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，否則else塊將被執(zhí)行。

TCL語言使用expr內(nèi)部命令，因此它不是明確地使用expr語句所需的。

流程圖

示例

#!/usr/bin/tclsh

set a 100

#check the boolean condition 
if {$a < 20 } {
   #if condition is true then print the following 
   puts "a is less than 20"
} else {
   #if condition is false then print the following 
   puts "a is not less than 20"
}
puts "value of a is : $a"

當(dāng)上述代碼被編譯和執(zhí)行時(shí)，它產(chǎn)生了以下結(jié)果：

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

if...else if...else 語句

if語句可以跟著一個(gè)可選的else if ... else語句，使用單個(gè)if 測試各種條件if...else if 聲明是非常有用的。

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

一個(gè)if可以有零或一個(gè)else，它必須跟在else if之后。
一個(gè)if語句可以有零到多個(gè)else if，并且它們必須在else之前。
一旦一個(gè) else if 成功，任何剩余else if 或else 不會(huì)再被測試。

語法

Tcl語言的 if...else if...else語句的語法是：

if {boolean_expression 1} {
   # Executes when the boolean expression 1 is true
} elseif {boolean_expression 2} {
   # Executes when the boolean expression 2 is true 
} elseif {boolean_expression 3} {
   # Executes when the boolean expression 3 is true 
} else {
   # executes when the none of the above condition is true 
}

示例

#!/usr/bin/tclsh

set a 100

#check the boolean condition
if { $a == 10 } {
   # if condition is true then print the following 
   puts "Value of a is 10"
} elseif { $a == 20 } {
   # if else if condition is true 
   puts "Value of a is 20"
} elseif { $a == 30 } {
   # if else if condition is true 
   puts "Value of a is 30"
} else {
   # if none of the conditions is true 
   puts "None of the values is matching"
}

puts "Exact value of a is: $a"

當(dāng)上述代碼被編譯和執(zhí)行時(shí)，它產(chǎn)生了以下結(jié)果：

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

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