The data types of function arguments are assumed to match the types of the formal parameters unless a function prototype declaration is present. In the presence of a function prototype, all arguments in the function invocation are compared for assignment compatibility to all parameters declared in the function prototype declaration. If the type of the argument does not match the type of the parameter but is assignment compatible, C converts the argument to the type of the parameter (see Section 6.11.1). If an argument in the function invocation is not assignment compatible to a parameter declared in the function prototype declaration, an error message is generated.

If a function prototype is not present, all arguments of type float are converted to double , all arguments of type char or short are converted to type int , all arguments of type unsigned char and unsigned short are converted to unsigned int , and an array or function name is converted to the address of the named array or function. The compiler performs no other conversions automatically, and any mismatches after these conversions are programming errors.

A function designator is an expression that has function type. Except when it is the operand of the sizeof operator or the unary & operator, a function designator with type "function returning type" is converted to an expression that has type "pointer to function returning type."

This section describes the following kinds of statements in the C programming language. Except as indicated in this chapter, statements are executed in the sequence in which they appear in a function body:

• Labeled statements ( Section 7.1)
• Compound statements ( Section 7.2)
• Expression statements ( Section 7.3)
• Null statements ( Section 7.4)
• Selection statements ( Section 7.5)
• Iteration statements ( Section 7.6)
• Jump statements ( Section 7.7)

A label is an identifier used to flag a location in a program as the target of a goto statement or switch statement. A label has the following syntax:

identifier : statement case constant-expression : statement default : statement

The scope of the label is the containing function body. Variables can have the same name as a label in the function because labels and variables have different name spaces (see Section 2.15).

There are three kinds of labeled statements in C:

• Any statement preceded by a label
• A case statement
• A default statement

The last two statements are discussed in Section 7.5.2 because they can appear only within a switch statement.

7.2 Compound Statements

A compound statement, or block, allows a sequence of statements to be treated as a single statement. A compound statement begins with a left brace, contains any mix of declarations and statements, and ends with a right brace, as shown in the following example:

{ int a; a = 1; int b; b = 2; }

Block declarations are local to the block, and, for the rest of the block, they supersede other declarations of the same name in outer scopes.

A block is entered normally when control flows into it, or when a goto statement transfers control to a label at the beginning of the block itself. Each time the block is entered normally, storage is allocated for auto or register variables. If, on the other hand, a goto statement transfers control to a label inside the block or if the block is the body of a switch statement, these storage allocations do not occur. For more information about storage classes, see Section 2.10.

Function definitions contain compound statements. The compound statement following the parameter declarations in a function definition is called the function body.

7.3 Expression Statements

Any valid expression can be used as a statement by following the expression with a semicolon, as shown in the following example:

i++;

This statement increments the value of the variable i . Note that i++ is a valid C expression that can appear in more complex C statements. For more information about the C expressions, see Chapter 6.

7.4 Null Statements

A null statement is used to provide a null operation in situations where the grammar of the language requires a statement, but the program requires no work to be done. The null statement consists of a semicolon:

;

The null statement is useful with the if , while , do , and for statements. The most common use of this statement is in loop operations in which all the loop activity is performed by the test portion of the loop. For example, the following statement finds the first element of an array that has a value of 0:

for (i=0; array[i] != 0; i++) ;

In this example, the for statement is executed for its side effects only; the loop body is a null statement. See Section 7.6 for more information about iteration statements.

The null statement is also useful where a label is needed just before a brace that terminates a compound statement. (A label cannot immediately precede the right brace; it must always be attached to a statement.) For example:

if (expression1) { ... goto label_1; /* Terminates this part of the if statement */ ... label_1: ; } else ...

7.5 Selection Statements

A selection statement selects among a set of statements depending on the value of a controlling expression. The selection statements are the if statement and the switch statement, which are discussed in the following sections.

7.5.1 The if Statement

The if statement has the following syntax:

if ( expression ) statement

elseopt else-statementopt

The statement following the control expression is executed if the value of the control expression is true (nonzero). An if statement can be written with an optional else clause that is executed if the control expression is false (0).

Consider the following example:

if (i < 1) funct(i); else { i = x++; funct(i); }

In this example, if the value of i is less than 1, then the statement funct(i) is executed and the compound statement following the keyword else is not executed. If the value of i is not less than 1, then only the compound statement following the keyword else is executed.

The control expression in a selection statement is usually a logical expression, but it can be any expression of scalar type.

When if statements are nested, an else clause matches the most recent if statement that does not have an else clause, and is in the same block. For example:

if (i < 1) { if (j < 1) funct(j); if (k < 1) /* This if statement is associated with */ funct(k); else /* this else clause. */ funct(j + k); }

7.5.2 The switch Statement

The switch statement executes one or more of a series of cases, based on the value of a controlling expression. The switch statement has the following syntax:

switch ( expression ) statement

The usual arithmetic conversions are performed on the control expression, but the result must have an integral type. For more information about data-type conversion, see Section 6.11. The switch statement is typically a compound statement, within which are one or more case statements executed if the control expression matches the case . The syntax for a case label and expression follows:

case constant-expression : statement

The constant expression must have an integral type. No two case labels can specify the same value. There is no limit on the number of case labels in a switch statement.

Only one statement in the compound statement can have the following label:

default :

The case and default labels can occur in any order, but it is common practice for the default statement to follow the case statements. Note that execution flows from the selected case into the cases following unless explicit action is taken, such as a break statement.

