Expression Versus Statement

Expression: Something which evaluates to a value. Example: 1+2/x
Statement: A line of code which does something. Example: GOTO 100

In the earliest general-purpose programming languages, like FORTRAN, the distinction was crystal-clear. In FORTRAN, a statement was one unit of execution, a thing that you did. The only reason it wasn't called a "line" was because sometimes it spanned multiple lines. An expression on its own couldn't do anything... you had to assign it to a variable.

1 + 2 / X

is an error in FORTRAN, because it doesn't do anything. You had to do something with that expression:

X = 1 + 2 / X

FORTRAN didn't have a grammar as we know it today—that idea was invented, along with Backus-Naur Form (BNF), as part of the definition of Algol-60. At that point the semantic distinction ("have a value" versus "do something") was enshrined in syntax: one kind of phrase was an expression, and another was a statement, and the parser could tell them apart.

Designers of later languages blurred the distinction: they allowed syntactic expressions to do things, and they allowed syntactic statements that had values. The earliest popular language example that still survives is C. The designers of C realized that no harm was done if you were allowed to evaluate an expression and throw away the result. In C, every syntactic expression can be a made into a statement just by tacking a semicolon along the end:

1 + 2 / x;

is a totally legit statement even though absolutely nothing will happen. Similarly, in C, an expression can have side-effects—it can change something.

1 + 2 / callfunc(12);

because callfunc might just do something useful.

Once you allow any expression to be a statement, you might as well allow the assignment operator (=) inside expressions. That's why C lets you do things like

callfunc(x = 2);

This evaluates the expression x = 2 (assigning the value of 2 to x) and then passes that (the 2) to the function callfunc.

This blurring of expressions and statements occurs in all the C-derivatives (C, C++, C#, and Java), which still have some statements (like while) but which allow almost any expression to be used as a statement (in C# only assignment, call, increment, and decrement expressions may be used as statements; see Scott Wisniewski's answer).

Having two "syntactic categories" (which is the technical name for the sort of thing statements and expressions are) can lead to duplication of effort. For example, C has two forms of conditional, the statement form

if (E) S1; else S2;

and the expression form

E ? E1 : E2

And sometimes people want duplication that isn't there: in standard C, for example, only a statement can declare a new local variable—but this ability is useful enough that the GNU C compiler provides a GNU extension that enables an expression to declare a local variable as well.

Designers of other languages didn't like this kind of duplication, and they saw early on that if expressions can have side effects as well as values, then the syntactic distinction between statements and expressions is not all that useful—so they got rid of it. Haskell, Icon, Lisp, and ML are all languages that don't have syntactic statements—they only have expressions. Even the class structured looping and conditional forms are considered expressions, and they have values—but not very interesting ones.

• an expression is anything that yields a value: 2 + 2
• a statement is one of the basic "blocks" of program execution.

Note that in C, "=" is actually an operator, which does two things:

• returns the value of the right hand subexpression.
• copies the value of the right hand subexpression into the variable on the left hand side.

Here's an extract from the ANSI C grammar. You can see that C doesn't have many different kinds of statements... the majority of statements in a program are expression statements, i.e. an expression with a semicolon at the end.

statement
    : labeled_statement
    | compound_statement
    | expression_statement
    | selection_statement
    | iteration_statement
    | jump_statement
    ;

expression_statement
    : ';'
    | expression ';'
    ;

http://www.lysator.liu.se/c/ANSI-C-grammar-y.html

An expression is something that returns a value, whereas a statement does not.

For examples:

1 + 2 * 4 * foo.bar()     //Expression
foo.voidFunc(1);          //Statement

The Big Deal between the two is that you can chain expressions together, whereas statements cannot be chained.

You can find this on wikipedia, but expressions are evaluated to some value, while statements have no evaluated value.

Thus, expressions can be used in statements, but not the other way around.

Note that some languages (such as Lisp, and I believe Ruby, and many others) do not differentiate statement vs expression... in such languages, everything is an expression and can be chained with other expressions.

For an explanation of important differences in composability (chainability) of expressions vs statements, my favorite reference is John Backus's Turing award paper, Can programming be liberated from the von Neumann style?.

