if(boolean_expression)
{
   /* statement(s) will execute if the boolean expression is true */
}
else
{
  /* statement(s) will execute if the boolean expression is false */
}

如果布爾表達(dá)式的值為true，那么if代碼塊將被執(zhí)行代碼，否則else塊將被執(zhí)行。

D編程語言假設(shè)任何非零和非空值作為true，如果是零或null，則假定為false。

流程圖：

例子：

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;
}

當(dāng)上面的代碼被編譯并執(zhí)行，它會產(chǎn)生以下結(jié)果：

a is not less than 20;
value of a is : 100

if...else if...else語句

一個(gè)if后面可以跟一個(gè)可選的if... else語句，如果測試各種條件下... else if語句，可使用單一的else語句。

當(dāng)使用 if , else if , else語句有幾點(diǎn)要記?。?/p>

if可以有零個(gè)或else，它必須跟在else if后面。
if 可以有零到多個(gè)else if，它們必須在else之前。
一旦一個(gè) else if 匹配成功，剩余else if不會被測試或計(jì)算。

語法

if...else if...else語句在D編程語言的語法是：

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 */
}

例子：

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;
}

當(dāng)上面的代碼被編譯并執(zhí)行，它會產(chǎn)生以下結(jié)果：

None of the values is matching
Exact value of a is: 100

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