Praise for C# 2.0: Practical Guide for Programmers 2005 phần 6 pot

112 Chapter 6: Statements and Exceptions ■

6.3.3 Iteration Statements

Iteration statements, or loops, allow a single statement or block to be executed repeat￾edly. The loop condition is a boolean expression that determines when to terminate the

loop. C# provides four kinds of loops: while, do-while, for, and foreach statements.

while Statement

EBNF The syntax of the while loop is:

WhileStmt = "while" "(" BooleanExpr ")" EmbeddedStmt .

EmbeddedStmt is executed zero or more times until the BooleanExpr evaluates to false.

Example:

Console.Write("Countdown: ");

int sec = 9;

while ( sec >= 0 )

Console.Write("{0} ", sec--);

Console.WriteLine("... Go!");

Output:

Countdown: 9 8 7 6 5 4 3 2 1 0 ... Go!

do-while Statement

EBNF The syntax of the do-while loop is:

DoStmt = "do" EmbeddedStmt "while" "(" BooleanExpr ")" ";" .

EmbeddedStmt is executed one or more times until the BooleanExpr evaluates to

false.

Example (giving the same output):

Console.Write("Countdown: ");

int sec = 9;

do

Console.Write("{0} ", sec--);

while ( sec >= 0 );

Console.WriteLine("... Go!");

■ 6.3 Embedded Statements 113

for Statement

The syntax of the for loop is: EBNF

ForStmt = "for" "(" ForInitializer? ";" ForCondition? ";" ForIterator? ")"

EmbeddedStmt .

and is equivalent to the following statements:

ForInitializer

"while" "(" ForCondition ")" "{"

EmbeddedStmt

ForIterator

"}"

where: EBNF

ForInitializer = LocalVarDecl | StmtExprList .

ForCondition = BooleanExpr .

ForIterator = StmtExprList .

Example (giving the same output):

Console.Write("Countdown: ");

for (int sec = 9; sec >= 0; --sec)

Console.Write("{0} ", sec);

Console.WriteLine("... Go!");

An infinite for loop that prints dots:

for (;;)

Console.Write(".");

is equivalent to the following while statement:

while (true)

Console.Write(".");

foreach Statement

The syntax of the foreach loop is: EBNF

ForeachStmt = "foreach" "(" Type Identifier "in" Expr ")" EmbeddedStmt .

The foreach statement enumerates the elements of a given collection and executes the

embedded statement for each one. The Type and Identifier declare a read-only itera￾tion variable to be used locally within the scope of the embedded statement. During the

loop execution, this iteration variable represents a collection element. A compilation error