Imperative languages (Fortran, C, Java, ...) emphasize statements for structuring programs, and have expressions as a sort of after-thought. Functional languages emphasize expressions. Purely functional languages have such powerful expressions than statements can be eliminated altogether.

Expressions can be evaluated to get a value, whereas statements don't return a value (they're of type void).

Function call expressions can also be considered statements of course, but unless the execution environment has a special built-in variable to hold the returned value, there is no way to retrieve it.

Statement-oriented languages require all procedures to be a list of statements. Expression-oriented languages, which is probably all functional languages, are lists of expressions, or in tha case of LISP, one long S-expression that represents a list of expressions.

Although both types can be composed, most expressions can be composed arbitrarily as long as the types match up. Each type of statement has its own way of composing other statements, if they can do that all. Foreach and if statements require either a single statment or that all subordinate statements go in a statement block, one after another, unless the substatements allow for thier own substatements.

Statements can also include expressions, where an expression doesn't really include any statements. One exception, though, would be a lambda expression, which represents a function, and so can include anything a function can iclude unless the language only allows for limited lambdas, like Python's single-expression lambdas.

In an expression-based language, all you need is a single expression for a function since all control structures return a value (a lot of them return NIL). There's no need for a return statement since the last-evaluated expression in the function is the return value.

Simply: an expression evaluates to a value, a statement doesn't.

Some things about expression based languages:

Most important: Everything returns an value

There is no difference between curly brackets and braces for delimiting code blocks and expressions, since everything is an expression. This doesn't prevent lexical scoping though: A local variable could be defined for the expression in which its definition is contained and all statements contained within that, for example.

In an expression based language, everything returns a value. This can be a bit strange at first -- What does (FOR i = 1 TO 10 DO (print i)) return?

Some simple examples:

• (1) returns 1
• (1 + 1) returns 2
• (1 == 1) returns TRUE
• (1 == 2) returns FALSE
• (IF 1 == 1 THEN 10 ELSE 5) returns 10
• (IF 1 == 2 THEN 10 ELSE 5) returns 5

A couple more complex examples:

• Some things, such as some function calls, don't really have a meaningful value to return (Things that only produce side effects?). Calling OpenADoor(), FlushTheToilet() or TwiddleYourThumbs() will return some sort of mundane value, such as OK, Done, or Success.
• When multiple unlinked expressions are evaluated within one larger expression, the value of the last thing evaluated in the large expression becomes the value of the large expression. To take the example of (FOR i = 1 TO 10 DO (print i)), the value of the for loop is "10", it causes the (print i) expression to be evaluated 10 times, each time returning i as a string. The final time through returns 10, our final answer

It often requires a slight change of mindset to get the most out of an expression based language, since the fact that everything is an expression makes it possible to 'inline' a lot of things

As a quick example:

 FOR i = 1 to (IF MyString == "Hello, World!" THEN 10 ELSE 5) DO
 (
    LotsOfCode
 )

is a perfectly valid replacement for the non expression-based

IF MyString == "Hello, World!" THEN TempVar = 10 ELSE TempVar = 5 
FOR i = 1 TO TempVar DO
(    
    LotsOfCode  
)

In some cases, the layout that expression-based code permits feels much more natural to me

Of course, this can lead to madness. As part of a hobby project in an expression-based scripting language called MaxScript, I managed to come up with this monster line

IF FindSectionStart "rigidifiers" != 0 THEN FOR i = 1 TO (local rigidifier_array = (FOR i = (local NodeStart = FindsectionStart "rigidifiers" + 1) TO (FindSectionEnd(NodeStart) - 1) collect full_array[i])).count DO
(
    LotsOfCode
)

A statement is a special case of an expression, one with void type. The tendency of languages to treat statements differently often causes problems, and it would be better if they were properly generalized.

For example, in C# we have the very useful Func<T1, T2, T3, TResult> overloaded set of generic delegates. But we also have to have a corresponding Action<T1, T2, T3> set as well, and general purpose higher-order programming constantly has to be duplicated to deal with this unfortunate bifurcation.

