Computer science 2.2

• Variable

  A value stored in memory that can change while the program is running.
• Constant

  A value that does not change while the program is running ,and is assigned when the program is designed.
• Assignment

  Giving a variable or a constant a value
• Casting

  Converting a variable from one data type to another.Eg integer to string.A variable can be a real/float, character, integer, String,Boolean
• Input

  A value is read from an input device eg keyboard
• Output

  Data generated by the computer and displayed to the user
• Advantages of constants

  1. They make a program easier to read as usually declared and assigned at the top of the program.
  2. Constants can easily be changed by the programmer in one place in a program.Instead of changing every instance of a value throughout a program.This leads to less chance of errors
  3. The complier can optimise the code.This makes a program run more quickly if constants are used instead of variables
• Why is casting needed

  Inputs form the keyboard are always characters. Multiple characters are called a string. However to perform a calculation the arithmetic logic unit must use numbers. Therefore a string has to be "cast" to a number. Makes sense as the character 1 is stored as the binary number 00110001(using the ASCII character set).The number 1 that the arithmetic logic unit needs for calculation is stored as the binary number 0000001.
• Integers to real


  Integers require less bits of memory than decimal point numbers. Integers would make the program more memory efficient. However it may be necessary to cast an integer to a real in a program. Some commands also require data in a particular data type.
• Integer

  Whole number

  Positive or Negative

  eg 6

  use=total+score

  Never used for telephone numbers only when arithmetic on the data is required
• Real/float

  Numbers with a decimal point

  eg 6.5

  use:cost=total+vat
• Character

  Single alphanumeric

  Use for menu choices

  eg "1" or "a"

  Use: Choice=input("enter your choice")
• String

  Multiple alphanumeric characters

  Use for words and telephone numbers

  eg"Craig"

  Forename="Craig"
• Sequence

  Sequence is executing one instruction after another

  
• Selection(Branching)

  Selection is a program branching depending on a condition

  
• Selection(branching) aware for exam


  Some programming languages support SWITCH/SELECT...CASE statements.Here a program can branch in more than directions depending on the value.

  
• Iteration(looping)

  Repeating sections of code.FOR loops (known as counter controlled loops)are used when the number of iterations needed is known ahead of the iteration executing.

  
• Iteration(looping) different loops

  WHILE loops(Condition controlled loops) are used when the number of iterations is not known because the variable used to determine when the iteration ends is changing within the iteration itself.

  Do..UNTIL loops are alternative to WHILE where the code executes at leat once before the condition is checked

  
• Common comparison operators

  
• Common arithmetic operators

  
• Boolean operators



  Or,And,Not

  
• example of Boolean Not operator

  end_of_file=False

  while not end_of_file.

  The same as while end_of_file==false:

  Use an umbrella if it is raining but NOT cold.
• example of Boolean AND operator


  end_of_file=False

  found=False

  while not end_of_file and not found

  Wear a coat if it's raining and cold
• example of Boolean OR operator


  if x==2 or x==4

  while choice<1 or choice>3

  Catch a bus if it's raining or cold.
• Create an infinite loop

  While loop
• What can variables and constants hold

  integer, Real/Float,Character,String,Boolean
• Importance of casting

  Necessary so that sub-problems receive data in expected format. Casting allows numbers to be manipulated as strings. Casting allows inputs that are always strings to become numbers.

  x=str(X) casts the variable x to a string

  x=int(x) casts the variable x to an integer
• To get the length of a string

  Returns the length of a string eg length=string.length
• Converting Cases

  uppercase=string.upper

  lowercase=string.lower
• To extract a substring

  Returns part of a string ,starting at the character of the first parameter and counting up by the number in the second parameter .

  String name.substring(starting position, number of characters)

  Chars=string.substring(3,1) hello returns "l"
• To extract from the left or right of a string

  Returns to the left most or right most characters from a string where the parameter indicates how many to return.

  Eg left = string.left(8)string ComputerScience returnComputer

  Eg right = string.right(7)string ComputerScience returnScience
• Concatenation

  + joins separate string values together

  eg print(stringA + stringB)
• ASCII Conversion

  Returns the ASCII value of a character .Returns a character from its ASCII number.ASC(),CHR()

  eg ASC(character) ascii=ASC("A") CHR=(ascii number) char=CHR(65) eg Ascii would be 65 eg char would be "A"
• Example of ASCII conversion


  someText="computer science"

  print(someText.length) 16

  print((someText.substring(3,3)).upper)PUT
• Get the length of the string

  len(string)
• Cases of a string

  U string=string.upper()

  l string=string.lower()
• Substring of a string


  string[x:i]
• Stages to write data to a file

  Open the file for creating/overwriting or appending to a file

  Write the data to a file

  Close the file
• Stages to read data from a file

  Open the file for reading data

  Assign a boolean variable to "false" to indicate that the end of the file is not reached.While the end of the file is false and the search item is not found:read the data from the file.If the data matches what is being searched for ,assign the data to variables or output.Check if the end of the file is reached and assign the boolean variable to "true".Close the file
• open

  Open(....)