如何使用 fscanf 跳过文件中的内容? [英] How to skip stuff in a file using fscanf?

问题描述

我正在学习 C 文件操作，我有一个特定的问题，我需要将字母(A-Z、a-z)存储为从文件中读取的字符串.所以，如果我有 abcd&*dvcd 那么 abcd 是一个字符串而 dvcd 是一个字符串.

I am learning C file operations and I have a specific problem where I need to store alphabets (A-Z, a-z) as a string read from a file. So, if I have abcd&*dvcd then abcd is a string and dvcd is a string.

基本上我有两个问题:

1. 我事先不知道要存储的字符串的大小.
2. 我需要跳过非字母字符

我想我可以通过使用 fscanf 格式字符串来解决这两个问题.这是我的想法.我需要以某种方式跳过使用 fscanf 格式字符串来查找我在文件中的位置.然后，我可以从保存的开始位置减去以找到我需要 malloc 的字符串.然后，我malloc指定长度的字符串，然后，我可以回到保存的位置，实际读取它.

I thought I would solve both by using fscanf format string. This is my thought. I need to somehow skip using fscanf format string to find my position in file. Then, I could subtract from the saved beginning position to find the string I need to malloc. Then, I malloc the string of specific length, and then, I could go back to the saved position and actually read it.

但是，我不知道如何告诉 fscanf 不存储扫描的字符串?

However, I have no idea how to tell fscanf to not store the scanned string?

或者是否有另一种方法可以跳过非字母字符?

Or if there is another method that allows to skip non-alphabetic characters?

推荐答案

因为@suresh 的回答是对我不起作用(输出应该是 asd gddf 而不是 asd asd)，我写了另一个版本，它不是那么短，但对我来说很好用:

Since @suresh' answer is not working for me (the output should be asd gddf and not asd asd), I have written another version, which is not as short but works fine for me:

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

int main(void) {
    char ch;
    while(scanf("%1c", &ch) == 1) { //while we have characters to read
        if(isalpha(ch)) { // if the next one is alphanumeric
            ungetc(ch, stdin); // we put it back
            char str[101];
            scanf("%100[a-zA-Z]", str); // to read the whole string (including that char)
            printf("found string %s\n", str);
        }
    }
    return 0;
}

这里是一个有效的 ideone 示例.我为此使用标准输入，但您可以轻松地调整它以使用您打开的另一个文件.我用于 scanf (%100[a-zA-Z]]) 的格式说明符意味着它最多应该读取 100 个字符(str 的大小)只包含小写或大写字母.%1c 表示单个字符，unget 将该字符放回缓冲区以避免在它不是符号时跳过它.

Here is a working ideone example. I use stdin for this one, but you can easily adapt it to use another file you have open. The format specifier I used for scanf (%100[a-zA-Z]]) means that it should read at most 100 characters (the size of str) containing only lowercase or uppercase letters. %1c means a single character, and unget puts the character back into the buffer to avoid skipping it if it's not a symbol.

正如@m-m 指出的那样，方括号内的 - 的使用不是标准的，在某些实现中可能不可用.如果它不适合您，您可以随时使用 %100[abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ].

as @m-m pointed out, the use of - inside the square brackets is not standard and may not be available in some implementations. If it is not available for you, you can always use %100[abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ].

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