尝试使用索引修改字符串时的语法错误 [英] Syntax error when attempting to amend a string with indexing

问题描述

我正在从此处学习APL.

为什么会出现此语法错误?

Why am I getting this syntax error?

      'computer' [ 1 2 3 ] ← 'COM'
SYNTAX ERROR
'computer'[1 2 3]←'COM'
          ^

但是，如果我将'computer'保存在变量中，则不会收到错误消息:

But if I save 'computer' in a variable I don't get the error:

      T ← 'computer'
      T
computer
      T[1 2 3] ← 'COM'
      T
COMputer

我在做什么错了?

推荐答案

'computer'是常量，您不能更改常量本身的值，只能更改变量的当前值.

'computer' is a constant, and you can't change the value of a constant itself, only the current value of a variable.

考虑一下:如果您可以分配给'computer'，那么下次编写'computer'时，您希望结果为COMputer吗? 2←3怎么样?显然，这没有任何意义.

Think about it: If you could assign to 'computer', then next time you wrote 'computer', would you expect the result to be COMputer? How about 2←3? Clearly, this doesn't make any sense.

但是，您可以使用相对较新的@"at"运算符(不包含在 Mastering Dyalog APL中，修改值而不将其分配给名称). em>，但该文档可在线获得)

However, you can amend a value without assigning it to a name, using the relatively new @ "at" operator (it isn't included in Mastering Dyalog APL, but the documentation is available online).

      'COM'@1 2 3⊢'computer'
COMputer

您可以在在计算机"一词的第1 2 3个索引中输入字母'COM'来阅读它..此处的⊢仅用于将1 2 3与'computer分开，因此@很清楚什么是索引，什么是要修改的数组.

You can read this as put the letters 'COM' at indices 1 2 3 of the word 'computer'. The ⊢ here only serves to separate 1 2 3 from 'computer so it is clear to @ what constitutes the indices and what is the array to be amended.

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