c - 骨架代码 : character pointers struggles

Question

这是读取此 csv 文件的骨架代码的一部分:

year,month,day,location,mintemp,maxtemp
2015,4,28,18,6.7,12.9
2015,4,28,22,12.7,19.1
2015,4,29,18,7.6,15.3
2015,4,29,22,13.4,21.9
2015,4,30,18,7.3,21.8
2015,4,30,22,13.2,23.2
2015,5,1,18,9.4,15.9
2015,5,1,22,16.1,27.2
2015,5,2,18,8.7,16.3
2015,5,2,22,14.2,21.4

此函数将文件作为字符串读取，并以某种方式将其分开，并将其放入名为“D”、类型为 csv_t 的结构中。

结构及其他:

typedef char input_line_t[LINELEN+1]; 

typedef struct {
    input_line_t labelstring;
    char *labs[MAXCOLS+1];
    int nrows;
    int ncols;
    double vals[MAXROWS][MAXCOLS];
} csv_t;

功能:

void
read_csv_file(char *fname, csv_t *D) {
    FILE *fp;   /* used to read from a named file */
    input_line_t line;
    int cols=0, rows=0, bytes=0;
    int c, i, j, chr, ncommas, empties=0;
    double x;
    double nan = 0.0/0.0;

    /* first argument on commandline should the data file name */
    if (fname==NULL) {
        /* and it wasn't there... */
        printf("No csv file specified on commandline\n");
        exit(EXIT_FAILURE);
    }

    /* try and open the named file for reading */
    if ((fp=fopen(fname,"r")) == NULL) {
        printf("Error: unable to open %s\n", fname);
        exit(EXIT_FAILURE);
    }

    /* file is open, can now use fp to access CSV data,
       start by reading the bytes of the header row */
    while ((c=getc(fp)) != '\n') {
        D->labelstring[bytes++] = c;
    }
    D->labelstring[bytes] = '\0';

    /* now process line again, breaking in to separate labels by
       replacing commas by nulls, and tracking the start of each of
       the column headings */
    D->labs[cols++] = D->labelstring;
    for (i=1; i<bytes; i++) {
        if (D->labelstring[i]==COMMA) {
            D->labelstring[i] = '\0';
            D->labs[cols++] = D->labelstring+i+1;
        }
        if (cols>MAXCOLS && i<bytes) {
            printf("Too many columns, limit is %d\n",
                MAXCOLS);
            exit(EXIT_FAILURE);
        }
    }
    D->labs[cols] = NULL;

    /* ok, that's the labels sorted, now for the data */
    while ((chr=getc(fp)) != EOF) {

        /* there is another row, because first character of it
           just got read, next step is to get the rest of them */
        i = 0;
        line[i++] = chr;
        ncommas = (chr==COMMA) ;
        while (((chr=getc(fp))!=EOF) && (chr!='\n')) {
            line[i++] = chr;
            ncommas += (chr==COMMA) ;
        }
        line[i] = '\0';
        if (ncommas!=cols-1) {
            printf("Data input error line %d\n", rows+2);
            exit(EXIT_FAILURE);
        }
        /* then process the line from the right end */
        j = i-1;
        for (c=cols-1; c>=0; c--) {
            /* look for next previous comma */
            while (j>=0 && line[j]!=COMMA) {
                j--;
            }
            /* access the value */
            if (sscanf(line+j+1, "%lf", &x) == 1) {
                D->vals[rows][c] = x;
            } else {
                D->vals[rows][c] = nan;
                empties++;
            }
            /* mark the new end of the string */
            line[j--] = '\0';
        }
        rows++;
        /* check to make sure don't overflow array */
        if (rows==MAXROWS) {
            /* time to stop reading data */
            printf("Too many rows, truncated at %d\n", MAXROWS);
            break;
        }
        /* if not full, go round and see if there is another data row */
    }

    /* either input has all been read or array is full */
    printf("file %s:\n    %d columns and %d rows of data\n",
            fname, cols, rows);
    if (empties) {
        printf("    %d entries were empty or non-numeric\n",
            empties);
    }
    /* finish building the structure */
    D->nrows = rows;
    D->ncols = cols;
    return;
}

我基本上了解发生了什么，除了从这一点开始，特别是这一点:

D->labs[cols++] = D->labelstring;
        for (i=1; i<bytes; i++) {
            if (D->labelstring[i]==COMMA/*=','*/) {
                D->labelstring[i] = '\0';
                D->labs[cols++] = D->labelstring+i+1;
            }

最后一段代码非常困惑。 labelstring 是一个字符串，但可以添加数字。它有什么作用？这如何给出列数。如果我明白了这一点，我就能理解整个代码。感谢帮助。非常感谢!!

Answer 1

其实很简单。假设您有 labelstring 包含字符串 "Foo"，那么它在内存中看起来像这样

+-----+-----+-----+------+| 'F' | 'o' | 'o' | '\0' |+-----+-----+-----+------+^|+-------------+| labelstring |+-------------+

换句话说，labelstring 指向字符串中的第一个字符。

如果您执行labelstring + 1，您将得到*第二个字符:

+-----+-----+-----+------+| 'F' | 'o' | 'o' | '\0' |+-----+-----+-----+------+      ^      |      +-----------------+      | labelstring + 1 |      +-----------------+

这称为指针算术。

是的，您也可以在数组上使用它，因为数组会衰减为指向其第一个元素的指针。

了解了上述内容后，也很容易理解 array[X] (对于任何数组或指针 array 以及对于任何 X ) 与 *(array + X) 相同。

真正让您大吃一惊的是，感谢 commutative property的addition ，最后一位表示 *(array + X) 等于 *(X + array) 这导致 array[X] 相等到X[数组]。