C# Essential Training: 1 Syntax and Object Oriented Programming(一)

打開檢視 > 其他視窗 > C#互動，可以不用編譯直接顯示程式碼結果。

1. 數字類型型別極限值

> int.MaxValue　// 2147483647

> int.MinValue　// -2147483648

> double.MaxValue　// 1.7976931348623157E+308

> double.MinValue　// -1.7976931348623157E+308

> decimal.MaxValue　// 79228162514264337593543950335

> decimal.MinValue　// -79228162514264337593543950335

2. stirng.Trim、string.Substring、string.Length 方法

> string text = "  abcde  ";

> text.Length　// 9。前後各有兩格空格。

> text.Trim()　// "abcde"。刪掉前後空格。另有TrimStart、TrimEnd。

> text.Trim().Length　// 5

> text.Substring(4)　// "cde  "。01234，所以對應到字串是「空格、空格、a、b、c...」的 'c'，'c' 以前的都字元都刪掉。

> text.Substring(3, 4)　// "bcde"。從index 3開始的4個字元。可以記成start, range(int startIndex, int length)

3. float的小數點要補'f'

> float pi = 3.14;

(1,12): error CS0664: Literal of type double cannot be implicitly converted to type 'float'; use an 'F' suffix to create a literal of this type

> decimal money = 10.99;

(1,17): error CS0664: Literal of type double cannot be implicitly converted to type 'decimal'; use an 'M' suffix to create a literal of this type

> float pi = 3.14f;　// pass.

> decimal money = 10.99m;　// pass.

4. string.Format 方法

> var city = "Dallas";

> var temperature = 103.4f;

> var currentDt = DateTime.Now();

(1,26): error CS1955: Non-invocable member 'DateTime.Now' cannot be used like a method.

> var currentDt = DateTime.Now;

> string.Format($"Welcome to {city}. The time is {currentDt}. The temperature is {temperature}.")

"Welcome to Dallas. The time is 2020/12/4 下午 04:36:31. The temperature is 103.4."

> string.Format($"Welcome to {city}. The time is {currentDt:t}. The temperature is {temperature}.")

"Welcome to Dallas. The time is 下午 04:36. The temperature is 103.4."

> string.Format($"Welcome to {city}. The time is {currentDt:T}. The temperature is {temperature}.")

"Welcome to Dallas. The time is 下午 04:36:31. The temperature is 103.4."

> temperature = 1000.25f

100.25

> temperature = 1000.25d

(1,15): error CS0664: Literal of type double cannot be implicitly converted to type 'float'; use an 'F' suffix to create a literal of this type

> temperature = 1000.25m

(1,15): error CS0266: Cannot implicitly convert type 'decimal' to 'float'. An explicit conversion exists (are you missing a cast?)

> string.Format($"Welcome to {city}. The time is {currentDt:T}. The temperature is {temperature:0,0.00}.")

"Welcome to Dallas. The time is 下午 04:36:31. The temperature is 1,000.25."

5. int.TryParse、string.Replace 方法

> int.Parse("15.2334")　// 調皮一下。

System.FormatException: 輸入字串格式不正確。+ System.Number.StringToNumber(string, System.Globalization.NumberStyles, ref System.Number.NumberBuffer, System.Globalization.NumberFormatInfo, bool) + ... + <Initialize>.MoveNext()

> string test = "15,234";

> test.Replace(',', '')　// 雷！不可以空字元！

(1,19): error CS1011: Empty character literal

> test.Replace(",", "")　// 空字串才可以!!

"15234"

> int result;

> int.TryParse("15,234", out result)　// true就指派給result

false

> int.TryParse("15234", out result)

true

> result

15234　// 死勾蟻爹死餒!

6. Math 類別

(需要背一下的靜態方法...)

Sqrt　// 平方根

Pow(value, power)　// 幾次方。印象中在C++ Math Library是value^power。

> Math.Pow(4, 0.5)　// 2。平方根也可以寫成二分之一次方。

> Math.Pow(8, 1/3d)　// 2。開三方，記得將power值切成double型別。

Round、Ceiling、Floor　// 四捨五入、無條件進位、捨去至整數

Abs　// 絕對值

Log(Double, Double)　// 傳回指定底數中指定數字的對數。

Log(Double)　// 傳回指定數字的自然對數 (底數為 e) 。

Log10(Double)　// 傳回指定數字的以 10 為底數的對數。

Log2(Double)　// 傳回指定數字的以 2 為底數的對數。

(未完待續...)