Now to print the name and telephone number:

if index >0 then

writeln('Telephone number of ',name[index],' is: ',tele[index]);

Program 10-1 first sets up an array of 7 names and 7 corresponding telephone numbers. This is done through constant declaration of the array elements. There are many ways to do this. Ideally, you would read the names and telephone numbers from a file which is updated as needed. However, for the purpose of the search and sort examples, this method of array declaration is sufficient. In order to find the telephone number of a person the following steps needs need to be followed:

PROGRAM 10-1

In this example, since the names were not alphabetized, we had to search through the entire list. This type of searching is known as the sequential search through an unordered list or linear search. However, if the list was alphabetized, like a telephone directory, we could stop looking after a certain name is passed. For example, you were looking for Annet Brooks; you looked through all names beginning with 'Broo' and now you are at Jerry Brown. You know that either you passed the name you are looking for or the name is not listed.

If you have an alphabetized list you could look at the name in the middle of that list and determine whether the name you are interested in should be in the first half or the second half of the list. If it is in the first half of the list, you could look at the middle name of the first half of the list and determine whether the name is in the first half or the second half of that portion, and so on. This way, you cut the size of the list to one half each time. Such a searching technique is known as the binary search. We will not be covering the binary search here.

SORTING

A list of ten random numbers could be sorted from the smallest to the largest several different ways. One approach might be to look through the list find the smallest number, place it a second list called the sorted list and scratch it from the original list. Now look for the smallest number in the remaining list, place it in the sorted list and scratch it from the original list. Continue this until only one number is left in the original list, move it to the sorted list, and you are done. This algorithm can be used to sort a list in Pascal.

PROGRAM 10-3

Since we can't scratch out a number from the memory, once a number is used, we would set it to a large number so it would be used as the smallest number again. Program 10-3 does the following:

1. places ten integers in an array of numbers.

2. prints these ten unsorted integers.

3. find the smallest number from the list 10 times.

4. when the smallest number is found save it's index.

5. move the smallest number to the sorted list.

6. scratch that number out from the original list.

(do this by making it a large number).

7. Print the sorted list.

The main disadvantage to this sort algorithm is that we have to have two arrays to do the sorting, the original array and the sorted one. The second disadvantage is that searching for the smallest should be done as many times as the total number of numbers. Each search goes through the whole list.

Another algorithm for sorting is known as the bubble sort. Bubble sort compares two adjacent numbers; if they are out of order, the numbers are swapped. Keep doing this until there are no more swaps. We will see that after one iteration of this bubbling process, the largest number would be at the bottom. Knowing this, the next time we only have to go through the list one less time. Let us modify the above program to the bubble sort.

Notice that in Program 10-4 we compared the first number to the second number and so on:

If numbers[count] > numbers[count+1] then ...

We need to be very careful that count+1 does not go over the total number of elements we have in the list. In order to do this we make sure that 'count' will never get larger than one less than the total number of elements.

pass := pass -1 ;