CS 1713/1711 Introduction to Computer Science Lecture 22

CS 1713 Lecture 22: Accumulating Sums and Related Problems
October 20, 1999

Accumulating sums Most operations (+, -, *, /, >, etc.) are binary. The operations can only be performed between two operands at a time. When you need to perform the operation on a list of items, the approach is to accumulate the result. As each item is processed, you update the result of everything processed so far. Examples:
- Find the sum of list of numbers.
- Find the largest in a list of numbers.
- Find the product of a list of numbers.
Example: Write a helper function to return the sum of the squares of the integers from 1 to n.
Example: Write a helper function to return n!.
Example: Write a helper function to return the average of n random numbers.

Example: Compare the code for the helper function findFirstVowel and the code for the howManyVowels helper function.

public int howManyVowels(String s) {
   int count = 0;
   int pos = 0;
   String vowels = new String("AEIOUaeiou");
   char thisChar;

   while (pos < s.Length()) {
      thisChar = s.charAt(pos);
      if (vowels.indexOf(thisChar) != -1)
          count++;
      pos++;
   }
   return count;
}

Review important methods for String class:
- length() - gives the number of characters in the string.
- charAt(i) - gives the character at position i in the string (starting from 0)
- indexOf(c) - gives the position of the first occurrence of the character c in the string or -1 if c isn't in the string.
- substring(offset) - returns the last part string starting with position offset.
- substring(offset, end) - returns the part of the string from offset to (but not including) end.
Example: Write a helper function to return the number of leading blanks in the String s.
Example: Write a helper function to return a new copy of the String s with the leading blanks removed.

Objective: Practice with accumulation.

Last revision: October 22, 1999 at 8:30 am