When the switch statement is executed, the following sequence takes place:

1. The switch control expression is evaluated (and integral promotions applied) and compared with the constant expressions in the case labels.
2. If the control expression's value matches a case label, control transfers to the statement following that label. If a break statement is encountered, the switch statement terminates; otherwise, execution continues into the following case or default statements until a break statement or the end of the switch statement is encountered (see Example 7-1).
   A switch statement can also be terminated by a return or goto statement. If a switch statement is inside a loop, the switch statement is terminated if a continue statement terminates the loop. See Section 7.7 for more information about these statements.
3. If the control expression's value does not match any case label, and there is a default label, control is transferred to the statement following that label. If a break statement does not end the default statement, and a case label follows, that case statement is executed.
4. If the control expression's value does not match any case label and there is no default label, execution of the switch statement terminates.

Example 7-1 uses the switch statement to count blanks, tabs, and new-line characters entered from the terminal.

Example 7-1 Using switch to Count Blanks, Tabs, and New Lines

/* This program counts blanks, tabs, and new lines in text * * entered from the keyboard. */ #include <stdio.h> main() { int number_tabs = 0, number_lines = 0, number_blanks = 0; int ch; while ((ch = getchar()) != EOF) switch (ch) { (1) case '\t': ++number_tabs; (2) break; case '\n': ++number_lines; break; case ' ' : ++number_blanks; break; default:; } printf("Blanks\tTabs\tNewlines\n"); printf("%6d\t%6d\t%6d\n", number_blanks, number_tabs,number_lines); }

Key to Example 7-1:

1. A series of case statements is used to increment separate counters depending on the character encountered.
2. The break statement causes control to return to the while loop. Control is passed to the while loop if the value of ch does not match any of the case constant expressions.

Without the break statements, each case would drop through to the next.

If variable declarations appear in the compound statement within a switch statement, initializers on auto or register declarations are ineffective. However, initializations within the statements following a case are effective. Consider the following example:

switch (ch) { int nx = 1; /* Initialization ignored */ printf("%d", n); /* This first printf is not executed */ case 'a' : { int n = 5; /* Proper initialization occurs */ printf("%d", n); break; } case 'b' : { break; } default : { break; } }

In this example, if ch == 'a' , then the program prints the value 5. If the variable equals any other letter, the program prints nothing because the initialization occurs outside of the case label, and statements outside of the case label are ineffective.

7.6 Iteration Statements

An iteration statement, or loop, repeatedly executes a statement, known as the loop body, until the controlling expression is false (0). The control expression must have a scalar type.

The while statement evaluates the control expression before executing the loop body (see Section 7.6.1).

The do statement evaluates the control expression after executing the loop body; at least one execution of the loop body is guaranteed (see Section 7.6.2).

The for statement executes the loop body based on the evaluation of the second of three expressions (see Section 7.6.3).

7.6.1 The while Statement

The while statement evaluates a control expression before each execution of the loop body. If the control expression is true (nonzero), the loop body is executed. If the control expression is false (0), the while statement terminates. The while statement has the following syntax:

while ( expression ) statement

Consider the following while statement:

n = 0; while (n < 10) { a[n] = n; n++; }

This statement tests the value of n ; if n is less than 10, it assigns n to the nth element of the array a and then increments n . The control expression (in parentheses) is then evaluated; if true (nonzero), the loop body is executed again; if false (0), the while statement terminates. If the statement n++ were missing from the loop body, this while statement would never terminate. If the statement n = 0 were replaced by the statement n = 10 , the control expression is initially false (0), and the loop body is never executed.

7.6.2 The do Statement

The do statement evaluates the control expression after each execution of the loop body. The do statement has the following syntax:

do statement while ( expression ) ;

The loop body is executed at least once. The control expression is evaluated after each execution of the loop body. If the control expression is true (nonzero), the statement is executed again. If the control expression is false (0), the do statement terminates.

7.6.3 The for Statement

The for statement evaluates three expressions and executes the loop body until the second controlling expression evaluates to false (0). The for statement is useful for executing a loop body a specified number of times. The for statement has the following syntax:

for ( expression-1opt ; expression-2opt ; expression-3opt) statement

The for statement is equivalent to the following code:

expression-1;

while ( expression-2 ) { statement expression-3 ; }

The for statement executes the loop body zero or more times. Semicolons (;) are used to separate the control expressions. A for statement executes the following steps:

1. expression-1 is evaluated once before the first iteration of the loop. This expression usually specifies the initial values for variables used in the loop.
2. expression-2 is any scalar expression that determines whether to terminate the loop. expression-2 is evaluated before each loop iteration. If the expression is true (nonzero), the loop body is executed. If the expression is false (0), execution of the for statement terminates.
3. expression-3 is evaluated after each iteration.
4. The for statement executes until expression-2 is false (0), or until a jump statement, such as break or goto , terminates execution of the loop.

Any of the three expressions in a for loop can be omitted:

• If expression-2 is omitted, the test condition is always true; that is, the while loop equivalent becomes while(1) . This is an infinite loop. For example:

  for (i = 0; ;i++) statement;

  
  Infinite loops can be terminated with a break , return , or goto statement within the loop body.

• If either expression-1 or expression-3 is omitted from the for statement, the omitted expression is evaluated as a void expression and is effectively dropped from the expansion. For example:

  n = 1; for ( ; n < 10; n++) func(n);

  
  In this example, n is initialized before the for statement is executed.

In relaxed ANSI C mode, the first clause of the for statement can be a declaration whose scope includes the remaining clauses of the for header and the entire loop body. This is normally used to declare and initialize a local loop control variable. For example:

for (int i=0; i<10; i++) printf("%d\n", i);

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