if语句由一个布尔表达式后跟一个或多个语句。
if语句在D编程语言的语法是：
| if(boolean_expression) |
| { |
| /* statement(s) will execute if the boolean expression is true */ |
| } |
如果布尔表达式的值为true，那么代码的if语句内的模块将被执行。如果布尔表达式计算为false，那么第一组代码的if语句(右大括号后)结束后，将被执行。
D编程语言假定任何非零和非空值作为true，如果它是零或为null，则假定为false。
顶替
| import std.stdio; |
| int main () |
| { |
| /* local variable definition */ |
| int a = 10; |
| /* check the boolean condition using if statement */ |
| if( a < 20 ) |
| { |
| /* if condition is true then print the following */ |
| writefln("a is less than 20" ); |
| } |
| writefln("value of a is : %d", a); |
| return 0; |
| } |
a is less than 20; value of a is : 10
一个if语句后面可以跟一个可选的else语句，该语句执行时的布尔表达式为false。
在D编程语言的if... else语句的语法是：
| if(boolean_expression) |
| { |
| /* statement(s) will execute if the boolean expression is true */ |
| } |
| else |
| { |
| /* statement(s) will execute if the boolean expression is false */ |
| } |
如果布尔表达式的值为true，那么if代码块将被执行代码，否则else块将被执行。
D编程语言假设任何非零和非空值作为true，如果是零或null，则假定为false。
| import std.stdio; |
| int main () |
| { |
| /* local variable definition */ |
| int a = 100; |
| /* check the boolean condition */ |
| if( a < 20 ) |
| { |
| /* if condition is true then print the following */ |
| writefln("a is less than 20" ); |
| } |
| else |
| { |
| /* if condition is false then print the following */ |
| writefln("a is not less than 20" ); |
| } |
| writefln("value of a is : %d", a); |
| return 0; |
| } |
a is not less than 20; value of a is : 100
一个if后面可以跟一个可选的if... else语句，如果测试各种条件下... else if语句，可使用单一的else语句。
当使用 if , else if , else语句有几点要记住：
if可以有零个或else，它必须跟在else if后面。
if 可以有零到多个else if，它们必须在else之前。
一旦一个 else if 匹配成功，剩余else if不会被测试或计算。
if...else if...else语句在D编程语言的语法是：
| if(boolean_expression 1) |
| { |
| /* Executes when the boolean expression 1 is true */ |
| } |
| else if( boolean_expression 2) |
| { |
| /* Executes when the boolean expression 2 is true */ |
| } |
| else if( boolean_expression 3) |
| { |
| /* Executes when the boolean expression 3 is true */ |
| } |
| else |
| { |
| /* executes when the none of the above condition is true */ |
| } |
| import std.stdio; |
| int main () |
| { |
| /* local variable definition */ |
| int a = 100; |
| /* check the boolean condition */ |
| if( a == 10 ) |
| { |
| /* if condition is true then print the following */ |
| writefln("Value of a is 10" ); |
| } |
| else if( a == 20 ) |
| { |
| /* if else if condition is true */ |
| writefln("Value of a is 20" ); |
| } |
| else if( a == 30 ) |
| { |
| /* if else if condition is true */ |
| writefln("Value of a is 30" ); |
| } |
| else |
| { |
| /* if none of the conditions is true */ |
| writefln("None of the values is matching" ); |
| } |
| writefln("Exact value of a is: %d", a ); |
| return 0; |
| } |
None of the values is matching Exact value of a is: 100
