Programming Ruby 1.9 & 2.0: The Pragmatic Programmers’ Guide (2013)

Part 1. Facets of Ruby

Chapter 9. Expressions

So far, we’ve been fairly cavalier in our use of expressions in Ruby. After all, a = b + c is pretty standard stuff. You could write a whole heap of Ruby code without reading any of this chapter.

But it wouldn’t be as much fun ;-).

One of the first differences with Ruby is that anything that can reasonably return a value does: just about everything is an expression. What does this mean in practice?

Some obvious things include the ability to chain statements together:

Perhaps less obvious, things that are normally statements in C or Java are expressions in Ruby. For example, the if and case statements both return the value of the last expression executed:

We’ll talk more about if and case later.

9.1 Operator Expressions

Ruby has the basic set of operators (+, -, *, /, and so on) as well as a few surprises. A complete list of the operators, and their precedences, is given in Table 13, Ruby operators (high to low precedence).

In Ruby, many operators are implemented as method calls. For example, when you write a*b+c, you’re actually asking the object referenced by a to execute the method *, passing in the parameter b. You then ask the object that results from that calculation to execute the + method, passing c as a parameter. This is the same as writing the following (perfectly valid) Ruby:

Because everything is an object and because you can redefine instance methods, you can always redefine basic arithmetic if you don’t like the answers you’re getting:

More useful is that classes you write can participate in operator expressions just as if they were built-in objects. For example, the left shift operator, <<, is often used to mean append to receiver. Arrays support this:

You can add similar support to your classes:

Produces:

Note that there’s a subtlety in this code—the << method explicitly returns self. It does this to allow the method chaining in the line scores << 10 << 20 << 40. Because each call to << returns the scores object, you can then call << again, passing in a new score.

As well as the obvious operators, such as +, *, and <<, indexing using square brackets is also implemented as a method call. When you write this:

you’re actually calling a method named [] on some_obj, passing it three parameters. You’d define this method using this:

Similarly, assignment to an element is implemented using the []= method. This method receives each object passed as an index as its first n parameters and the value of the assignment as its last parameter:

Produces:

9.2 Miscellaneous Expressions

As well as the obvious operator expressions and method calls and the (perhaps) less obvious statement expressions (such as if and case), Ruby has a few more things that you can use in expressions.

Command Expansion

If you enclose a string in backquotes (sometimes called backticks ) or use the delimited form prefixed by %x, it will (by default) be executed as a command by your underlying operating system. The value of the expression is the standard output of that command. Newlines will not be stripped, so it is likely that the value you get back will have a trailing return or linefeed character.

You can use expression expansion and all the usual escape sequences in the command string:

The exit status of the command is available in the global variable $?.

Redefining Backquotes

In the description of the command output expression, we said that the string in backquotes would “by default” be executed as a command. In fact, the string is passed to the method called Object#` (a single backquote). If you want, you can override this. This example uses $?, which contains the status of the last external process run:

Produces:

9.3 Assignment

Just about every example we’ve given so far in this book has featured assignment. Perhaps it’s about time we said something about it.

An assignment statement sets the variable or attribute on its left side (the lvalue ) to refer to the value on the right (the rvalue ). It then returns that rvalue as the result of the assignment expression. This means you can chain assignments, and you can perform assignments in some unexpected places:

Ruby has two basic forms of assignment. The first assigns an object reference to a variable or constant. This form of assignment is hardwired into the language:

The second form of assignment involves having an object attribute or element reference on the left side. These forms are special, because they are implemented by calling methods in the lvalues, which means you can override them.

We’ve already seen how to define a writable object attribute. Simply define a method name ending in an equals sign. This method receives as its parameter the assignment’s rvalue. We’ve also seen that you can define [] as a method:

As this example shows, these attribute-setting methods don’t have to correspond with internal instance variables, and you don’t need an attribute reader for every attribute writer (or vice versa).

In older Rubys, the result of the assignment was the value returned by the attribute-setting method. As of Ruby 1.8, the value of the assignment is always the value of the parameter; the return value of the method is discarded. In the code that follows, older versions of Ruby would set result to 99. Now result will be set to 2.

