學習C#

理解C#中的if-else語句：初學者指南

Layla Porter

7m 37s

在C#程式設計中，根據條件進行決策是一個建立動態和響應式應用程式的基本方面。這就是if-else語句的用武之地。 C#中的if-else語句允許程式根據指定條件的真或假來執行不同的程式碼塊。

此指南受影片'C# for Beginners Part 2 - Learn how to write an 'if-else' statement and a 'while' loop'由LaylaCodesIt啟發，將介紹C#中if-else語句的基礎知識，使用條件和邏輯運算子編寫布林條件，展示如何使程式與使用者輸入智能互動。

介紹條件邏輯

條件邏輯允許程式根據某些條件做出決策。 Layla的影片介紹了if-else語句的概念和語法，這是一個條件邏輯的例子，它根據指定條件的真或假執行不同的程式碼塊。

在下面的程式碼範例中，Layla編寫了一個問候訊息，詢問使用者是否喜歡起司：

// Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?");
string doTheyLikeCheese = Console.ReadLine();

// Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?");
string doTheyLikeCheese = Console.ReadLine();

' Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?")
Dim doTheyLikeCheese As String = Console.ReadLine()

' Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?")
Dim doTheyLikeCheese As String = Console.ReadLine()

Simple If-Else Statement in C#

然後，這個if-else條件語句檢查使用者的回答並根據使用者的回答改變其輸出響應。條件是使用條件運算子構建的，並根據其結果執行if-else塊中的語句。這是一個單行語句的簡單範例：

// Checking the user's response and providing appropriate output
if (doTheyLikeCheese == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

// Checking the user's response and providing appropriate output
if (doTheyLikeCheese == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

' Checking the user's response and providing appropriate output
If doTheyLikeCheese = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

' Checking the user's response and providing appropriate output
If doTheyLikeCheese = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

如果使用者說'yes'，if語句內的程式碼將顯示一條訊息，提供一塊起司。如果使用者的回應不是'yes'，那麼else塊將輸出'這是一塊煤'。

區分大小寫問題

使用基本的if-else語句，可能會出現一個常見問題：區分大小寫。如果使用者輸入'Yes'而不是'yes'，程式無法識別這是有效輸入。為了解決這個問題，Layla使用ToLower()方法，自動將使用者的輸入轉換為小寫，避免了區分大小寫問題。

// Using ToLower() to handle case sensitivity in user input
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

// Using ToLower() to handle case sensitivity in user input
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

' Using ToLower() to handle case sensitivity in user input
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

' Using ToLower() to handle case sensitivity in user input
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

處理多個條件

Layla 然後解釋如何使用else-if語句處理多於兩個可能的回應。在此，程式檢查多個條件語句 - 如果使用者回應'yes'、'no'或其他任何東西。

// Checking multiple conditions using else-if
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else if (doTheyLikeCheese.ToLower() == "no")
{
    Console.WriteLine("Here is a lump of coal.");
}
else
{
    Console.WriteLine("Please give a yes or no answer.");
}

// Checking multiple conditions using else-if
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else if (doTheyLikeCheese.ToLower() == "no")
{
    Console.WriteLine("Here is a lump of coal.");
}
else
{
    Console.WriteLine("Please give a yes or no answer.");
}

' Checking multiple conditions using else-if
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
ElseIf doTheyLikeCheese.ToLower() = "no" Then
    Console.WriteLine("Here is a lump of coal.")
Else
    Console.WriteLine("Please give a yes or no answer.")
End If

' Checking multiple conditions using else-if
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
ElseIf doTheyLikeCheese.ToLower() = "no" Then
    Console.WriteLine("Here is a lump of coal.")
Else
    Console.WriteLine("Please give a yes or no answer.")
End If

如前所述，如果if語句中的布林表達式為真，則當使用者說'yes'時，程式將提供一塊起司。但是，而不是向任何其他回應提供一塊煤，在else-if語句中，它現在只在使用者回答'no'時這麼做。如果使用者的回答不是'yes'或'no'，則程式的else部分將要求使用者提供一個'yes'或'no'的答案。讓我們看看上面程式碼的運行效果：（圖片可以改善）

使用迴圈

為了確保使用者提供有效的回答，引入了迴圈。迴圈可以反复要求使用者輸入，直到他們提供正確答案。這稱為'while'迴圈，以下範例顯示其運作方式：

// Using a while loop to repeatedly ask for valid input
bool hasAnswered = false;

while (!hasAnswered)
{
    // Checking the user's response and setting hasAnswered to true for valid responses
    if (doTheyLikeCheese.ToLower() == "yes")
    {
        Console.WriteLine("Here is a lump of cheese.");
        hasAnswered = true;
    }
    else if (doTheyLikeCheese.ToLower() == "no")
    {
        Console.WriteLine("Here is a lump of coal.");
        hasAnswered = true;
    }
    else
    {
        Console.WriteLine("Please give a yes or no answer.");
        doTheyLikeCheese = Console.ReadLine();
    }
}

// Using a while loop to repeatedly ask for valid input
bool hasAnswered = false;

while (!hasAnswered)
{
    // Checking the user's response and setting hasAnswered to true for valid responses
    if (doTheyLikeCheese.ToLower() == "yes")
    {
        Console.WriteLine("Here is a lump of cheese.");
        hasAnswered = true;
    }
    else if (doTheyLikeCheese.ToLower() == "no")
    {
        Console.WriteLine("Here is a lump of coal.");
        hasAnswered = true;
    }
    else
    {
        Console.WriteLine("Please give a yes or no answer.");
        doTheyLikeCheese = Console.ReadLine();
    }
}

' Using a while loop to repeatedly ask for valid input
Dim hasAnswered As Boolean = False

While Not hasAnswered
    ' Checking the user's response and setting hasAnswered to true for valid responses
    If doTheyLikeCheese.ToLower() = "yes" Then
        Console.WriteLine("Here is a lump of cheese.")
        hasAnswered = True
    ElseIf doTheyLikeCheese.ToLower() = "no" Then
        Console.WriteLine("Here is a lump of coal.")
        hasAnswered = True
    Else
        Console.WriteLine("Please give a yes or no answer.")
        doTheyLikeCheese = Console.ReadLine()
    End If
End While

' Using a while loop to repeatedly ask for valid input
Dim hasAnswered As Boolean = False

While Not hasAnswered
    ' Checking the user's response and setting hasAnswered to true for valid responses
    If doTheyLikeCheese.ToLower() = "yes" Then
        Console.WriteLine("Here is a lump of cheese.")
        hasAnswered = True
    ElseIf doTheyLikeCheese.ToLower() = "no" Then
        Console.WriteLine("Here is a lump of coal.")
        hasAnswered = True
    Else
        Console.WriteLine("Please give a yes or no answer.")
        doTheyLikeCheese = Console.ReadLine()
    End If
End While

在上面的例子中，迴圈不斷提示使用者，直到他們回應'yes'或'no'。這是Layla影片中程式碼執行時的運行效果：