Trivial example - a function that checks whether a reference is null before calling onto another function:

TResult IfNotNull<TValue, TResult>(TValue value, Func<TValue, TResult> func)
                  where TValue : class
{
    return (value == null) ? default(TValue) : func(value);
}

Could the compiler deal with the possibility of TResult being void? Yes. All it has to do is require that return is followed by an expression that is of type void. The result of default(void) would be of type void, and the func being passed in would need to be of the form Func<TValue, void> (which would be equivalent to Action<TValue>).

A number of other answers imply that you can't chain statements like you can with expressions, but I'm not sure where this idea comes from. We can think of the ; that appears after statements as a binary infix operator, taking two expressions of type void and combining them into a single expression of type void.

The de-facto basis of these concepts is:

Expressions: A syntactic category whose instance can be evaluated to a value.

Statement: A syntactic category whose instance may be involved with evaluations of an expression and the resulted value of the evaluation (if any) is not guaranteed available.

Besides to the very initial context for FORTRAN in the early decades, both definitions of expressions and statements in the accepted answer are obviously wrong:

• Expressions can be unvaluated operands. Values are never produced from them.
  • Subexpressions in non-strict evaluations can be definitely unevaluated.
    • Most C-like languages have the so-called short-circuit evaluation rules to conditionally skip some subexpression evaluations not change the final result in spite of the side effects.
  • C and some C-like languages have the notion of unevaluated operand which may be even normatively defined in the language specification. Such constructs are used to avoid the evaluations definitely, so the remained context information (e.g. types or alignment requirements) can be statically distinguished without changing the behavior after the program translation.
    • For example, an expression used as the operand of the sizeof operator is never evaluated.
• Statements have nothing to do with line constructs. They can do something more than expressions, depending on the language specifications.
  • Modern Fortran, as the direct descendant of the old FORTRAN, has concepts of executable statements and nonexecutable statements.
  • Similarly, C++ defines declarations as the top-level subcategory of a translation unit. A declaration in C++ is a statement. _{(This is not true in C.)} There are also expression-statements like Fortran's executable statements.
  • To the interest of the comparison with expressions, only the "executable" statements matter. But you can't ignore the fact that statements are already generalized to be constructs forming the translation units in such imperative languages. So, as you can see, the definitions of the category vary a lot. The (probably) only remained common property preserved among these languages is that statements are expected to be interpreted in the lexical order (for most users, left-to-right and top-to-bottom).

_{(BTW, I want to add [citation needed] to that answer concerning materials about C because I can't recall whether DMR has such opinions. It seems not, otherwise there should be no reasons to preserve the functionality duplication in the design of C: notably, the comma operator vs. the statements.)}

(The following rationale is not the direct response to the original question, but I feel it necessary to clarify something already answered here.)

Nevertheless, it is doubtful that we need a specific category of "statements" in general-purpose programming languages:

• Statements are not guaranteed to have more semantic capabilities over expressions in usual designs.
  • Many languages have already successfully abandon the notion of statements to get clean, neat and consistent overall designs.
    • In such languages, expressions can do everything old-style statements can do: just drop the unused results when the expressions are evaluated, either by leaving the results explicitly unspecified (e.g. in RⁿRS Scheme), or having a special value (as a value of a unit type) not producible from normal expression evaluations.
    • The lexical order rules of evaluation of expressions can be replaced by explicit sequence control operator (e.g. begin in Scheme) or syntactic sugar of monadic structures.
    • The lexical order rules of other kinds of "statements" can be derived as syntactic extensions (using hygienic macros, for example) to get the similar syntactic functionality. (And it can actually do more.)
  • On the contrary, statements cannot have such conventional rules, because they don't compose on evaluation: there is just no such common notion of "substatement evaluation". _{(Even if any, I doubt there can be something much more than copy and paste from existed rules of evaluation of expressions.)}
    • Typically, languages preserving statements will also have expressions to express computations, and there is a top-level subcategory of the statements preserved to expression evaluations for that subcategory. For example, C++ has the so-called expression-statement as the subcategory, and uses the discarded-value expression evaluation rules to specify the general cases of full-expression evaluations in such context. Some languages like C# chooses to refine the contexts to simplify the use cases, but it bloats the specification more.
• For users of programming languages, the significance of statements may confuse them further.
  • The separation of rules of expressions and statements in the languages requires more effort to learn a language.
  • The naive lexical order interpretation hides the more important notion: expression evaluation. (This is probably most problematic over all.)
    • Even the evaluations of full expressions in statements are constraint with the lexical order, subexpressions are not (necessarily). Users should ultimately learn this besides any rules coupled to the statements. (Consider how to make a newbie get the point that ++i + ++i is meaningless in C.)
    • Some languages like Java and C# further constraints the order of evaluations of subexpressions to be permissive of ignorance of evaluation rules. It can be even more problematic.
      • This seems overspecified to users who have already learned the idea of expression evaluation. It also encourages the user community to follow the blurred mental model of the language design.
      • It bloats the language specification even more.
      • It is harmful to optimization by missing the expressiveness of nondeterminism on evaluations, before more complicated primitives are introduced.
    • A few languages like C++ (particularly, C++17) specify more subtle contexts of evaluation rules, as a compromise of the problems above.
      • It bloats the language specification a lot.
      • This goes totally against to simplicity to average users...

