sas MACRO & 符号 [英] sas MACRO ampersand

问题描述

%let test = one;
%let one = two;

%put &test;
%put &&test;
%put &&&test;
%put &&&&test;
%put &&&&&test;

嗯.我完全被这些&符号打败了.我不明白为什么他们在宏变量之前需要这么多&符号.有没有什么技巧可以掌握&符号的用法?BTW，对应的五个结果是什么?

Well. I'm TOTALLY BEATEN by these ampersands. I don't understand why they need SO MANY ampersands before a macro variable. Is there any trick to master the usage of ampersand? BTW, what are the five results, correspondingly?

推荐答案

用一组 & 符号，你得到的东西很无聊；一后，奇数个与号导致解析两次，偶数个与号解析一次.因此，您使用 1 个和号来解析一次，使用 3 个和号来解析两次，除非您拥有拥有与号权利的公司的股票.

With a single set of ampersands, what you get is pretty boring; after one, odd number of ampersands leads to resolving twice, even number of ampersands resolves once. So you use 1 ampersand to resolve once and 3 ampersands to resolve twice, unless you have stock in the company that owns rights to the ampersand.

更有趣的是以下测试，它说明了为什么偶数个 & 符号具有价值:

More interesting is the following test, which shows why even numbers of ampersands have value:

%let test = one;
%let testtwo = one;
%let one = two;
%let two=three;

%put &test&one;
%put &&test&one;
%put &&&test&one;
%put &&&&test&one;
%put &&&&&test&one;
%put &&&&&&test&one;

基本上，每次通过，SAS 都会执行以下操作:

Basically, each pass through, SAS does the following:

• 将任何单个与号加文本解析为宏变量引用.
• 将任何对与号解析为一个与号.

那些是同时迭代完成的，直到所有的&符号都消失了，并且每个结果都保留用于下一次迭代，并且不会影响当前迭代.所以，&test&one 变成了 onetwo 因为 &test-> one 和 &one ->二.剩余步骤:

Those are done simultaneously and iteratively until all ampersands are gone, and each result is kept for the next iteration and does not affect the current iteration. So, &test&one becomes onetwo because &test-> one and &one -> two. Steps for the remaining:

• &&test&one -> &testtwo -> one.&&|test|&one.测试前的双 && 变成了 &，测试仍然存在，而 &one 解析为 two.剩下的 &testtwo 用于解析为 one 的第二遍.
• &&&test&one -> &onetwo -> 无法解析.&&|&test|&one -> &|one|two -> DNR.
• &&&&test&one -> &&testtwo -> &testtwo -> 一.&&|&&|test|&one -> &&|testtwo -> &testtwo -> 一.两对都解析为一对，形成一对，然后再解析为一对，剩下 &testtwo 来解析.
• &&&&&test&one 类似于三个与号的情况，但多了一对.
• &&&&&&test&one 解析为 &&&testtwo 解析为 &one 解析为两个.&&|&&|&&|test|&one -> &&|&testtwo -> &一 -> 二.奇数对意味着我们又得到了一组解析.

• &&test&one -> &testtwo -> one. &&|test|&one. The double && before test becomes &, test remains, and &one resolves to two. That leaves &testtwo for the second pass which resolves to one.
• &&&test&one -> &onetwo -> does not resolve. &&|&test|&one -> &|one|two -> DNR.
• &&&&test&one -> &&testtwo -> &testtwo -> one. &&|&&|test|&one -> &&|testtwo -> &testtwo -> one. Two pairs each resolve down to one, making one pair, which then resolves to one, which leaves &testtwo to resolve.
• &&&&&test&one is similar to three ampersand case, but with one extra pair.
• &&&&&&test&one resolves to &&&testtwo resolves to &one resolves to two. &&|&&|&&|test|&one -> &&|&testtwo -> &one -> two. The odd number of pairs means we get one more set of resolves.

归根结底，你需要记住的是:

At the end of the day, what you need to remember:

• 1 & 符号解析宏变量一次，就是这样.
• 2 & 符号对于复合宏变量很有用，即前缀加上宏驱动的后缀 (&&prefix&suffix).
• 3 &符号对于解析单个宏变量(&&&var -> &var2 -> var3).
• 6 & 符号对于解析两层深复合宏变量(即组合 2 和 3)很有用 ([&prefix=var, &suffix=2>] &&&&&&prefix&suffix -> &&&var2 -> &var3code> -> 4).

• 1 ampersand resolves the macro variable once and that's it.
• 2 ampersands is useful for composite macro variables, ie, a prefix plus a macro-driven suffix (&&prefix&suffix).
• 3 ampersands is useful for going two deep in resolving a single macro variable (&&&var -> &var2 -> var3).
• 6 ampersands is useful for resolving a two-deep composite macro variable (ie, combining 2 and 3) ([&prefix=var, &suffix=2] &&&&&&prefix&suffix -> &&&var2 -> &var3 -> 4).

除此之外，4 个或更多(6 个除外)仅对特别复杂的组合有用；额外的级别将用于将解析延迟到特定时间.

Beyond that, 4 or more (other than 6) are useful only for particularly complex combinations; the extra levels would be used to delay resolution until particular times.

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