Pascal Programming

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Pascal Programming

• Making decisions- Selection Statements
• National Certificate Unit 4
• Carl Smith

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Selection

• In most programs there are actions which are to
  be executed only when certain conditions are
  true. Conditions are expressions which, when
  evaluated, produce a result of either True or
  False
• e.g.
• Is the quantity less than 500?
• Is the print at the end of a page?
• Has the employee worked more than 40 hours?

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Selection

• The ability to write a program to make such
  decisions is an important tool to add to your
  problem-solving techniques.
• A decision structure is a programming construct
  that allows a deviation from the sequential
  processing using a condition to assist in the
  selection of one or more possible execution
  paths.

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Selection Representation

• Testing the condition of an expression within
  program flow is represented in a flow chart like
  this-

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The If statement

• Pascal and most other languages provide the if
  statement to allow the programmer to make
  decisions

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Conditions

• A conditional expression compares two quantities,
  normally of the same type and returns a value of
  TRUE if the relationship is valid, and FALSE if
  it is not.
• The operators used for comparison are called
  relational operators. They may be one of the
  following
• OPERATOR MEANING
• IS EQUAL TO
• ltgt IS NOT EQUAL TO
• lt IS LESS THAN
• gt IS GREATER THAN
• lt LESS THAN OR EQUAL TO
• gt GREATER THAN OR EQUAL TO

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Expressions and Values
Boolean Expression Boolean Value
7 (2 3.5) True
-18 lt -15 True
13 lt 100 True
0.012 gt 0.013 False
A lt B True
B gt A True
-17.32 ltgt -17.32 False
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Multiple Conditions

• Sometimes an action depends on the evaluation of
  more than one condition e.g.
• an extra mileage allowance is paid by a company
  if the employee is an executive and the engine
  size of the car is greater than 1500 cc.
• In PASCAL it is possible to combine expressions
  together using the Boolean operators AND and OR.
  e.g.
• IF (driver exec') AND (engine gt 1500) THEN ...
• IF (age lt 18) OR (age gt 60) THEN ...

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AND and OR

• The following table illustrates the possible
  outcomes of combining two conditions using AND
  and OR.
• Condition A Condition B A AND B A OR B
• false false false false
• false true false true
• true false false true
• true true true true

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The NOT operator

• The NOT operator is used to NEGATE the boolean
  value of an expression.
• E.g.
• IF NOT EOF if not at end of file
• (18 (10 8)) true
• NOT (18 (10 8)) false

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The IF.. THEN statement in Pascal

• IF condition THEN
• BEGIN
• Statements
• END
• more statements
• If the condition evaluates as true then the
  statements after the THEN clause (between the
  Begin and End) are executed otherwise execution
  continues at the statement after the semicolon.

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Example

• The actions specified after the THEN and ELSE
  clauses can be any imperative PASCAL statement
  e.g.
• IF month 12 THEN
• BEGIN
• Monthname December
• END
• More than one statement enclosed by BEGIN and END
  is known as a statement block.
• Note If a statement block consists of only 1
  statement it is not essential to use a BEGIN and
  END clause (and therefore is no longer a
  statement block..!). However be careful with the
  use of semicolons.

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The IF..THEN..ELSEstatement in Pascal

• Sometimes a further alternative action is
  required if the condition evaluates as false.
  This can be done using the ELSE clause of the IF
  statement. e.g.
• IF condition THEN
• statement block
• ELSE
• a different statement block

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if then else - Code sample

• IF quantity lt 500 THEN
• begin
• quantity quantity 500
• end
• ELSE
• begin
• quantity quantity 1.05
• Writeln(quantity)
• end

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The Case statement

• Decisions can also be programmed using the Case
  statement.
• Case is useful when a number of decisions have to
  be made
• Examine this example for a club membership-
• Is the member aged under 18?
• Is the member aged between 19 and 30?
• Is the member aged between 30 and 50?
• Is the member aged over 50?
• Etc

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The Case Statement

• The Case structure uses an expression to transfer
  program control to any one of a collection of
  possible paths.
• CASE selector OF selector is the expression)
• case value 1 statement or statement block
• case value 2 statement or statement block
• case value n statement or statement block
• ELSE statement or statement block
• END
• Note there is no BEGIN to the statement block

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How CASE works

• If the values of the expression and a case
  constant match, the associated statement or
  statement block is executed. If none of the cases
  match, the statement(s) associated with the ELSE
  are executed. The ELSE statement is optional.
• CASE groupnumber OF
• l premium 97
• 2 premium 115
• 3 premium 165
• 4 premium 176
• ELSE premium 204
• END
• If there is no ELSE and none of the cases match,
  execution continues at the statement following
  the end

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Case Example

• Case values can be expressed as single values, a
  group of values or a range of values e.g.
• CASE marks OF
• 1 . . 37 writeln (outright fail)
• 38, 39 writeln(near miss)
• 100 writeln (full marks !)
• ELSE writeln (passed)
• END

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Case Example

• Readln(keypress)
• CASE keypress OF
• A,E,I,O,U writeln(vowel)
• 1..9 writeln(digit)
• , -, /, writeln(operator)
• writeln(space)
• ELSE writeln(other character)
• END
• Note If more than one statement is needed for
  the action statements use the BEGIN and END
  delimiters.

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Indentation

• To make your code easier to read use
  indentation in loops and selection statements
  e.g.
• IF month 12 THEN
• BEGIN
• Monthname December
• END

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Summary

• Making decisions
• Boolean Logic
• The If statement
• The If Then Else statement
• CASE and CASE-ELSE