文件在球拍中的位置
我想知道 Racket 中是否有某些东西可以操纵输入端口中的文件位置。 我的情况是，我需要使文件指针之类的东西返回到一个位置，以防我读到我不应该读的内容。
例如 “我很困惑。”在文件 test.txt 中。 那么文件指针就在混淆之前。 （我猜） 那么我可以做些什么，让下次读取字符串时，我得到“am”，而不是“.”？
I am wondering whether there is something in Racket to manipulate the file position in an input port.
My case is that I need to make the something like file pointer goes back to one position in case I read stuff that I should not have read.
e.g.
"I am confused." in the file test.txt.
then the file pointer is after confused before . (I guess)
So is there something I can do in the way that next time I read-string, I get "am", rather than "."??
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考虑
peek-string。它与 read-string 相同，只是返回的字符保留在端口中以供将来读取。Consider
peek-stringfor example. It's identical toread-string, except that the returned characters are preserved in the port for future reads.为了完整起见，有一个
file-position函数，可以检索当前位置或更改它。但您通常应该避免它，因为它使代码仅在您可以实际更改位置的端口上工作 - 但情况并非总是如此。正如 Yasir 指出的那样，对于所有阅读功能，通常都有一个“窥视”版本，可以在不移动位置的情况下读取内容，这是一个更好的解决方案。For the sake of completion, there is a
file-positionfunction that can retrieve the current position or change it. But you should generally avoid it, since it makes the code work only on ports where you can actually change a position -- and that's not always true. As Yasir pointed out, for all of the reading functions there is usually a "peek" version that reads something without moving the position, and that's a better solution.