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Example Program: Printing Outliers in Data

The requirement specification for a program is:

A set of positive data values (200) are available. It is required to find the average value of these values and to count the number of values that are more than 10% above the average value.

Since the data values are all positive a negative value can be used as a sentinel to signal the end of data entry. Obviously this is a problem in which an array must be used since the values must first be entered to find the average and then each value must be compared with this average. Hence the use of an array to store the entered values for later re-use.

An initial algorithmic description is:

initialise.
enter elements into array and sum elements.
evaluate average.
scan array and count number greater than
                       10% above average.
output results.
This can be expanded to the complete algorithmic description:
set sum to zero.
set count to zero.
set nogt10 to zero.
enter first value.
while value is positive
   {
    put value in array element with index count.
    add value to sum.
    increment count.
    enter a value.
   }
average = sum/count.
for index taking values 0 to count-1
    if array[index] greater than 1.1*average
       then increment nogt10.
output average, count and nogt10.
In the above the variable nogt10 is the number greater than 10% above the average value. It is easy to argue that after exiting the while loop, count is set to the number of positive numbers entered. Before entering the loop count is set to zero and the first number is entered, that is count is one less than the number of numbers entered. Each time round the loop another number is entered and count is incremented hence count remains one less than the number of numbers entered. But the number of numbers entered is one greater than the number of positive numbers so count is therefore equal to the number of positive numbers.

A main() program written from the above algorithmic description is given below:

void main()
{
  const int NE = 200;  // maximum no of elements in array
  float sum = 0.0;     // accumulates sum
  int count = 0;       // number of elements entered
  int nogt10 = 0;      // counts no greater than 10%
                       // above average
  float x;             // holds each no as input
  float indata[NE];    // array to hold input
  float average;       // average value of input values
  int i;               // control variable

      // Data entry, accumulate sum and count
      // number of +ve numbers entered
  cout << "Enter numbers, -ve no to terminate: " << endl;
  cin >> x;
  while (x >= 0.0)
    {
      sum = sum + x;
      indata[count] = x;
      count = count + 1;
      cin >> x;
    }

      // calculate average
  average = sum/count;

      // Now compare input elements with average
  for (i = 0; i < count; i++)
    {
      if (indata[i] > 1.1 * average)
        nogt10++;
    }

      // Output results
  cout << "Number of values input is " << n;
  cout << endl
       << "Number more than 10% above average is "
       << nogt10 << endl;
}
Download program.
Download sample data.

Since it was assumed in the specification that there would be less than 200 values the array size is set at 200. In running the program less than 200 elements may be entered, if n elements where n < 200 elements are entered then they will occupy the first n places in the array indata. It is common to set an array size to a value that is the maximum we think will occur in practice, though often not all this space will be used.


Next: Example Program: Test of Up: Arrays Previous: Initialisation of arrays