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Variables and assignments

1.2. Variables and assignments#

  • A variable is a name that refers to a value

  • Variable names:

    • Can contain most unicode characters, but cannot begin with a number

    • Are case sensitive (e.g. Tau is different from tau)

    • Should ideally be a good description of what the variable represents

  • An assignment statement has the form variable_name = expression, and it creates a new variable and gives it the value of the evaluated expression

my_number = 1.2            # Assignment

1.2

pi                         # Pre-defined variable

π = 3.1415926535897...

My_number                  # Error - was defined without capital M

UndefVarError: `My_number` not defined

δ = 0.001                  # Unicode characters allowed (type \delta and press tab)

0.001

my_number_2 = (my_number + 1) * δ     # Use pre-defined variables in expressions

0.0022

1.2.1. Updating operators#

  • The following updating operators make it convenient to operate on a variable and assign the result back to the variable:

+=  -=  *=  /=  \=  ÷=  %=  ^=

counter = 1               # counter = 1

1

counter += 1              # counter = counter + 1

2

counter ^= 4              # counter = counter ^ 4

16

angle = 60                # degrees

60

angle *= π / 180          # convert to radians

1.0471975511965976

1.2.2. Numerical literal coefficients#

  • Julia allows variables to be immediately preceded by a numeric literal, implying multiplication

  • This can make expressions easier to read, and look more like mathematical notation

x = 3
println(2x^2 - 3x + 1)
println(1.5x^2 - .5x + 1)
println(2(x-1)^2 - 3(x-1) + 1)

10
13.0
3

  • Numeric literal coefficients have higher precedence than * and /:

println(360 / 2*π)  # Left first - equals 180*π
println(360 / 2π)   # Right first - equals 180/π

565.4866776461628
57.29577951308232

  • For exponentiation ^, numeric literal coefficients behave similarly to unary operators:

println(2^2x)       # 2^(2x), not (4)x
println(2x^2)       # 2*(x^2), not (2x)^2

64
18

1.2.3. Comments#

  • Comments start with the # symbol, and everything from the # to the end of the line is ignored by Julia

  • Alternatively, you can write multi-line comments by wrapping the text inside #= and =#

  • Comments are very important, but should be descriptive. Do not comment on obvious things.

  • Typical uses of comments:

    • Start each program (script) with a concise description of what it does

    • Define each important variable/constant

    • Top a block of code for a specific task with a concise comment

1.2.4. Example of comments#

The comments in this code make it easy to understand exactly what it does

#=
   Experiments with spheres
=#

radius1 = 100     # Outer sphere radius
radius2 = 50      # Inner sphere radius 

# Compute the volume of each sphere
vol1 = 4π * radius1^3 / 3
vol2 = 4π * radius2^3 / 3

# Compute the volume of a hollow sphere
vol_hollow = vol1 - vol2

3.665191429188092e6
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