Grade 10 Computing, Week 27, Lesson 51: Using check boxes in an application

Using check boxes in an application

Textbook: pp. 110–115

GoalUses the CheckBox object, tests the Checked property, declares a global array and fills it with initial values in the OnCreate event.

New words

CheckBox: a box that can be ticked or cleared · CheckBoxChecked: True if the box is ticked, otherwise False · Checkedglobal variable: a variable declared outside procedures, available everywhere in the unit · global o‘zgaruvchiarray: numbered variables of one type under one name · massiv

Explanation

A CheckBox (Standard tab) is either ticked or cleared; this state is held in the logical property Checked, and its label in Caption. Any number of CheckBoxes can be ticked at the same time, so in a survey with them the number of answers can be larger than the number of participants. To remember while the application runs how many people chose what, variables are needed, but a local variable disappears when the procedure ends. So a global variable (for example an array) is declared: in the code window, in the implementation section, write var n: array[0..3] of Integer;, and the elements are called n[0], n[1], n[2], n[3]. It is convenient to give the initial values when the application window is created: select Form1, double-click OnCreate on the Events tab of the Object Inspector and set the elements to zero with a loop. Delphi sets global variables to zero at first, but writing it openly in OnCreate makes the program text clearer.

Worked examples

A “Habit diary”: CheckBox1 – I read a book, CheckBox2 – I did sport, CheckBox3 – I got up early. Every evening the boxes are ticked and the “End of day” button is pressed: procedure TForm1.Button1Click(Sender: TObject); ⏎ begin ⏎ n[0] := n[0] + 1; ⏎ if CheckBox1.Checked then n[1] := n[1] + 1; ⏎ if CheckBox2.Checked then n[2] := n[2] + 1; ⏎ if CheckBox3.Checked then n[3] := n[3] + 1; ⏎ CheckBox1.Checked := False; ⏎ CheckBox2.Checked := False; ⏎ CheckBox3.Checked := False; ⏎ end; n[0] counts the days and n[1]–n[3] count on how many days each habit was done; the ticks are cleared for the next day.

Six days were recorded: reading was ticked on 4 days, sport on 2 and getting up early on 3. n[0] = 6, n[1] = 4, n[2] = 2, n[3] = 3. The sum of the ticks is 4+2+3 = 9, which is more than the number of days (6), because several habits can be ticked on one day. The share of days with reading is 4/6 ≈ 67%.

Class activity

“The class survey”: a group builds a 4-option application on its own topic (for example a favourite subject); it enters 8 invented “votes” and compares the totals with the hand-calculated result.

Practice

Which property holds whether a CheckBox is ticked?

How many elements does var n: array[0..3] of Integer; have?

Five students took part; one option got 3 ticks and another 4. What is the sum of ticks for the two options?

Why can the number of ticks exceed the number of participants?

CheckBox and the array

Using check boxes in an application