from input file to array using malloc and realloc

Question

I am trying to read input from a file and put each string in an array using malloc and realloc. So if the input file is :

alex
john
jane
smith

Answer 1

You can try the following or just study how it's done. It works fine on my linux machine. Let me know if you have any questions.

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

const int INITIAL_MAX_LINES = 2;
const int MAX_LINES_INC = 2;

const int INITIAL_MAX_LINE_LENGTH = 2;
const int MAX_LINE_LENGTH_INC = 2;

int main (int argc, char *argv[])
{
  int nlines = 0, i;
  FILE *fp = fopen(argv[1], "r");
  char **inputFile, *buffer;
  int max_lines, c, buflen, bufpos, end_of_line;

  if (argc < 2) {
    printf("No enough arguments.\n");
    return -1;
  }

  max_lines = INITIAL_MAX_LINES;

  inputFile = (char **) malloc(max_lines * sizeof(char*));
  if (fp==0) {
    fprintf(stderr, "Cannot open file!\n");
    return -1;
  }
  else{
    /* Start with a buffer. */
    bufpos = 0;
    buflen = INITIAL_MAX_LINE_LENGTH;
    buffer = (char *) malloc(buflen * sizeof(char *));

    c = 0;
    while (c != EOF) {

      end_of_line = 0;

      c = fgetc(fp);

      if (c == EOF || c == '\n' || c == '\r') {
        end_of_line = 1;
       /* Discard this character. */
      }
      else {
        /* Put this character in the buffer. */
        /* But check if we have enough memory first! */
        /* Leave room for the null character at the end. */
        if (bufpos >= buflen - 1) {
          buflen += MAX_LINE_LENGTH_INC;
          buffer = (char *) realloc(buffer, buflen * sizeof(char));
        }
        buffer[bufpos] = c;
        bufpos++;
      }

      if (end_of_line) {
        /* Remember this line and get a new buffer. */
        /* Check if we need more memory. */
        if (nlines >= max_lines) {
          max_lines += MAX_LINES_INC;
          inputFile = (char **) realloc(inputFile, max_lines * sizeof(char*));
        }

        /* Null terminate the buffer.*/
        buffer[bufpos++] = 0;

        inputFile[nlines] = buffer;
        nlines++;

        bufpos = 0;
        buflen = INITIAL_MAX_LINE_LENGTH;
        buffer = (char *) malloc(buflen * sizeof(char *));
      }
    }
  }

  printf("%d lines\n", nlines);
  for (i=0; i<nlines; i++) {
    printf("%s\n", inputFile[i]);
  }
}

Answer 2

you would want to allocate an array of pointers first with an initial size say 10:

int size = 10;
char **inputFile= malloc(sizeof*inputFile*size);

Then for each word you read you allocate more memory for it and insert it into the array:

char line[100];
fscanf(file, "%s", line);
inputFile[index++] = strdup(line);

Now check if you need more words then you realloc the array:

if (index==size) {
   size += 10;
   inputFile = realloc(inputFile, sizeof*inputFile*size);
}

So you end up with something like this:

[0]->"alex"
[1]->"john"
[2]->"jane"
[3]->"smith"

When you're done you need to loop over the array and free each string and then free the array, this part is left as an exercise :)