Parallel Assignment

During your first week in a programming course (or the second semester if it was a party school), you may have had to write code to swap the values in two variables:

You can do this much more cleanly in Ruby:

Ruby lets you have a comma-separated list of rvalues (the things on the right of the assignment). Once Ruby sees more than one rvalue in an assignment, the rules of parallel assignment come into play. What follows is a description at the logical level: what happens inside the interpreter is somewhat hairier. Users of older versions of Ruby should note that these rules have changed in Ruby 1.9.

First, all the rvalues are evaluated, left to right, and collected into an array (unless they are already an array). This array will be the eventual value returned by the overall assignment.

Next, the left side (lhs) is inspected. If it contains a single element, the array is assigned to that element.

If the lhs contains a comma, Ruby matches values on the rhs against successive elements on the lhs. Excess elements are discarded.

Splats and Assignment

If Ruby sees any splats on the right side of an assignment (that is, rvalues preceded by an asterisk), each will be expanded inline into its constituent values during the evaluation of the rvalues and before the assignment to lvalues starts:

Exactly one lvalue may be a splat. This makes it greedy—it will end up being an array, and that array will contain as many of the corresponding rvalues as possible. So, if the splat is the last lvalue, it will soak up any rvalues that are left after assigning to previous lvalues:

If the splat is not the last lvalue, then Ruby ensures that the lvalues that follow it will all receive values from rvalues at the end of the right side of the assignment—the splat lvalue will soak up only enough rvalues to leave one for each of the remaining lvalues.

As with method parameters, you can use a raw asterisk to ignore some rvalues:

Nested Assignments

The left side of an assignment may contain a parenthesized list of terms. Ruby treats these terms as if they were a nested assignment statement. It extracts the corresponding rvalue, assigning it to the parenthesized terms, before continuing with the higher-level assignment.

Other Forms of Assignment

In common with other languages, Ruby has a syntactic shortcut: a = a + 2 may be written as a += 2. The second form is converted internally to the first. This means that operators you have defined as methods in your own classes work as you’d expect:

Something you won’t find in Ruby are the autoincrement (++) and autodecrement (--) operators of C and Java. Use the += and -= forms instead.

9.4 Conditional Execution

Ruby has several different mechanisms for conditional execution of code; most of them should feel familiar, and many have some neat twists. Before we get into them, though, we need to spend a short time looking at boolean expressions.

Boolean Expressions

Ruby has a simple definition of truth. Any value that is not nil or the constant false is true—"cat", 99, 0, and :a_song are all considered true.

In this book, when we want to talk about a general true or false value, we use regular Roman type: true and false. When we want to refer to the actual constants, we write true and false.

The fact that nil is considered to be false is convenient. For example, IO#gets, which returns the next line from a file, returns nil at the end of file, enabling you to write loops such as this:

However, C, C++, and Perl programmers sometimes fall into a trap. The number zero is not interpreted as a false value. Neither is a zero-length string. This can be a tough habit to break.

And, Or, and Not

Ruby supports all the standard boolean operators. Both the keyword and and the operator && return their first argument if it is false. Otherwise, they evaluate and return their second argument (this is sometimes known as shortcircuit evaluation). The only difference in the two forms is precedence (and binds lower than &&).

Thus, && and and both return a true value only if both of their arguments are true, as expected.

Similarly, both or and || return their first argument unless it is false, in which case they evaluate and return their second argument.

As with and, the only difference between or and || is their precedence. To make life interesting, and and or have the same precedence, but && has a higher precedence than ||.

A common idiom is to use ||= to assign a value to a variable only if that variable isn’t already set:

This is almost, but not quite, the same as var = var || "default value". It differs in that no assignment is made at all if the variable is already set. In pseudocode, this might be written as var = "default value" unless var or as var || var = "default value".

not and ! return the opposite of their operand (false if the operand is true and true if the operand is false). And, yes, not and ! differ only in precedence.

All these precedence rules are summarized in Table 13, Ruby operators (high to low precedence).

defined?

The defined? operator returns nil if its argument (which can be an arbitrary expression) is not defined; otherwise, it returns a description of that argument. If the argument is yield, defined? returns the string “yield” if a code block is associated with the current context.

Comparing Objects

In addition to the boolean operators, Ruby objects support comparison using the methods ==, ===, <=>, =~, eql?, and equal? (see Table 5, Common comparison operators). All but <=> are defined in class Object but are often overridden by descendants to provide appropriate semantics. For example, class Array redefines == so that two array objects are equal if they have the same number of elements and the corresponding elements are equal.

