Assignment Statements

An assignment statement creates a new variable and gives it a value:

julia> message = "And now for something completely different"
"And now for something completely different"
julia> n = 17
17
julia> π_val = 3.141592653589793
3.141592653589793

This example makes three assignments. The first assigns a string to a new variable named message, the second assigns the integer 17 to n, and the third assigns the (approximate) value of $\pi$ to π_val (pi TAB).

A common way to represent variables on paper is to write the name of each with an arrow pointing to its value. This kind of figure is called a state diagram because it shows what state each of the variables is in (think of it as the variable’s state of mind). Figure 2-1 shows the result of the previous example.

Figure 2-1. State diagram

Variable Names

Programmers generally choose names for their variables that are meaningful—they document what the variable is used for.

Variable names can be as long as you like. They can contain almost all Unicode characters (see “Characters”), but they can’t begin with a number. It is legal to use uppercase letters, but it is conventional to use only lowercase for variable names.

Unicode characters can be entered via tab completion of LaTeX-like abbreviations in the Julia REPL.

The underscore character, _, can appear in a name. It is often used in names with multiple words, such as your_name or airspeed_of_unladen_swallow.

If you give a variable an illegal name, you get a syntax error:

julia> 76trombones = "big parade"
ERROR: syntax: "76" is not a valid function argument name
julia> more@ = 1000000
ERROR: syntax: extra token "@" after end of expression
julia> struct = "Advanced Theoretical Zymurgy"
ERROR: syntax: unexpected "="

76trombones is illegal because it begins with a number. more@ is illegal because it contains an illegal character, @. But what’s wrong with struct?

It turns out that struct is one of Julia’s keywords. The REPL uses keywords to recognize the structure of the program, and they cannot be used as variable names.

Julia has these keywords:

abstract type    baremodule   begin      break       catch
const            continue     do         else        elseif
end              export       finally    for         false
function         global       if         import      in
let              local        macro      module      mutable struct
primitive type   quote        return     true        try
using            struct       where      while

You don’t have to memorize this list. In most development environments, keywords are displayed in a different color; if you try to use one as a variable name, you’ll know.

Expressions and Statements

An expression is a combination of values, variables, and operators. A value all by itself is considered an expression, and so is a variable, so the following are all legal expressions:

julia> 42
42
julia> n
17
julia> n + 25
42

When you type an expression at the prompt, the REPL evaluates it, which means that it finds the value of the expression. In this example, n has the value 17 and n + 25 has the value 42.

A statement is a unit of code that has an effect, like creating a variable or displaying a value:

julia> n = 17
17
julia> println(n)
17

The first line here is an assignment statement that gives a value to n. The second line is a print statement that displays the value of n.

When you type a statement, the REPL executes it, which means that it does whatever the statement says.

Script Mode

So far we have run Julia in interactive mode, which means that you interact directly with the REPL. Interactive mode is a good way to get started, but if you are working with more than a few lines of code, it can be clumsy.

The alternative is to save code in a file called a script and then run Julia in script mode to execute the script. By convention, Julia scripts have names that end with .jl.

If you know how to create and run a script on your computer, you are ready to go. Otherwise I recommend using JuliaBox again. Open a text file, write the script, and save the file with a .jl extension. The script can be executed in a terminal with the command julia name_of_the_script.jl.

Because Julia provides both modes, you can test bits of code in interactive mode before you put them in a script. But there are differences between interactive mode and script mode that can be confusing.

For example, if you are using Julia as a calculator, you might type:

julia> miles = 26.2
26.2
julia> miles * 1.61
42.182

The first line assigns a value to miles and displays the value. The second line is an expression, so the REPL evaluates it and displays the result. It turns out that a marathon is about 42 kilometers.

But if you type the same code into a script and run it, you get no output at all. In script mode an expression, all by itself, has no visible effect. Julia actually evaluates the expression, but it doesn’t display the value unless you tell it to:

miles = 26.2
println(miles * 1.61)

This behavior can be confusing at first.

A script usually contains a sequence of statements. If there is more than one statement, the results appear one at a time as the statements execute.

For example, the script:

println(1)
x = 2
println(x)

produces the output:

1
2

The assignment statement produces no output.

5
x = 5
x + 1

Operator Precedence

When an expression contains more than one operator, the order of evaluation depends on the operator precedence. For mathematical operators, Julia follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

• Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2*(3-1) is 4, and (1+1)^(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even if it doesn’t change the result.

