Can the C preprocessor perform simple string manipulation?

This 开发者_开发问答is C macro weirdness question.

Is it possible to write a macro that takes string constant X ("...") as argument and evaluates to sting Y of same length such that each character of Y is [constant] arithmetic expression of corresponding character of X.

This is not possible, right ?

No, the C preprocessor considers string literals to be a single token and therefore it cannot perform any such manipulation.

What you are asking for should be done in actual C code. If you are worried about runtime performance and wish to delegate this fixed task at compile time, modern optimising compilers should successfully deal with code like this - they can unroll any loops and pre-compute any fixed expressions, while taking code size and CPU cache use patterns into account, which the preprocessor has no idea about.

On the other hand, you may want your code to include such a modified string literal, but do not want or need the original - e.g. you want to have obfuscated text that your program will decode and you do not want to have the original strings in your executable. In that case, you can use some build-system scripting to do that by, for example, using another C program to produce the modified strings and defining them as macros in the C compiler command line for your actual program.

As already said by others, the preprocessor sees entire strings as tokens. There is only one exception the _Pragma operator, that takes a string as argument and tokenizes its contents to pass it to a #pragma directive.

So unless your targeting a _Pragma the only way to do things in the preprocessing phases is to have them written as token sequences, manipulate them and to stringify them at the end.