Table 5. Common comparison operators

Both == and =~ have negated forms, != and !~. Ruby first looks for methods called != or !~, calling them if found. If not, it will then invoke either == or =~, negating the result.

In the following example, Ruby calls the == method to perform both comparisons:

Produces:

If instead we explicitly define !=, Ruby calls it:

Produces:

You can use a Ruby range as a boolean expression. A range such as exp1..exp2 will evaluate as false until exp1 becomes true. The range will then evaluate as true until exp2 becomes true. Once this happens, the range resets, ready to fire again. We show some examples of this later.

Prior to Ruby 1.8, you could use a bare regular expression as a boolean expression. This is now deprecated. You can still use the ~ operator (described in the reference section) to match $_ against a pattern, but this will probably also disappear in the future.

if and unless Expressions

An if expression in Ruby is pretty similar to if statements in other languages:

The then keyword is optional if you lay out your statements on multiple lines:

However, if you want to lay out your code more tightly, you must separate the boolean expression from the following statements with the then keyword:[48]

You can have zero or more elsif clauses and an optional else clause. And notice that there’s no e in the middle of elsif.

As we’ve said before, an if statement is an expression—it returns a value. You don’t have to use the value of an if statement, but it can come in handy:

Ruby also has a negated form of the if statement:

The unless statement does support else, but most people seem to agree that it’s clearer to switch to an if statement in these cases.

Finally, for the C fans out there, Ruby also supports the C-style conditional expression:

A conditional expression returns the value of the expression either before or after the colon, depending on whether the boolean expression before the question mark is true or false. In the previous example, if the duration is greater than three minutes, the expression returns 0.35. For shorter durations, it returns 0.25. The result is then assigned to cost.

if and unless Modifiers

Ruby shares a neat feature with Perl. Statement modifiers let you tack conditional statements onto the end of a normal statement:

For an if modifier, the preceding expression will be evaluated only if the condition is true. unless works the other way around:

Because if itself is an expression, you can get really obscure with statements such as this:

This path leads to the gates of madness.

9.5 case Expressions

The Ruby case expression is a powerful beast: a multiway if on steroids. And just to make it even more powerful, it comes in two flavors.

The first form is fairly close to a series of if statements; it lets you list a series of conditions and execute a statement corresponding to the first one that’s true:

The second form of the case statement is probably more common. You specify a target at the top of the case statement, and each when clause lists one or more comparisons to be tested against that target:

As with if, case returns the value of the last expression executed, and you can use a then keyword if the expression is on the same line as the condition:[49]

case operates by comparing the target (the expression after the keyword case) with each of the comparison expressions after the when keywords. This test is done using comparison === target. As long as a class defines meaningful semantics for === (and all the built-in classes do), objects of that class can be used in case expressions.

For example, regular expressions define === as a simple pattern match:

Ruby classes are instances of class Class. The === operator is defined in Class to test whether the argument is an instance of the receiver or one of its superclasses. So (abandoning the benefits of polymorphism and bringing the gods of refactoring down around your ears), you can test the class of objects:

9.6 Loops

Don’t tell anyone, but Ruby has pretty primitive built-in looping constructs.

The while loop executes its body zero or more times as long as its condition is true. For example, this common idiom reads until the input is exhausted:

The until loop is the opposite; it executes the body until the condition becomes true:

As with if and unless, you can use both of the loops as statement modifiers:

Back in the section on boolean expressions, we said that a range can be used as a kind of flip-flop, returning true when some event happens and then staying true until a second event occurs. This facility is normally used within loops. In the example that follows, we read a text file containing the first ten ordinal numbers (“first,” “second,” and so on) but print only the lines starting with the one that matches “third” and ending with the one that matches “fifth”:

Produces:

You may find folks who come from Perl writing the previous example slightly differently:

Produces:

This uses some behind-the-scenes magic behavior: gets assigns the last line read to the global variable $_, the ~ operator does a regular expression match against $_, and print with no arguments prints $_. This kind of code is falling out of fashion in the Ruby community and may end up being removed from the language.

