从.txt文件读取随机行 [英] Read random line from .txt file

问题描述

我正在尝试通过读取.txt文件中的随机单词来升级我的Hangman游戏.问题是，我不知道如何从.txt文件中读取随机行. .txt文件的每一行都包含一个单词.

I'm trying to upgrade my Hangman game by reading random words from a .txt file. Thing is, I can't figure out how to read a random line from the .txt file. There are single words on every new line of the .txt file.

void ler_palavras()
{
    FILE *words;

    if ((words = fopen("words.txt", "r")) == NULL) {
        printf("Error! opening file");
        exit(1);
    }

    // reads text until newline
    fscanf(words,"%[^\n]", word);
    fclose(words);
}

推荐答案

如果由于某种原因，您不能只将整个行集加载到内存中(太大或其他大小)，则可以选择一种方法流条目集合中的随机条目.它不会无限扩展，并且会表现出较小的偏差，但这是一个游戏，而不是加密技术，因此不应该成为破坏交易的事情.

If, for some reason, you can't just load the whole set of lines into memory (too big or whatever), there is a way to select a random entry from a streaming set of entries. It won't scale indefinitely, and it will exhibit small biases, but this is a game, not cryptography, so that shouldn't be a dealbreaker.

逻辑是:

1. 声明一个缓冲区来容纳单词
2. 打开文件
3. 对于每行:
   • 增加一个计数器，指示您所在的行
   • 生成随机的double(例如，使用 drand48 或其他任何PRNG工具可供您使用)
   • 如果是1.0 / lineno > randval，请用当前行中的单词替换当前存储的单词(因此第一行是自动存储的，第二行替换的可能性为50％，第三行替换的可能性为33％，等)

1. Declare a buffer to hold the word
2. Open the file
3. For each line:
   • Increment a counter indicating which line you're on
   • Generate a random double (e.g. with drand48 or whatever PRNG facilities are available to you)
   • If 1.0 / lineno > randval, replace the currently stored word with the word from the current line (so the first line is auto stored, the second line is 50% likely to replace it, the third is 33% likely to do so, etc.)

假设行数足够小(并且PRNG产生的double的范围足够细)，这将使选择给定行的可能性尽可能接近；两行，每行有50/50的命中率，三行是33.33 ...％，依此类推.

Assuming the number of lines is small enough (and the range of doubles produced by your PRNG is fine-grained enough), this gives as close as possible to an equal likelihood of any given line being selected; for two lines, each has a 50/50 shot, for three, 33.33...%, etc.

我现在缺少C编译器，但是基本代码如下:

I lack a C compiler right now, but the basic code would look like:

/* Returns a random line (w/o newline) from the file provided */
char* choose_random_word(const char *filename) {
    FILE *f;
    size_t lineno = 0;
    size_t selectlen;
    char selected[256]; /* Arbitrary, make it whatever size makes sense */
    char current[256];
    selected[0] = '\0'; /* Don't crash if file is empty */

    f = fopen(filename, "r"); /* Add your own error checking */
    while (fgets(current, sizeof(current), f)) {
        if (drand48() < 1.0 / ++lineno) {
            strcpy(selected, current);
        }
    }
    fclose(f);
    selectlen = strlen(selected);
    if (selectlen > 0 && selected[selectlen-1] == '\n') {
        selected[selectlen-1] = '\0';
    }
    return strdup(selected);
}

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