Difference between statement and function

Out of the three language tags you've chosen, I'm only very familliar with Python, but I believe many other languages have a similar view of these concepts. All the example code here is Python.

A statement is a thing that is executed; an "instruction to do something" that the language implementation understands. e.g.

print "Hello World"

pass

def foo(n):
    return n + 1

if condition:
    print 'yay'
else:
    print 'doh'

The above block contains a print statement, a pass statement, a function definition statement, and an if/else statement. Note that the function definition and the if/else statement are compound statements; they contain other statements (possibly many of them, and possibly other compound statements).

An expression is something that can be evaluated to produce a value. e.g.

1

"foo"

2 * 6

function(argument)

None

The above contains a numeric literal expression, a string literal expression, an expression involving numeric operators, a function call expression, and the literal None expression. Other than literals and variables, expressions are made up of other expressions. In function(argument), function and argument are also both expressions.

The key difference is that statements are instructions that tell the language implementation to "go do something". Expressions are evaluated to a value (which possibly requires to language implementation to "go do something" on the way).

A consequence of this is that anywhere you see a value (including an expression), you could substitute any other expression and you would still get something that makes some sort of sense. It may fail to compile, or throw exceptions at runtime, or whatever, but on at least some level you can understand what's going on.

A statement can never appear inside an expression (I believe this is not true in Ruby and Javascript in some sense, as they allow literal code blocks and functions which are then used as a value as a whole, and functions and code blocks contain statements; but that's kind of different from what I'm talking about). An expression must have a value (even if it's an uninteresting one like None). A statement is a command; it doesn't make sense for it to appear as part of an expression, because it has no value.

Many languages also allow expressions to be used as statements. The usual meaning of this is "evaluate this expression to get a value, then throw it away". In Python, functions that always return None are usually used this way:

write_to_disk(result)

It's used as a "command", so it looks like a statement, but technically it's an expression, we just don't use the value it evaluates to for anything. You can argue that a "bare expression" is one of the possible statements in a language (and they're often parsed that way).

Some languages though distinguish between functions that must be used like statements with no return value (often called procedures) and functions that are used like an expression, and give you errors or warnings for using a function like a statement, and definitely give you an error for using a procedure as an expression.

So, if foo is an expression, I can write 1 + foo and while it may be result in a type error, it at least makes that much sense. If foo is a statement, then 1 + foo is usually a parse error; the language implementation won't even be able to understand what you're trying to say.

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A function on the other hand, is a thing you can call. It's not really either an expression or a statement in itself. In Python, you use a def statement to create a function, and a function call is an expression. The name bound to the function after you create it is also an expression. But the function itself is a value, which isn't exactly an expression when you get technical, but certainly isn't a statement.

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So, for loops. This is a for loop in Python:

for thing in collection:
    do_stuff(thing)

Looks like a statement (a compound statement, like an if statement). And to prove it, this is utterly meaningless (and a parse error):

1 + for thing in collection:
    do_stuff(thing)

In some languages though, the equivalent of a for loop is an expression, and has a value, to which you can attempt to add 1. In some it's even a function, not special syntax baked into the language.

A for loop is a not usually a function, it is a special kind of statement called a flow control structure.

A statement is a command. It does something. In most languages, statements do not return values. Example:

print "Hello World"

A function is a subroutine that can be called elsewhere in the program. Functions often (but not necessarily) return values. Example:

function(a) { return a * 2 }

A control structure, also known as a compound statement, is a statement that is used to direct the flow of execution. Examples:

if (condition) then { branch_1 } else { branch_2 }

for (i = 0; i < 10; i += 1) { ... }

Also worth noting is that an expression is a piece of code that evaluates to a value. Example:

2 + 2

All examples are in pseudocode, not tied to any particular language. Also note that these are not exclusive categories, they can overlap.