The start and end of a range used in a boolean expression can themselves be expressions. These are evaluated each time the overall boolean expression is evaluated. For example, the following code uses the fact that the variable $. contains the current input line number to display line numbers 1 through 3 as well as those between a match of /eig/ and /nin/:

Produces:

You’ll come across a wrinkle when you use while and until as statement modifiers. If the statement they are modifying is a begin...end block, the code in the block will always execute at least one time, regardless of the value of the boolean expression:

Produces:

Iterators

If you read the beginning of the previous section, you may have been discouraged. “Ruby has pretty primitive built-in looping constructs,” it said. Don’t despair, gentle reader, for we have good news. Ruby doesn’t need any sophisticated built-in loops, because all the fun stuff is implemented using Ruby iterators.

For example, Ruby doesn’t have a for loop—at least not the kind that iterates over a range of numbers. Instead, Ruby uses methods defined in various built-in classes to provide equivalent, but less error-prone, functionality.

Let’s look at some examples:

Produces:

It’s easy to avoid fence-post and off-by-one errors; this loop will execute three times, period. In addition to times , integers can loop over specific ranges by calling downto and upto , and all numbers can loop using step . For instance, a traditional “for” loop that runs from 0 to 9 (something like for(i=0; i < 10; i++)) is written as follows:

Produces:

A loop from 0 to 12 by 3 can be written as follows:

Produces:

Similarly, iterating over arrays and other containers is easy if you use their each method:

Produces:

And once a class supports each , the additional methods in the Enumerable module become available. (We talked about this back in the Modules chapter, and we document it fully in Enumerable.) For example, the File class provides an each method, which returns each line of a file in turn. Using the grep method in Enumerable, we could iterate over only those lines that end with a d:

Produces:

Last, and probably least, is the most basic loop of all. Ruby provides a built-in iterator called loop :

The loop iterator calls the associated block forever (or at least until you break out of the loop, but you’ll have to read ahead to find out how to do that).

for ... in

Earlier we said that the only built-in Ruby looping primitives were while and until. What’s this for thing, then? Well, for is almost a lump of syntactic sugar.

When you write this:

Ruby translates it into something like this:

The only difference between the for loop and the each form is the scope of local variables that are defined in the body. This is discussed in Section 9.7, Variable Scope, Loops, and Blocks.

You can use for to iterate over any object that responds to the method each , such as an Array or a Range:

Produces:

As long as your class defines a sensible each method, you can use a for loop to traverse its objects:

Produces:

break, redo, and next

The loop control constructs break, redo, and next let you alter the normal flow through a loop or iterator.[50]

break terminates the immediately enclosing loop; control resumes at the statement following the block. redo repeats the current iteration of the loop from the start but without reevaluating the condition or fetching the next element (in an iterator). next skips to the end of the loop, effectively starting the next iteration:

These keywords can also be used within blocks. Although you can use them with any block, they typically make the most sense when the block is being used for iteration:

Produces:

A value may be passed to break and next. When used in conventional loops, it probably makes sense only to do this with break, where it sets the value returned by the loop. (Any value given to next is effectively lost.) If a conventional loop doesn’t execute a break, its value is nil.

If you want the nitty-gritty details of how break and next work with blocks and procs, take a look at the reference description. If you are looking for a way of exiting from nested blocks or loops, take a look at Object#catch.

9.7 Variable Scope, Loops, and Blocks

The while, until, and for loops are built into the language and do not introduce new scope; previously existing locals can be used in the loop, and any new locals created will be available afterward.

The scoping rules for blocks (such as those used by loop and each) are different. Normally, the local variables created in these blocks are not accessible outside the block:

Produces:

However, if at the time the block executes a local variable already exists with the same name as that of a variable in the block, the existing local variable will be used in the block. Its value will therefore be available after the block finishes. As the following example shows, this applies to normal variables in the block but not to the block’s parameters:

Note that the assignment to the variable doesn’t have to be executed; the Ruby interpreter just needs to have seen that the variable exists on the left side of an assignment:

You can list block-local variables in the block’s parameter list, preceded by a semicolon. Contrast this code, which does not use block-locals:

Produces:

with the following code, which uses a block-local variable, so square in the outer scope is not affected by a variable of the same name within the block:

Produces:

If you are concerned about the scoping of variables with blocks, turn on Ruby warnings, and declare your block-local variables explicitly.

Footnotes