# Rust if語句

`if`語句確定條件是否爲真。如果條件爲`true`，則執行`if`塊，否則，控制會跳過`if`塊。

• if塊
• if-else塊
• if else-if階梯塊
• 嵌套if語句

## if語句

`if`語句塊的語法 -

``````if condition
{
//block statements;
}``````

`if`語句的流程圖 -

``````fn main(){

let a=1;
if a==1
{
println!("a is equal to 1");
}
}``````

``a is equal to 1``

## if-else語句

if-else語句的語法

``````if condition
{
//block statements
}else{
//block statements
}``````

if-else的流程圖

``````fn main()
{
let a=3;
let b=4;
if a>b
{
println!("a is greater than b");
}
else
{
println!("a is smaller than b");
}
}``````

``a is smaller than b``

## else-if語句

`else-if`語句的語法 -

``````if condition 1
{
//block statements
}else if condition 2
{
//block statements
}
.
.
else{
//block statements
}``````

`else if`語句塊的流程圖 -

``````fn main(){

let num= -5;
if num>0
{
println!("number is greater than 0");
}
else if num<0
{
println!("number is less than 0 ");
}
else
{
println!("number is not equal to 0");
}
}``````

``number is less than 0``

## 嵌套if-else語句

`if-else`語句出現在另一個`if``else`塊的主體內時，它稱爲嵌套`if-else`

``````if condition 1
{
// 嵌套if/else塊
if condition 2
{
//block statements
}else
{
//block statements
}
}else{
//block statements
}``````

``````fn main(){

let a=5;
let b=6;
if a!=b
{
if a>b
{
println!("a is greater than b");
}else{
println!("a is less than b");
}
}

else
{
println!("a is equal to b");
}
}``````

``a is less than b``