Cats learn Swift variables and constants (2)

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Address: http://blog.csdn.net/u013357243#

Constants: Let

Let is a keyword, a language keyword, used to declare constants, what is a keyword? GA do not want to explain, Java Public,oc in the beginning of the @, JS in the Var and so on. ）
constants, which are often used (as you understand), are like men, women, π=3.14, etc., which are common in life. These cannot be changed, and the constants in swift cannot change, so remember this is enough.

//常量：let20

If you want to change the constants, you will get an error.

Calculation of constants

let1    //整数默认类型是Intlet0.8  //小数默认类型是Doublelet num = num1 + num2

The above code will cause an error because Swift is a type-specific voice that is not implicitly convertible under any circumstances.

But what do we have to do about it? As long as you follow the same type principle.

let1    //整数默认类型是Intlet0.8  //小数默认类型是Double//let num = num1 + num2let num3 = Double(num1) + num2let num4 = num1 + Int(num2)

It is important to note that double int is not rounded, but is discarded after the decimal point.

Variable var

Variables are declared with Var. After the declaration can be modified

var v = 20v = 30

But note that you can only change the same type. With object-oriented thinking, the initialization is equivalent to birth, and after a pig is born you can't make him a person ... Can't just become a person can change that is eight commandments ...

Tip, we cannot copy the string type to the int type, which means that Swift is a very strict type of language. (a value will never be automatically converted to another type)

Display definition type, conversion type

Definition is to use
var variable name: type = value
Let variable name: type = value
You can display the definition type, such as

var a:Double = 3

We can press and hold the keyboard option and click on the variable A to see his type, when you can see his type is double

, although we did not do a display type, Swift would automatically deduce that he is a double type

Conversion type

var3var b = (Int)(a)

, we force the conversion type, and the initialized B automatically derives the int type.

Summarize:

1: Variable var can be modified after initialization
2: Solid let cannot be modified after initialization
3: Note: In the daily development, should be preferred let, in the need to modify at this time, in the Var

Cats Learn Swift Variables and constants (2)