So why statements? Anyway, the history is already a mess. It seems most language designers do not take their choice carefully.

Worse, it even gives some type system enthusiasts (who are not familiar enough with the PL history) some misconceptions that type systems must have important things to do with the more essential designs of rules on the operational semantics.

Seriously, reasoning depending on types are not that bad in many cases, but particularly not constructive in this special one. Even experts can screw things up.

For example, someone emphasizes the well-typing nature as the central argument against the traditional treatment of undelimited continuations. Although the conclusion is somewhat reasonable and the insights about composed functions are OK (but still far too naive to the essense), this argument is not sound because it totally ignores the "side channel" approach in practice like _Noreturn any_of_returnable_types (in C11) to encode Falsum. _{And strictly speaking, an abstract machine with unpredictable state is not identical to "a crashed computer".}

In a statement-oriented programming language, a code block is defined as a list of statements. In other words, a statement is a piece of syntax that you can put inside a code block without causing a syntax error.

Wikipedia defines the word statement similarly

In computer programming, a statement is a syntactic unit of an imperative programming language that expresses some action to be carried out. A program written in such a language is formed by a sequence of one or more statements

Notice the latter statement. (although "a program" in this case is technically wrong because both C and Java reject a program that consists of nothing of statements.)

Wikipedia defines the word expression as

An expression in a programming language is a syntactic entity that may be evaluated to determine its value

This is, however, false, because in Kotlin, throw new Exception("") is an expression but when evaluated, it simply throws an exception, never returning any value.

In a statically typed programming language, every expression has a type. This definition, however, doesn't work in a dynamically typed programming language.

Personally, I define an expression as a piece of syntax that can be composed with an operator or function calls to yield a bigger expression. This is actually similar to the explanation of expression by Wikipedia:

It is a combination of one or more constants, variables, functions, and operators that the programming language interprets (according to its particular rules of precedence and of association) and computes to produce ("to return", in a stateful environment) another value

But, the problem is in C programming language, given a function executeSomething like this:

void executeSomething(void){
    return;
}

Is executeSomething() an expression or is it a statement? According to my definition, it is a statement because as defined in Microsoft's C reference grammar,

You cannot use the (nonexistent) value of an expression that has type void in any way, nor can you convert a void expression (by implicit or explicit conversion) to any type except void

But the same page clearly indicates that such syntax is an expression.

A statement is a block of code that doesn't return anything and which is just a standalone unit of execution. For example-

if(a>=0)
printf("Hello Humen,I'm a statement");

An expression, on the other hand, returns or evaluates a new value. For example -

 if(a>=0)
    return a+10;//This is an expression because it evalutes an new value;

or

 a=10+y;//This is also an expression because it returns a new value. 

Expression

A piece of syntax which can be evaluated to some value. In other words, an expression is an accumulation of expression elements like literals, names, attribute access, operators or function calls which all return a value. In contrast to many other languages, not all language constructs are expressions. There are also statements which cannot be used as expressions, such as while. Assignments are also statements, not expressions.

Statement

A statement is part of a suite (a “block” of code). A statement is either an expression or one of several constructs with a keyword, such as if, while or for.