JavaScript Numbers and Expressions

1
JavaScript Numbers and Expressions

• Recitation, Week 5
• September 23, 2005

2
Precedence of operators

• Recall JavaScript variables can store values of
  different data types
• String, number, boolean (True or false)
• Operators for numbers
• Multiplication(), division(/), remainder()
• have higher precedence than
• addition (), subtraction (-)

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Precedence Associativity of operators

• Example 1
• x 3 27 5 7 20 ? x 145
• x 3 (27 5) (7 20) ? x 145
• Example 2
• Same type of operators processed from left to
  right
• x 20 7 5 x ? 8
• x 20 (7 5) x ? 18
• Mixing both
• x 3 6 8 / 2 (10 5) x ? 9

4
Number of operands

• Previous arithmetic operators are binary
  operators They take 2 operands.
• Unary operators They take a single operand.
• Negation(-) It has higher precedence than the
  binary arithmetic operators.
• t 4
• u t 6 8
• z -1 (v u / -t) - 13
• document.write(t t) ? t 4 (negation
  doesnt affect t)
• document.write(u u) ? u 52
• document.write(v v) ? v -13
• document.write(z z) ? z 0

5
Number of operands

• Unary operators
• Increment () Increases the value of its
  operand by 1.
• x 23
• x or also x is the same as x x 1 x
  ? 24
• decrement (--) Decreases the value of its
  operand by 1
• y 15
• --y or also y-- is the same as y y 1 y
  ? 1
• They also have higher precedence over arithmetic
  operations.

6
Increment and decrement

• Note x and x have different affects when
  they are used on the right hand side of
  assignments
• First case
• x 8
• y x ? y 8
• Second case
• x 8
• y x ? y 9
• Same situation applies for decrement (--)

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Alternative Assignment Ways

• Some shorthand notations
• x y is the same as x x y
• -
• x - y ? x x y
• x y ? x x y
• /
• x / y ? x x / y

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Number of operands

• Example
• x 8
• y x x
• x 1 ? x becomes 9 now
• z (y x) 10
• document.write(z z) ? z 5

9
Predefined Math functions

• Math.random()
• This function generates a random number between 0
  and 1 (1 not included)
• To map the generated numbers between 0 and X we
  multiply the number by X.
• To obtain whole numbers following functions are
  useful
• Math.round(a) gives the integer closest to a
• Math.floor(a) rounds a down to the closest
  integer to a.
• Math.ceil(a) rounds a up to the closest
  integer to a,
• Note If a is already an integer the result of
  these functions is a itself.

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Math.random

• Check out this site about obtaining whole numbers
  by using previous functions
• http//www.shawnolson.net/a/789/
• Now lets see an example random.htm
• We used the following pattern to come up with
  integers between 1 and 10
• rand Math.random()
• number1 10 rand 1
• number2 Math.floor(number1)

11
Diagnostic Write Statements

• We have an example on calculating the area of a
  right triangle by using only the length of the
  hypotenuse and one of the sides.
• There is a logic error in the program, so we will
  use diagnostic write statements to find the
  error.
• 1) errored one
• 2) the one with diagnostic write statements
• 3) corrected one