模拟if语句

问题描述

我想做的是在任何谓词为真时在控制台上写一些东西.一个例子是:

write('hello') IF 成员('a',['a','b']).

解决方案

一个连词 a, b 为真(成功)如果 a 为真，并且 b 是真的.顺序很重要:

写hello"并检查x是否在[a,b]

?- write(hello), member(x, [a,b]).你好错误的.

检查x是否在[a,b]中，如果是就写hello"

?- member(x, [a,b]), write(hello).错误的.

检查a是否在[a,b]中，如果是，写hello"

?- member(a, [a,b]), write(hello).你好 % 按 Enter真的 .

if-then-else 结构 Condition ->然后 ;Else 略有不同，仅根据您的问题，我不知道您是否真的需要它.例如，为了避免这些示例中的额外选择点，您可能需要这样写:

?- memberchk(a, [a,b]), write(hello).你好真的.

当然，您实际上可能需要查看每个成员.例如:

只打印字符列表中的大写字母

?- member(X, [a, b, 'C', 'D', e, f, 'G']), char_type(X, upper).X = 'C' ;X = 'D' ;X = 'G'.

你应该仔细看看这个.没有if-then-else，也没有打印.我几乎可以说您正在尝试解决一个非问题.一般而言，您不应该经常需要 if-then-else 或使用 write 或 format 打印.

if-then-else 可以通过使用为您做的谓词来避免，如上所示的 member/2 与 memberchk/2.

打印由顶层完成.我很难想出 write 和 format 的有效用法，除非您需要写入文件/流.即便如此，先准备好最终结果，然后一次性将其写入文件也更清晰、更容易.

What I'm trying to do is to write something on the console if any predicate is true.An example would be:

write('hello') IF member('a',['a','b']).

解决方案

A conjunction a, b is true (succeeds) if a is true, and b is true. The order is significant:

?- write(hello), member(x, [a,b]).
hello
false.

?- member(x, [a,b]), write(hello).
false.

?- member(a, [a,b]), write(hello).
hello % press Enter
true .

The if-then-else construct Condition -> Then ; Else is something slightly different and based on your question alone, I don't know if you really need it. For example, to avoid the extra choice point in these examples, you might want to write:

?- memberchk(a, [a,b]), write(hello).
hello
true.

Of course, you might actually need to look at each member. For example:

?- member(X, [a, b, 'C', 'D', e, f, 'G']), char_type(X, upper).
X = 'C' ;
X = 'D' ;
X = 'G'.

You should look at this carefully. There is no if-then-else, nor printing. I'd almost say that you are trying to solve a non-problem. Very generally speaking, you shouldn't need if-then-else or printing with write or format too often.

The if-then-else can be avoided by using predicates that do it for you, as member/2 vs. memberchk/2 shown above.

The printing is done by the top level. I have trouble coming up with valid uses of write and format, unless you need to write to a file/stream. Even then, it is cleaner and easier to first get the final result ready, then write it to a file in one go.

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