CS208 Software programming

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CS208 Software programming

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Introduction

• Software
• Instructions that tell the computer what to do
  (ie. a program)

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Languages

• Machine Language
• zeros and ones
• Assembly Languages -
• Use mnemonics to operations and sometimes memory
  locations
• Each is specific to a particular hardware
  architecture
• Translated by an Assembler

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Languages

• High Level Languages -
• Use English-like statements to instruct the
  computer what to do.
• Are more portable (will run on different kinds of
  hardware)
• Translated by a compiler (produces object code)
• Linked via linker to other code and library
  routines (produces executable code)

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Languages

• High Level Languages
• Examples
• -    FORTRAN - scientific/engineering, math
• -    COBOL - business
• -    BASIC - education and PCs
• -    Pascal - education
• -    C - Application development
• - C - Application development
• - Java - Application development
•  

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Structured Programming

• Program is subdivided into modules (one logical
  thought, approx. one page long)
•  Each page is organized into recognizable
  paragraphs, using indentation to show nesting and
  subordinate clauses.
•  Embedded comments describe the function of each
  variable and purpose of each module.
•  Straightforward, easily readable code is
  preferred over slightly more efficient, but
  obtuse, code.

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Structured Programming

•  Final program is created via top-down design.
• Main code with empty modules, slowly refined to
  lowest level
• Keeps programmer from being overwhelmed by job,
  and allows easy division of work (modules
  assigned to different programmers)

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Benefits of Structured Programming

• More readable
• Easier to maintain
• More likely to be correct on first run
• Easier to "prove" by systematic program
  verification

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Eight-Statement Pascal

•  program to name the program
• const to define constants
• var to declare variables
• readln to read input
• writeln to write output
• assignment () to perform calculations
• if-then-else to make decisions
• while-do to loop

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What a Pascal program looks like

• program Warning (output)
• header - required
• begin required
• writeln ('DANGER!!')
• end. required

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Declarations (Constants/Variables)

• Primitive (Basic) Data Types
• Real, Integer, and Char
•         Integer - whole numbers
• No commas in numbers
•         Real - numbers with fractional parts
• Must have digit before after decimal pt
•         Char - a single ASCII character
• enclosed in single quotes

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Constants

• Value known prior to start of program AND value
  doesn't change
• Format const
• constant-name value

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Constants

• Examples
• const
• Pi 3.14
• TaxRate 0.28
• MaxItems 50
• BestGrade 'A'

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Variables

• allocates memory cell, types it, gives it a name
•  
• Format
• var variable-list type
•  Examples
• var
• Initial char
• I, Count integer
• GPA real

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Variables

• NOT case-sensitive.
• good identifiers make code "Self-Documenting"
• must begin with a Letter, followed by
  letters/digits

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Executable Statements

• Between "begin" and "end" (separated by
  semicolons )
•  Statements can take up more than one line.
•  Extra blanks ignored - use for readability.
• Use comments

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 Assignment Statements

• assigns a value to an identifier
•   id 5 (id "gets" 5)
• types must be compatible
•  Operators - / DIV MOD translate
  several math formulas. (/ for real nums only, DIV
  MOD for integers)

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Assignment Statements

• Swap Example
• temp a
• a b
• b temp
•  
• Increment Example a a 1

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I/O Procedure statements

• Writeln (output list)
• procedure call to write output to standard
  output device (w/CR)
• Writeln writes a blank line with no output on
  it.
•  
• Examples Writeln ('Hello')
• Writeln ('Please enter a number')
• Writeln ('The answer is', sum)
•  

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I/O Procedure statements

• Readln (input list)
• procedure call to get input from standard input
  device
•  
• Examples Readln (Pay)
• Readln (Num1, Num2)

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Semi-Colons ()

• used to separate statements
• do not use AFTER Begin or BEFORE End

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Example

• program CalcAge (input, output)
• Computes and prints age
• const
• CurYr 2001
• var
• BirthYr, Age integer
• begin
• writeln ('Enter year of birth ')
• readln (BirthYr)
• Age CurYr - BirthYr
• writeln ('Your age after this year''s birthday
  is ', Age)
• end.

