添加到Erlang中的现有值 [英] Adding to an existing value in Erlang

问题描述

我正在尝试创建一个将数字存储到记录中的函数，然后在函数运行时将值X添加到该数字。

I am attempting to create a function that stores a number into a record and then adds value X to that number every time the function runs.

Value: 5
Run Function (Add One):   1
Value should be: 6
Run Function (Add One):   1
value should be 7

我尝试使用记录：

-record(adder,{value :: integer()}).

---function 
   Number = random:uniform(6),
        L=#added{value = Number + #added.value}.

这不起作用，因为它每次都会重置值。任何建议？

This does not work as it resets the value every time. Any suggestions?

推荐答案

看看这段代码：

-module(test).
-export([add/1]).

-record(adder, {value=6}).

add(X) ->
    #adder{value = X + #adder.value}.

如果您在shell中编译，任何对add（3）的调用将导致 {adder，5}，而不在{adder，9}中。看看：

If you compile this in your shell, any call to "add(3)" will result in "{adder,5}" and not in "{adder, 9}". Take a look:

Eshell V6.4  (abort with ^G)
1> c(test).
{ok,test}
2> test:add(3).
{adder,5}
3> test:add(3).
{adder,5}

怎么来？这是因为记录实际上是元组。最后一行中的表达式＃adder.value被计算为加法器元组中的字段值的位置，这是2.我们有一些证明。更改记录的定义：

How come? This is because records are actually tuples. The expression "#adder.value" in the last line is evaluated as the position of the field "value" in your adder tuple, which is 2. Let's have some proof. Change the definition of your record:

-module(test).
-export([add/1]).

-record(adder, {field1, field2, value=6}).

add(X) ->
    #adder{value = X + #adder.value}.

现在，重新编译代码并再次调用add（3）会导致

Now, recompiling your code and calling add(3) again would result in

1> c(test).
{ok,test}
2> test:add(3).
{adder,undefined,undefined,7}

我问自己，你怎么样提出了这个问题。你不希望你的功能是这样的吗？

I've asked myself, how you came up with this question. Didn't you want your function to be something like this?

add2(#adder{value = V} = R, X) ->
    R#adder{value = V + X}.

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