Erlang学习进度情况(四)

目前发现在Erlang有些特性需要注意，相关内容如下：

第一个就是Short-Circuit Boolean Expressions

有两种语法结构如下：

Expr1 orelse Expr2

表示：This first evaluates Expr1. if Expr1 evaluates to true, Expr2 is not evaluated. If Expr1 evaluates to false, Expr2 is evaluated.

Expr1 andalso Expr2

表示：This first evaluates Expr1. if Expr1 evaluates to true, Expr2 is evaluated. if Expr1 evaluates to false, Expr2 is not evaluated.

在erlang语言里需要注意，A or B; A and B.两个条件都是要执行的。even if the truth value of the expression can be determined by evaluating only the first expression.

第二个就是Term Comparisons

格式如下：

number<atom<reference<fun<port<pid<tuple<list<binary

上面表示的意思是如下：

This means that, for example, a number(any number) is defined to be smaller than an atom(any atom),

that a tuple is greater than an atom, and so on.

下面时8个比较操作：

X>Y         X is greater than Y.

X<Y         X is less than Y.

X<=Y       X is equal to or less than Y.

X>=Y       X is greater than or equal to Y.

X==Y       X is equal to Y.

X/=Y        X is not equal to Y.

X=:=Y      X is identical to Y.

X=/=Y      X is not identical to Y.

这里有以下几个注意地方：

关于=:=和=/=两个操作符

behave in the following way if their arguments are numbers:

if one argument is a integer and the other is a float, then the integer is converted to a float before the comparison is performed.

if both arguments are integers or if both arguments are floats, then the arguments are used "as is", that is without conversion.

关于==和=:=两个操作符

== is useful only when comparing floats with integers.  =:= is for testing whether two terms are identical.

关于Underscore Variables

There's one more thing to say about variables. The special syntax _VarName is used for a normal variable,not an anonymous variable.

关于这种变量的使用场景有2个地方：

第一种：To name a variable that we don't intend to use.

                 That is, writing open(File,_Mode) makes the program more readable than writing open(File,_).

第二种：For debugging purposes.举例说明如下：

                some_func(X)->

                          {P,Q}=some_other_func(X),

  io:formar("Q = ~p~n",[Q]),

  P.

                  This compiles without an error message.

              Now comment out the format statement:

                   some_func(X)->

                           {P,Q}=some_other_func(X),

                            %%   io:formar("Q = ~p~n",[Q]),

                            P.

                    if we compile this, the compiler will issue a warning that the variable Q is not used.

    if we rewrite the function like this:

                     some_func(X)->

                          {P,_Q}=some_other_func(X),

  io:formar("_Q = ~p~n",[_Q]),

  P.

                      then we can comment out the format statement, and the compiler will not complain.