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Statements 语句

1. Definition of a Statement

In C, a statement is the smallest standalone element of an imperative programming language that expresses an action to be carried out. * Concept: If identifiers are "words" and expressions are "phrases," a statement is a complete "sentence." * Syntax: Most statements end with a semicolon (;), which acts as the terminator. * Purpose: It instructs the CPU to perform an action at runtime (e.g., calculation, control flow, function call).

2. Categories of Statements

Standard C classifies statements into five main types:

  1. Expression Statements An expression followed by a semicolon. It is typically executed for its side effects (modifying the state).

    • a = 10; (Assignment)
    • func(); (Function call)
    • ; (Null statement)

  2. Compound Statements (Blocks) A group of statements enclosed in braces { }. Syntactically, the entire block is treated as a single statement.

  3. Selection Statements Used to select a path of execution.

    • Keywords: if, switch.

  4. Iteration Statements (Loops) Used to execute code repeatedly.

    • Keywords: while, do...while, for.

  5. Jump Statements Used to unconditionally transfer control.

    • Keywords: break, continue, return, goto.

3. Scope and Boundaries of Statements

A crucial concept in C is that control flow statements (like if, while, for) encompass their bodies.

The "Whole Unit" Rule

A selection or iteration statement is not just the line with the keyword (e.g., if (x>0)). It is defined as the keyword, the condition, plus the sub-statement (body) that follows it.

  • Syntax: if ( expression ) statement
  • Implication: The "statement" at the end can be a single line (ending in ;) or a compound block ({...}). In both cases, they are legally part of the if statement itself.

Example: Nested Logic without Braces

The following code is valid because the entire if...else structure is considered one single statement by the while loop.

while (1)           // Controls the ONE statement immediately following it
    if (x > 5)      // Start of the nested statement
        print("A");
    else            // Still part of the same nested statement
        print("B");

4. What is NOT a Statement?

It is vital to distinguish executable statements from directives and declarations.

Preprocessor Directives

Lines beginning with # (e.g., #define, #include) are not statements. * Reason: They are processed by the Preprocessor at compile-time (text replacement), not by the CPU at runtime.

Declarations (int a; is NOT a statement)

Although int a; ends with a semicolon and appears inside functions, it is grammatically a Declaration, not a Statement.

  • The Difference:
    • Declaration (int a;): Tells the compiler "what exists". It introduces an identifier and reserves memory (stack/global) for it.
    • Statement (a = 10;): Tells the CPU "what to do". It executes an instruction to move data into that memory.
  • Evidence (C89 Standard): In the original C89 standard, declarations had to be placed at the very top of a block, before any statements. c void func() { int a; // Declaration a = 10; // Statement int b; // Error in C89! (Declaration cannot follow a statement) } If int a; were a statement, the code above would have been valid in C89 (statement following statement). The fact that it was forbidden proves they are distinct syntactic categories.

5. Common Pitfalls

The Null Statement Trap

Accidentally placing a semicolon after a control structure terminates the statement immediately.

if (x > 0);  // The 'if' statement ends here with an empty action.
{
    // This block is no longer controlled by the 'if'.
    // It will execute unconditionally.
    printf("Bug");
}

Assignment vs. Equality

Using = instead of == inside a condition.

if (x = 5)  // Assigns 5 to x. Expression evaluates to 5 (True).
  • Result: The condition is always true, and x is silently modified.

The do...while Semicolon

This is the unique loop that requires a semicolon at the end. * while (...) { ... } (No semicolon) * do { ... } while (...); (Required semicolon) * Here, the semicolon marks the end of the entire iteration statement.