CIS 234: Order of Operations, Shortcut

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CIS 234 Order of Operations, Shortcut Other
Operators

• Dr. Ralph D. Westfall
• February, 2004

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Some Things to Think About

• order of operations
• what gets calculated 1st?
• type casting
• changing data from one type of variable to a
  different type
• shortcut operators
• doing the same calculations with less typing

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Order of Operations

• order in which calculations and other operations
  are performed by language
• 2 4 / 2 // is result 3 or 4?
• operations that are done first have "higher
  precedence"
• pp. 45-46, 188
• "Please excuse my dear aunt Sally"

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Order of Operations - 2

• operations within parentheses are always done
  first
• inner before outer parentheses e.g.,
• (5 (2 3)) ("please")
• method calls next (lower precedence)
• Math.pow(2, 3) // Java's exponentiation
• // 222 ("excuse")

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Order of Operations - 3

• following all have the same precedence
• "unary" minus
• unary minus doesn't subtract from anything
• value -5
• value - x // reverses sign
• increment or decrement and --
• type casting (changes data type)
• value (double) 5 / 2

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Type Casting

• can change data type of a variable by putting a
  different data type in front
• in parentheses e.g., (int), (char), (float)
• can also put parentheses around an expression,
  and a cast before that, to change the data type
  of the result
• double a (int) (3.0 / 2) // a?
• // how many conversions in above line?

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Order of Operations - 4

• then multiply ("my"), regular division ("dear"),
  and modulus operations
• , /,
• then addition ("aunt"), and subtraction ("Sally")
• , -

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Order of Operations - 5

• then comparison operations (in order)
• lt, lt, gt, gt // highest among these
• , !
• (AND)
• (OR) // lowest among these
• then assignment (equals)
• e.g., height 60
• or shortcut assignment e.g., x y
• p. 188

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Practice

• evaluate the following
• x 3 5 4 / 2 Math.min(3, 8)
• y (9 2) (7 4)
• z (double) 3 / 2 //unifying type?
• a (float) 5.0 / 2 //a decimal type
• b (int) (7 / 2.0)

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Shortcut Operators

• way to write code with less typing
• result is exactly same
• code may actually run faster
• examples
• a // exactly same as a a 1
• a 4 keystrokes (including semicolon)
• aa1 6 keystrokes
• a a 1 10 keystrokes (4 spaces)

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Addition and Subtraction

• is the "increment operator"
• adds 1
• -- is the "decrement operator"
• subtracts 1
• both are "unary" operators, because they only
  work on 1 variable
• contra a b, c d, e f, etc.

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Prefix versus Postfix

• prefix means put the operator before the variable
  e.g., a, --b
• increments or decrements a variable before it is
  used in an expression
• a 2
• x a
• now x is 3, a is 3

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Prefix versus Postfix - 2

• postfix means put the operator after the variable
  e.g., a--, b
• decrements or increments a variable after it is
  used in an expression
• a 2
• x a
• now x is 2, a is 3

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Practice

• a4
• b3
• xa // x? a?
• ya-- // y? a?
• x a b-- // x? a? b?
• y a-- - (b) // y? a? b?

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Other Shortcut Operators

• can add, subtract, multiply, divide, or get
  modulus
• not limited to add or subtract just 1
• a 2 // same as a a 2
• a - 2 // same as a a - 2
• a 2 // same as a a 2
• a / 2.0 // same as a a / 2.0

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Other Shortcut Operators - 2

• modulus remainder after integer division
  ("clock arithmetic")
• use symbol
• 5 2 1
• if num 2 0, num is an even number
• shortcut operator for modulus
• a 2 // same as a a 2

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Practice

• a4 b5 // a?
• a 5 // a?
• b - 4 // b?
• b 4 // b?
• a b // a? b?

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Shortcuts in Totals

• total 0.00
• sale 3.00
• total sale
• / would work well in a loop read a line from a
  file into sale variable, add it onto total, read
  another line into sale, etc. /