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    <p><strong>Expression:</strong> Something which evaluates to a value. Example: <em>1+2/x</em><br> <strong>Statement:</strong> A line of code which does something. Example: <em>GOTO 100</em></p> <p>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.</p> <blockquote> <p><code>1 + 2 / X</code></p> </blockquote> <p>is an error in FORTRAN, because it doesn't do anything. You had to do something with that expression:</p> <blockquote> <p><code>X = 1 + 2 / X</code></p> </blockquote> <p>FORTRAN didn't have a grammar as we know it today&mdash;that idea was invented, along with Backus-Naur Form (BNF), as part of the definition of Algol-60. At that point the <em>semantic</em> distinction ("have a value" versus "do something") was enshrined in <em>syntax</em>: one kind of phrase was an expression, and another was a statement, and the parser could tell them apart.</p> <p>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:</p> <blockquote> <p><code>1 + 2 / x;</code></p> </blockquote> <p>is a totally legit statement even though absolutely nothing will happen. Similarly, in C, an expression can have <em>side-effects</em>&mdash;it can change something.</p> <blockquote> <p><code>1 + 2 / callfunc(12);</code></p> </blockquote> <p>because <code>callfunc</code> might just do something useful.</p> <p>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</p> <blockquote> <p><code>callfunc(x = 2);</code></p> </blockquote> <p>This evaluates the expression x = 2 (assigning the value of 2 to x) and then passes that (the 2) to the function <code>callfunc</code>.</p> <p>This blurring of expressions and statements occurs in all the C-derivatives (C, C++, C#, and Java), which still have some statements (like <code>while</code>) 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 <a href="https://stackoverflow.com/a/20771/8554766">Scott Wisniewski's answer</a>).</p> <p>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</p> <pre><code>if (E) S1; else S2; </code></pre> <p>and the expression form</p> <pre><code>E ? E1 : E2 </code></pre> <p>And sometimes people <em>want</em> duplication that isn't there: in standard C, for example, only a statement can declare a new local variable&mdash;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.</p> <p>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 <em>syntactic</em> distinction between statements and expressions is not all that useful&mdash;so they got rid of it. Haskell, Icon, Lisp, and ML are all languages that don't have syntactic statements&mdash;they only have expressions. Even the class structured looping and conditional forms are considered expressions, and they have values&mdash;but not very interesting ones.</p>
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    1. POExpression Versus Statement
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    1. USJoel Spolsky
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    1. POExpression Versus Statement
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    1. COIf I'm not mis-interpreting you here, you seem to claim that "(setf (third foo) 'goose)" is an expression, not a statement, both because it's Lisp, which "doesn't have statements," and because Lisp is more than a decade older than C, which was the "earliest popular language to blur the lines [between expressions and statements]." Could explain the details of that to me?
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      1. USCurt J. Sampson
    2. CO@Curt Sampson, have you asked that as a separate question?
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      1. USKelly S. French
    3. COIf I'm not mistaken, `callfunc(x = 2);` passes `x` to `callfunc`, not `2`. If `x` is a float, `callfunc(float)` will be called, not `callfunc(int)`. And in C++, if you pass `x=y` to `func`, and `func` takes a reference and changes it, it changes `x`, not `y`.
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      1. USGabriel
 

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