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Boolean Expressions

• A relational operator tests the relationship
  between two expressions and returns a Boolean
  (TRUE or FALSE) result.
•  
• Syntax (ltexprgt ltrelopgt ltexprgt)

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Boolean Expressions

• operator example result
• ------------------------------------------
• A B TRUE if A is equal to
  B
• ltgt A ltgt B TRUE if A is not equal
  to B
• lt A lt B TRUE if A is less
  than B
• lt A lt B TRUE if A is less than
  or equal to B
• gt A gt B TRUE if A is greater
  than B
• gt A gt B TRUE if A is greater
  than or equal to B

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If Selection Structure

• Syntax One Alternative
• if condition then statementT
• do statement if condition is TRUE
• Two Alternatives
• if condition
• then statementT
• do if TRUE
• else statementF
• do if FALSE

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If Selection Structure

• Examples
• if Age gt 18 then
• writeln ('Can Vote')
•  
• if Divisor ltgt 0.0 then
• Answer Num / Divisor
•  
• if MorD 'M' then if MorD 'M'
• writeln ('Hi Mom') then writeln ('Hi Mom')
• else writeln ('Hi Dad')

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If Selection Structure

• if Num gt 0
• then writeln ('Positive')
• else writeln ('Negative')
•  
• if Temp gt 60
• then writeln ('Tennis Anyone?')
• else writeln ('Too cold for tennis.')

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If Selection Structure

• program CalcAge Computes and prints age
• Apptype Console
• const
• CurYr 2001 LastMon 1
• var
• BirthYr, BirthMon, Age integer
• begin
• writeln ('Enter year of birth ') readln
  (BirthYr)
• writeln ('Enter month of birth ') readln
  (BirthMon)
• Age CurYr - BirthYr
• if BirthMon gt LastMon then
• Age Age - 1 Subtract 1 if birthday has not
  yet occurred this year
• writeln ('Your age is ', Age)
• writeln(Push return to exit)
• readln
• end.

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If Selection Structure

• Exercise
• Write a program that reads in 3 integers and
  outputs the smallest.
• example
• 13 8 12
• The smallest number is 8

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If Selection Structure

• program SmallNum
• Apptype Console
• var
• Num1, Num2, Num3, Smallest integer
• begin
• Writeln ('Enter three numbers ') Readln
  (Num1, Num2, Num3)
•  
• if Num1 lt Num2 then
• Smallest Num1
• else Smallest Num2
•  
• if Num3 lt Smallest then
• Smallest Num3
•  
• Writeln ('Smallest ', Smallest1)
• Writeln(Press return to exit) Readln
• end.

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Conditional Loop Structure

• Conditional Loop Structure the WHILE statement
•   
• Syntax WHILE (boolean-expression) DO
• begin
• statement1
• statementN
• end
•  
• NOTE begin/end not needed if only one
  statement

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Conditional Loop Structure

• Example 1 Display multiples of 5, from 5 to 50.
•  
• Count 1
• WHILE Count lt 10 DO
• begin
• Mult 5 Count
• Writeln (Mult)
• Count Count 1
• end

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Conditional Loop Structure

• Example 2
• Error check input, to be sure a positive number
  is entered.
•  
• Writeln ('Enter a positive number')
• Readln (Num)
• WHILE Num lt 0 DO
• begin
• Writeln ('Invalid entry. Try Again.')
• Readln (Num)
• end

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Conditional Loop Structure

• Student Exercise
• Write a WHILE loop that will add a series of
  positive integers as they are entered by the
  user. Stop adding and display the total when the
  user enters a negative number.
• Example
• 10
• 20
• -1
• Total is 30

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Conditional Loop Structure

• Sum 0
• Num 0
• WHILE Num gt 0 DO
• begin
• Sum Sum Num
• Writeln ('Enter a positive number (negative to
  exit)')
• Readln (Num)
• end
• Writeln ('The total is ', sum)