Data Types & Variables Grade 10

Every value in Delphi has a data type. Variables are the containers that hold these values while your program runs. Constants hold values that never change.

Data Types

Data Type	What it stores	Examples	Quotes?
`Integer`	Whole numbers (positive & negative, no decimals)	`3`, `-15`, `1250`	No
`Real`	Numbers with decimals (floating point)	`3.14`, `-13.24`, `125.89`	No
`String`	One or more characters — text	`'Banana'`, `'23.45'`, `'@!jkl'`	Single quotes `'…'`
`Char`	A single character only	`'S'`, `'!'`, `'a'`	Single quotes `'…'`
`Boolean`	Only `True` or `False`	`True`, `False`	No

Variables

A variable is a named storage location that holds a value during program execution. Think of it as a labelled box — you can put something in, use it, and swap it out.

Variables must be: Declared → Populated → Used.

Naming Convention — Prefixes

Prefix	Data Type	Example
`i`	Integer	`iAge`
`r`	Real	`rPrice`
`s`	String	`sName`
`c`	Char	`cGrade`
`b`	Boolean	`bFound`

Declaring Variables

Declared in the var section using: variableName : DataType;

Local Declaration (inside a procedure)

Delphi — Local

procedure TForm1.btnClickClick(Sender: TObject);
var
  sName:   string;
  iNumber: integer;
begin
  // code here
end;

Global Declaration (available to all procedures)

Delphi — Global

var
  Form1:   TForm1;
  sName:   string;   // declared here, below existing var block
  iNumber: integer;

Populating (Assigning) Variables

Use the assignment operator := to put a value into a variable.

Delphi

sName   := 'Jannie';
sName   := edtName.Text;        // from a text box
iNumber := 15;
iNumber := sedNum.Value;         // from a spin edit
bFlag   := True;

Initialising Variables

Always Initialise

Set a starting value before using a variable, especially counters and totals — otherwise Delphi may assign a random garbage value.

Delphi

i        := 0;
iTotal   := 0;
sReverse := '';
cLetter  := '';

Using Variables

Delphi

lblFullName.Caption := sName + ' ' + sSurname;
memOut.Lines.Add(sName + ' ' + sSurname);
iAnswer := iNum1 + iNum2;
memOut.Lines.Add(IntToStr(iNum1) + ' + ' + IntToStr(iNum2)
               + ' = ' + IntToStr(iAnswer));

Constants

A constant holds a value that is known before the program runs and cannot change while it runs. Use const instead of var. No data type is needed, and use = not :=.

Delphi

const
  VAT       = 0.15;
  MAX_ITEMS = 100;

// Using a constant — exactly like a variable
rVatAmount := rCostPrice * VAT;
rTotal     := rCostPrice + rVatAmount;

Type Conversion

Components like TEdit only hold strings. You must convert to the right type before using in calculations.

Function	Converts	Example
`StrToInt()`	String → Integer	`iAge := StrToInt(edtAge.Text);`
`StrToFloat()`	String → Real	`rPrice := StrToFloat(edtPrice.Text);`
`IntToStr()`	Integer → String	`lblOut.Caption := IntToStr(iTotal);`
`FloatToStr()`	Real → String	`lblOut.Caption := FloatToStr(rAverage);`

The Val procedure — converting safely

StrToInt crashes (runtime error) if the text isn't a valid number. The Val procedure converts a string to a number without crashing: it gives back the converted value AND an error code that tells you whether the conversion worked (0 = success, otherwise the position of the first bad character).

Delphi — Val(sourceString, numberResult, errorPosition)

var
  iValue, iCode : Integer;
begin
  Val(edtAge.Text, iValue, iCode);
  if iCode = 0 then
    ShowMessage('Valid number: ' + IntToStr(iValue))
  else
    ShowMessage('Not a valid number!');   // iCode = position of the bad character
end;

Formatting Real Numbers

Delphi

// FloatToStrF(value, format, totalDigits, decimals)
lblPrice.Caption := FloatToStrF(rPrice, ffCurrency, 4, 2);  // → R24.50
lblPrice.Caption := FloatToStrF(rPrice, ffFixed,    4, 2);  // → 24.50

Types of Errors

Error Type	When it occurs	Example
Syntax Error	Code is typed incorrectly — won't compile	Missing semicolon, missing `end`
Runtime Error	Crashes while the program is running	Dividing by zero
Logical Error	Runs, but gives wrong answer	Calculating average before total, infinite loop

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