C 解析带换行符的逗号分隔值

Question

我有一个 CSV 数据文件，其中包含以下数据:

H1,H2,H3
a,"b
c
d",e

当我通过 Excel 打开 CSV 文件时，它能够显示列标题为 H1、H2、H3 的工作表，列值为:a for H1 ,

multi line value as
b
c
d
for H2 

和 c 用于 H3 我需要使用 C 程序解析该文件并像这样选取值。 但是，我的以下代码片段将不起作用，因为我的列有多行值:

char buff[200];
char tokens[10][30];
fgets(buff, 200, stdin);
char *ptok = buff; // for iterating
char *pch; 
int i = 0;
while ((pch = strchr(ptok, ',')) != NULL) {
  *pch = 0; 
  strcpy(tokens[i++], ptok);
  ptok = pch+1;
}
strcpy(tokens[i++], ptok);

如何修改此代码片段以适应列的多行值？ 请不要被字符串缓冲区的硬编码值所困扰，这是 POC 测试代码。 我不想使用任何第三方库，而是想从第一原则出发以困难的方式来做这件事。 请帮忙。

Answer 1

在 C 中解析“格式良好”的 CSV 的主要复杂性恰恰是对可变长度字符串和数组的处理，而使用固定长度字符串和数组则可以避免这种情况。 (另一个复杂问题是处理格式不正确的 CSV。)

如果没有这些复杂性，解析实际上非常简单:

(未经测试)

/* Appends a non-quoted field to s and returns the delimiter */
int readSimpleField(struct String* s) {
  for (;;) {
    int ch = getc();
    if (ch == ',' || ch == '\n' || ch == EOF) return ch;
    stringAppend(s, ch);
  }
}

/* Appends a quoted field to s and returns the delimiter.
 * Assumes the open quote has already been read.
 * If the field is not terminated, returns ERROR, which
 * should be a value different from any character or EOF.
 * The delimiter returned is the character after the closing quote
 * (or EOF), which may not be a valid delimiter. Caller should check.
 */
int readQuotedField(struct String* s) {
  for (;;) {
    int ch;
    for (;;) {
      ch = getc();
      if (ch == EOF) return ERROR;
      if (ch == '"') {
        ch = getc();
        if (ch != '"') break;
      }
      stringAppend(s, ch);
    }
  }
}

/* Reads a single field into s and returns the following delimiter,
 * which might be invalid.
 */
int readField(struct String* s) {
  stringClear(s);
  int ch = getc();
  if (ch == '"') return readQuotedField(s);
  if (ch == '\n' || ch == EOF) return ch;
  stringAppend(s, ch);
  return readSimpleField(s);
}

/* Reads a single row into row and returns the following delimiter,
 * which might be invalid.
 */
int readRow(struct Row* row) {
  struct String field = {0};
  rowClear(row);
  /* Make sure there is at least one field */
  int ch = getc();
  if (ch != '\n' && ch != EOF) {
    ungetc(ch, stdin);
    do {
      ch = readField(s);
      rowAppend(row, s);
    } while (ch == ',');
  }
  return ch;
}

/* Reads an entire CSV file into table.
 * Returns true if the parse was successful.
 * If an error is encountered, returns false. If the end-of-file
 * indicator is set, the error was an unterminated quoted field; 
 * otherwise, the next character read will be the one which
 * triggered the error.
 */
bool readCSV(struct Table* table) {
  tableClear(table);
  struct Row row = {0};
  /* Make sure there is at least one row */
  int ch = getc();
  if (ch != EOF) {
    ungetc(ch, stdin);
    do {
      ch = readRow(row);
      tableAppend(table, row);
    } while (ch == '\n');
  }
  return ch == EOF;
}

<小时/>

以上是“基本原则”——它甚至不使用标准 C 库字符串函数。但需要花一些功夫去理解和验证。就我个人而言，我会使用 (f)lex 甚至 yacc/bison (尽管这有点过分)来简化代码并使预期的语法更加明显。但在 C 中处理可变长度结构仍然需要是第一步。