• Exponentiation has the next highest precedence, so 1+2^3 is 9, not 27, and 2*3^2 is 18, not 36.

• Multiplication and Division have higher precedence than Addition and Subtraction. So, 2*3-1 is 5, not 4, and 6+4/2 is 8, not 5.

• Operators with the same precedence are evaluated from left to right (except exponentiation). So in the expression degrees / 2 * π, the division happens first and the result is multiplied by π. To divide by $2\pi$, you can use parentheses, or write degrees / 2 / π or degrees / 2π.

String Operations

In general, you can’t perform mathematical operations on strings, even if the strings look like numbers, so the following are illegal:

"2" - "1"    "eggs" / "easy"    "third" + "a charm"

But there are two exceptions, * and ^.

The * operator performs string concatenation, which means it joins the strings by linking them end-to-end. For example:

julia> first_str = "throat"
"throat"
julia> second_str = "warbler"
"warbler"
julia> first_str * second_str
"throatwarbler"

The ^ operator also works on strings; it performs repetition. For example, "Spam"^3 is "SpamSpamSpam". If one of the values is a string, the other has to be an integer.

This use of * and ^ makes sense by analogy with multiplication and exponentiation. Just as 4^3 is equivalent to 4*4*4, we expect "Spam"^3 to be the same as "Spam"*"Spam"*"Spam", and it is.

Comments

As programs get bigger and more complicated, they get more difficult to read. Formal languages are dense, and it is often difficult to look at a piece of code and figure out what it is doing, or why.

For this reason, it is a good idea to add notes to your programs to explain in natural language what the program is doing. These notes are called comments, and they start with the # symbol:

# compute the percentage of the hour that has elapsed
percentage = (minute * 100) / 60

In this case, the comment appears on a line by itself. You can also put comments at the end of a line:

percentage = (minute * 100) / 60   # percentage of an hour

Everything from the # to the end of the line is ignored—it has no effect on the execution of the program.

Comments are most useful when they document nonobvious features of the code. It is reasonable to assume that the reader can figure out what the code does; it is more useful to explain why.

This comment is redundant with the code and useless:

v = 5   # assign 5 to v

This comment contains useful information that is not in the code:

v = 5   # velocity in meters/second

Debugging

Three kinds of errors can occur in a program: syntax errors, runtime errors, and semantic errors. It is useful to distinguish between them in order to track them down more quickly:

Syntax error

“Syntax” refers to the structure of a program and the rules about that structure. For example, parentheses have to come in matching pairs, so (1 + 2) is legal, but 8) is a syntax error.

If there is a syntax error anywhere in your program, Julia displays an error message and quits, and you will not be able to run the program. During the first few weeks of your programming career, you might spend a lot of time tracking down syntax errors. As you gain experience, you will make fewer errors and find them faster.

Runtime error

The second type of error is a runtime error, so called because the error does not appear until after the program has started running. These errors are also called exceptions because they usually indicate that something exceptional (and bad) has happened.

Runtime errors are rare in the simple programs you will see in the first few chapters, so it might be a while before you encounter one.

Semantic error

The third type of error is “semantic,” which means related to meaning. If there is a semantic error in your program, it will run without generating error messages, but it will not do the right thing. It will do something else. Specifically, it will do what you told it to do.

Identifying semantic errors can be tricky because it requires you to work backward by looking at the output of the program and trying to figure out what it is doing.

Glossary

variable

A name that refers to a value.

assignment

A statement that assigns a value to a variable.

state diagram

A graphical representation of a set of variables and the values they refer to.

keyword

A reserved word that is used to parse a program; you cannot use keywords like if, function, and while as variable names.

expression

A combination of variables, operators, and values that represents a single result.

evaluate

To simplify an expression by performing the operations in order to yield a single value.

statement

A section of code that represents a command or action. So far, the statements we have seen are assignments and print statements.

execute

To run a statement and do what it says.

interactive mode

A way of using the Julia REPL by typing code at the prompt.

script mode

A way of using Julia to read code from a script and run it.

script

A program stored in a file.

operator precedence

Rules governing the order in which expressions involving multiple mathematical operators and operands are evaluated.

concatenate

To join two strings end-to-end.

comment

Information in a program that is meant for other programmers (or anyone reading the source code) and has no effect on the execution of the program.

syntax error

An error in a program that makes it impossible to parse (and therefore impossible to interpret).

runtime error or exception

An error that is detected while the program is running.

semantics

The meaning of a program.

semantic error

An error in a program that makes it do something other than what the programmer intended.

Exercises