在C中使用字符串数组数组来解析文本文件

Question

I would like to read from N text files (having similar structure: a few lines, each line having the same small number of words) and store in a string matrix the words read, in such a way that in each (row, col) position I have one word.

A simple (two lines, three words per line) specimen for the files is the following:

line1word1 line1word2 line1word3
line2word1 line2word2 line2word3

Delimiter for the words is space.

I have attempted this code:

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

#define MAX_STRING_LENGTH 1000
#define MAX_TOKS 100
#define DELIMITERS " "

// line parsing utility
int parseString(char* line, char*** argv) {

  char* buffer;
  int argc;

  buffer = (char*) malloc(strlen(line) * sizeof(char));
  strcpy(buffer,line);
  (*argv) = (char**) malloc(MAX_TOKS * sizeof(char**));

  argc = 0;  
  (*argv)[argc++] = strtok(buffer, DELIMITERS);
  while ((((*argv)[argc] = strtok(NULL, DELIMITERS)) != NULL) &&
     (argc < MAX_TOKS)) ++argc;
  return argc; 
}


int main() {

  char S[MAX_STRING_LENGTH];
  char **A;

  int  n,i,j,l;

  FILE *f;
  char file[50];

  char ***matrix;
  matrix = malloc(MAX_TOKS * sizeof(char**));

 //memory allocation for matrix
 for (i = 0; i < MAX_TOKS; i++)
     {
       matrix[i] = malloc(MAX_TOKS * sizeof(char *));
       for (j = 0; j < MAX_TOKS; j++)
           {
           matrix[i][j] = malloc(MAX_TOKS * sizeof(char));
           }
     }

  int NFILE = 10; // number of files to be read

  for(i=0;i<NFILE;i++) 
    {  
    sprintf(file,"file%d.txt",i); 
    f = fopen(file,"r");

    l=0; // line-in-file index
    while(fgets(S,sizeof(S),f)!=NULL) {
          n = parseString(S,&A);
          for(j=0;j<n;j++) {
            matrix[i][l]=A[j];
            printf("%s\t%s\n",matrix[i][l],A[j]); 
            } 
        l++;
        } 
 fclose(f); 
    }

free(matrix);
free(A);    
return(0);  
}

The problem I can't solve is that there when checking for correspondance between the arrays (in order to be sure I am storing the single words correctly) using

printf("%s\t%s\n",matrix[i][l],A[j]);

I find that the last word (and only the last one) of each line, regardless of the file number, is not stored in matrix . That is to say, line1word1 and line1words of file0 are correctly stored in matrix[0][0][0] and matrix[0][0][1] , but in the field matrix[0][0][2] there isn't line1word3 , even if A[2] has it!

What am I doing wront? Any suggestion?

Many thanks in advance, cheers

Answer 1

char ***matrix doesn't declare a three dimensional array. Your matrix would need to be something like char *matrix[a][b] to hold a two dimensional array of string pointers. In order to calculate addresses within an array, the compiler needs to know the all of dimensions but one. If you think about it, you will probably see why...

If you have two arrays:

1 2 3        1  2  3  4  5  6  7
4 5 6        8  9 10 11 12 13 14
7 8 9       15 16 17 18 19 20 21

You can see that item[1][1] is NOT the same item. Regardless of the dimensions in your array, the elements are typically arranged sequentially in memory, with each row following the previous (or possible column, depending on language, I suppose.) If you have an array of pointers, the actual content may be elsewhere, but the points would be arranged like this. So, in my examples above, you must provide the compiler with the number of columns so that it can find members (the number of rows can be variable.) In a three dimensional array, you must provide the first TWO dimensions so that the compiler may calculate item offsets.

I hope that helps.

EDIT: You can have truly dynamic array dimensions by creating your own function to process all array item accesses. The function would need to know the dynamic dimensions and the item index(s) so that it could calculate the appropriate address.

Answer 2

This looks wrong: buffer = (char*) malloc(strlen(line) * sizeof(char));

Firstly, there is no need to cast malloc in C. If your code doesn't compile without the cast, there are two possible reasons:

1. There is no prototype for malloc. Obviously this can cause problems, because no prototype means the function returns a default type: int , or an error occurs. This can cause your program to misbehave. To avoid this, #include <stdlib.h> .
2. You're using a C++ compiler. Stop. Either program in C++ (stop using malloc) or use a C compiler. If you want to use this project in a C++ project, compile your C code with a C compiler and link to it in your C++ compiler.

Secondly, sizeof(char) is always 1. There is no need to multiply by it.

Thirdly, a string is a sequence of characters ending at the first '\\0'. This means a string always occupies at least 1 character, even if it is an empty string. What does strlen("") return? What is sizeof("") ? You need to add 1 to make room for the '\\0': buffer = malloc(strlen(line) + 1); .

This looks slightly wrong: (*argv) = (char**) malloc(MAX_TOKS * sizeof(char**));

malloc returns a pointer to an object. *argv is a char ** , which means it points to a char * . However, in this case malloc returns a pointer to char ** objects. The representation isn't required to be identical. To avoid portability issues assosciated with this, follow this pattern variable = malloc(n * sizeof *variable); ... in this case, *argv = malloc(MAX_TOKS * **argv);

It gets more gritty as it goes. Forget everything you think you know about your code; Pretend you're going to come back to this in 24 months. What are you going to think of this?

argc = 0;  
(*argv)[argc++] = strtok(buffer, DELIMITERS);
while ((((*argv)[argc] = strtok(NULL, DELIMITERS)) != NULL) &&
   (argc < MAX_TOKS)) ++argc;

There's actually an off-by-one here, too. Assuming argc == MAX_TOKS , your loop would attempt to assign to (*argv)[MAX_TOKS] . This loop is where I believe your problem lies, and the solution is to express your intent more clearly rather than attempting to cram as much code into one line as possible. How would you rewrite this? Here's what I'd do, in this situation:

char *arg;
size_t argc = 0;
do {
    arg = strtok(buffer, DELIMITERS);
    buffer = NULL;

    (*argv)[argc] = arg;
    argc++;
} while (argc < MAX_TOKS && arg != NULL);

The problem is that your parsing loop doesn't increment when strtok returns NULL. Hence, your function returns the position of the last item. Supposing you had two tokens, your parsing function would return 1. Your display loop displays items up to, but not including this position: for(j=0;j<n;j++) . You could use the suggested improvement, or change your loop: for (j = 0; j <= n; j++) . Either way, you'll need to fix those off-by-ones.

Out of curiosity, which book are you reading?