我丢失了结构（c）中的值

Question

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

#define stock_dir "/Users/myname/prices/"
#define file_list "/Users/myname/trade/trade/nasdaq100_stock_list.txt"
#define look_back_period 3
#define num_stocks 103
#define days_of_data 21

int main()
{
    FILE *stocks, *stk;
    char stock[11], fullpath[50] = "\0", header[25];
    char line_of_data[40];
    char *sclose, *svol;
    int n = 0, i = 0;
    typedef struct daily_data {
        char  *date;
        float close;
        int vol;
    }data;
    
    sclose = (char*) malloc(20*sizeof(char));
    svol =  (char*) malloc(20*sizeof(char));
    
    data** day_data = (data**) malloc(num_stocks*sizeof(data*)  );
   
    if (day_data == NULL)
        {
        printf("day_data not allocated\n");
        exit(0);
        }

    for(i = 0; i < num_stocks; i++)
        if ((day_data[i] = (data*)malloc(days_of_data*sizeof(data))) == NULL)
        {
        printf("data[%d] not allocated\n", i);
        exit(0);
        }

    for(i = 0; i < num_stocks; i++)
      for(n = 0; n < days_of_data; n++)
       
          if ((day_data[i][n].date = (char*)malloc(20)) == NULL)
          {  printf("data[%d][%d] not allocated\n", i,n);
             exit(0);
          }

    /* ... code omitted ... */

    if ( (stocks = fopen(file_list, "r") )== NULL)
        printf("didn't open file list\n");
    i = 0;
 
    while (fgets(stock, sizeof(stock), stocks) != NULL)
    {
       printf("%s",stock);
       strcpy(fullpath,stock_dir);
        strcat(fullpath,stock);
        fullpath[strcspn(fullpath, "\n")] = 0;
        if ( (stk = fopen(fullpath, "r") )== NULL)
               printf("didn't open quote list\n");
        fgets(header,sizeof(header),stk);
        n=0;
        while(fgets(line_of_data, sizeof(line_of_data),stk) !=NULL)
            {
                fgets(line_of_data,sizeof(line_of_data),stk);
                day_data[i][n].date = strtok(line_of_data, ",");
                sclose = strtok(NULL,",");
                day_data[i][n].close = atof(sclose);
                svol = strtok(NULL, ",");
                day_data[i][n].vol = atoi(svol);;
                printf("%s %f %d\n",day_data[i][n].date,day_data[i][n].close,day_data[i][n].vol);
              n++;
            }
    
        fclose(stk);
        i++;
    }
             
   for (n = look_back_period - 1; n  < (days_of_data  - look_back_period); n++)
      printf("%d %s  %f %d\n",n, day_data[1][n].date, day_data[1][n].close, day_data[1][n].vol);
    
}

The print statement in the while(fgets(line_of_data, sizeof(line_of_data),stk) !=NULL) loop shows that everything went into the right place. But when I print values outside they're mostly wrong. I'm supposed to add more details but I don't know what else to say. I lose the values in the struct when I leave the loop.

Answer 1

You overwrite the same data again and again.

Take a look at your structure:

    typedef struct daily_data {
        char  *date;  ///< a pointer without own storage
        float close;
        int vol;
    }data;

while processing your file you read each line into line_of_data

while(fgets(line_of_data, sizeof(line_of_data),stk) !=NULL)

you tokenize the line_data and assign the pointer to data->date

day_data[i][n].date = strtok(line_of_data, ",");

What tokenize ( strtok reference ) does is inserting terminators into your input string and returning the pointer to the start of the new part of your input. So no new memory is allocated at this point. the returned pointer points into your input string.

So effectively you assigning the local variable pointer to your data storage structure.

Additionally to this you lose the pointer to your initially allocated memory for the date pointer.

I would suggest you to remove the a priory allocation of date and allocate the required memory at the point you really know the required length or if you are sure, you know the maximum length, then you can just make the date member an array.

So you either have to allocate new memory and copy the tokenized data or if you made date a fixed size array, just copy the tokenized data.

on the first variant it would look like this

  char * tok = strtok(line_of_data, ",");
  day_data[i][n].date = malloc(strlen(tok)+1);
  strcpy(day_data[i][n].date, tok); 

(+ remove the pre allocation of the date member)

or the second variant:

change data to

    typedef struct daily_data {
        char  date[20];
        float close;
        int vol;
    }data;

and the processing code looks like this:

  char * tok = strtok(line_of_data, ",");
  strcpy(day_data[i][n].date, tok); 

(+ (of course) remove the pre allocation of the date member)

You also should in any case add error handling if the tokenized string exceeds the max length or the format of the lines does not match the expectation (missing delimiters, wrong/invalid number(formats), ...).