of?

• What number you were currently adding up -- that's the index variable!
• One more piece of information -- the sum-so-far. Thats an “accumulator” variable -- it's the accumulated info from all the previous times through the loop.

Let's write the process in pseudo-code: English instructions for your (perhaps rather dense) roommate to follow. Note that your roommate might get interrupted and stop to text a friend; they'll remember which instruction they're working on but won't remember anything else. Have them keep local variables to store any info they need to remember!

• Keep a variable i, which is currently zero. (Or, “nextNumToAdd”)
• Keep a variable sum-so-far, which is currently zero.
• While i is less than 10:
•     Add i to sum-so-far, and make that the new sum-so-far.
•     Add 1 i to i, and make that the new i.
• sum-so-far now contains your answer.

• Find the sum of the integers in [0,n). (That is, add up i for each i in [0,n).)

  Note: If we want to time our loop, we can use java.lang.System#currentTimeMillis.

• Find the sum of the square root of the integers in [0,n). (That is, add up √i for each i in [0,n).)

• Example: “fingerprinting” a string. Add up the character codes of each letter in "Mississippi".

  For a string s, write a function which computes a “fingerprint” as follows: it's the sum of the i'th character code times (i+1), keeping the sum remainder 1000.

We reached here.

---

• Out of n random numbers (in the range 0...1000), find the sum.

  With a friend: look up the documentation for java.util.Random, and use that class and its methods to generate some random numbers in the Code Pad.
  (Note that Java could have named the class more accurately as “RandomNumberGenerator”, since that's what the object actually is -- it's not itself a random object!)

  Write a small function that just prints three random numbers in the range [-10,+10), as well as the sum of those three numbers. (Double-check that your answer is correct -- note that it's difficult to write automatic test cases when using random numbers!)
  (If you want to combine this problem with using a java.util.Scanner from last week's lab, you can modify the problem to prompt the user for the upper and lower bound.)

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©2010, Ian Barland, Radford University Last modified 2010.Oct.27 (Wed)

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