Arrays and Text Files

I find that I save some time teaching arrays and text file together from the start.

1. It was mentioned that learners have to test if the file exists even if the paper does not ask you to do so.
2. If an array is declared as [1..30] your while loop needs to test, using the counter, that it does not exceed this number. For example: while not eof(tFile) and (iCount < 30) do
3. It is best to start with your iCount as 0 and inc(iCount); before you add a value to the array. If you use the method of setting iCount to 1 before the loop your counter will be one higher than the number of items in your array which will be needed in another event.
4. Remember that iCount will be global and may not be re-declared as a local variable or you will lose its contents.
5. I use one very large program to code many examples together.
6. First step is to write a Procedure to ReadFromFile. Using the pos, copy, delete method to read from the text file works in all scenarios. Their homework is to study this code and then write a written test every day until they get full marks. Before they switch their computers on I give them a piece of paper and they have to write the code from testing if the file exists up to the CloseFile(tFile); until they get full marks. It does not take long to mark as you can stop the moment you find a mistake. As soon as a learner gets full marks they do not need to write again but those that did not will continue to do so everyday until they also get full marks. I tell them that they are reading from a text file named 'data.txt' and that the format of the data is John#Smith#50 which needs to be read into three parallel arrays declared as [1..30] with arrName and arrSurname being string and arrMark being an integer.
7. Sometimes the question requires you to read only some of the data from the text file into the array. In this case it is best to use the pos function to determine if the value needs to be read into the array. It is important to then only increment the array counter inside of this if. See example below.
8. What do they need to be able to do with arrays:
   • Display in neat columns - we also use a procedure for this in the example program.
   • Calculate the total and average.
   • Using counter arrays like arrCount :array ['A'..'Z'] of integer; to count the number of characters in arrays using just three lines of code. See screenshot below.
   • Searching:
     1. Many - use a for loop if more than one item can meet the condition. For example, display all the John's: for k := 1 to iCount do if arrName[k] = 'John' then redOutput.Lines.Add(arrName[k]+#9+arrSurname[k]);
     2. Finding only one record / making the loop stop when the item has been found / using the most effective way of searching - use a while loop with a boolean flag to make the loop stop once the item has been found. It is not good programming techniques to use a break / exit inside of loops to make loops stop. You may lose many marks using one of these methods in finals.
     3. Learners do not need to be able to code the binary search method. There could be questions in theory about the binary search so they need to understand how it works.
     4. Remember that pos can be used to find part of a sentence in a text file or array. See screenshot below.
   • Find the highest / lowest values - I use the method of keeping the index number of the highest / lowest value which allows me to display all the other arrays parallel to this array by just using the index number of the highest / lowest value. See screenshot below.
   • Sorting - Learners do not need to be able to code the bubble sort method. There could be questions in theory about the bubble sort, so they need to understand how it works.
   • Insert into an array:
     1. Add an item to the end of the array.
     2. Add an item at a certain position
     3. Insert an item into a sorted array.
     4. Writing the new array to a text file / adding the new item to the end of the text file. It is at this time that I teach writing to a text file.
   • Delete:
     1. A single item
     2. Multiple items
   • Removing duplicates - creating new arrays without duplicates is the easiest method for all learners to understand. Depending on the question you might have to replace the original arrays with these new non-duplicate arrays. An example of this type of question can be found in the Exemplar 2018 Q 4.1. The display procedure needed to be called after removing duplicates which were displaying the original array and therefor needed to be replaced with the new array. See screenshot below.
   • Union and intersection.
   • Mode and median.