Statements & Expressions

Aug 5 2016

Ever been confused about the difference between and expression and a statement? Maybe you used them interchangeably?

I have.

It wasn’t until I had been programming for a few years that I began to grasped the difference. My delay was largely due the set of languages I started out with, Python and PHP, both of which are imperative.

First, let’s define statement and expression

statement

the smallest standalone element of an imperative programming language that expresses some action to be carried out1.

In Java, Python, PHP, and other imperative languages, these are some common statements:

expression

a combination of one or more explicit values, constants, variables, operators, and functions that the programming language interprets […] and computes to produce […] another value2.

This can be paraphrased as anything that can be evaluated to a single value

Making the distinction

I like to use this “rule” to help me identify if something is an expression or a statement.

An expression is anything that can be assigned to a variable, and conversely, a statement is anything that cannot be assigned to a variable.

Let’s look at an some examples of expressions, and statements.

Expressions

I like to look at statements through the lens of expressions, so to get some context, let’s look at a few expressions.

1 + 1

", ".join(["Hello", "world!"])

objectA.equals(objectB)

Each of the above evaluate to a single value. Each of which can be assigned to a variable.

Statements

Now let’s look at some statements, and then try to coerce them into an expression.

assignment statement

x = 1 + 1

If it were an expression, you could do this.

z = (x = 1 + 1)

if statement

if True:
    foo()
else:
    bar()

An if statement does not return a value, it only controls the flow of execution. If it were an expresssion, we could do something like this.

result = if True:
             foo()
         else:
             bar()

Note: Some languages have if expressions where this is possible. Ruby is one such language. Also Elixir and Clojure. This example is in Python, which only has if statements.

return statement

return myVar;

Similar to the assignment statement, if it were an expression, we could do something like

Object myNewVar = return myVar;

for loop

for (var i = 0; i < 10; i++) {
    i *= 2;
}

Similar to the if statement, for loops only control the flow of execution. Though, if it were an expression…

var foo = for (var i = 0; i < 10; i++) {
    i *= 2;
}

Some of these might be hard to reason about. For instance, what would a for loop evaluate to? It might be easy to say that our example would evaluate to 14, but what if that for loop was iterating over a list of objects and mutating them? What would it evaluate to then?

It isn’t always statements and expressions

Not every language has statements. There are languages where everything is an expression, such language may not have imperative constructs like for-loops and while-loops though. Many functional languages will be absent of statements, and only contain expressions.

Conclusion

Expressions and statements can be easy to mix up, especially for newcomers. As a programmers, we have a wealth of terminology, which can be overwhelming.

Learning a functional programming language such as Elixir or Clojure, where everything is an expression, may help illuminate the differences between statements and expressions in imperative languages, like Java and Python.

Addendum

I omitted the definition of an imperative language, because there is probably enough to fill a post of its own.

Sources

  1. Statement (computer science). (2016, August 3). In Wikipedia, The Free Encyclopedia. Retrieved 14:20, August 5, 2016, from https://en.wikipedia.org/w/index.php?title=Statement(computerscience)&oldid=732881180

  2. Expression (computer science). (2016, March 31). In Wikipedia, The Free Encyclopedia. Retrieved 14:08, August 5, 2016, from https://en.wikipedia.org/w/index.php?title=Expression(computerscience)&oldid=712921965

tags:programming languagesexpressionsstatements

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