CODEWITHSUNDEEP
cfilescanf
Drake Owen
Drake Owen

Reputation: 87

Fscanf Reading From Middle of File C

I am creating a program that reads numbers from a file, gets the amount of numbers, creates an array and displays those numbers along with the amount of numbers in the file. I am using fscanf to get the the data from the file. I finally got the program to compile and output but it only reads 4 out of the 10 numbers in the files. If I increase the amount of numbers it still only reads 4 number. If I decrease the amount of numbers to below 4 its still tries to read 4 numbers and the prints random numbers from memory. I think that its a pointer issue but I'm still new and don't fully understand pointers. Help is appreciated.

Code:

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

int ReadSize (FILE*);
void readData(FILE* fp, int size, int* arr1);
int* allocateMemory(int);

int main (int argc, char* argv[])
{
    FILE * in = fopen(argv[1], "r"); //Gets file to read
    int size = ReadSize(in);

    printf("Size: %i\n", size); //Temp Line to Print Size

    int *arr1 = allocateMemory(size);
    readData(in, size, arr1);

    printf("Data: ");

    for (int i = 0; i < size; i++)
    {
        printf("%d, ", arr1[i]);
    }
}

int ReadSize(FILE* fp)
{
    int size = 0;
    fscanf(fp, "%d", &size);
    return size;
}

void readData(FILE* fp, int size, int* arr1)
{
  int i;

  for(i = 0; i < size; i++)
  {
      fscanf(fp, "%d", &arr1[i]);
  }
}

int* allocateMemory(int sz)
{
  int *temp = (int*)malloc(sizeof(int)*sz);
  return temp;
}

Input File:

4 2
-1 2 
1 2
2 -1
-1 -2

Upvotes: 0

Views: 231

Answers (2)

paulsm4
paulsm4

Reputation: 121649

I think you're misinterpreting the file format. You read "4" from the file, then your loop iterates 4 times: hence 4 numbers, not 10.

But we can only guess what you SHOULD read.

Perhaps something like this:

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

int main (int argc, char* argv[])
{
    FILE *in;
    if ((fp = fopen(argv[1], "r")) == 0) {
      printf("Error: unable to open file!\n");
      return -1;
    }

    int nitems, items_per_line;
    if (fscanf(fp, "%d %d", &nitems, &items_per_line) != 2) {
       printf ("Error reading dimensions!\n");
       return -1;
    }

    int *arr1 = malloc (sizeof(int) * nitems * items_per_line);
    if (arr1 == 0) {
      printf("Error: unable to malloc array!\n");
       return 1;
    }
    for (int i = 0; i < nitems * items_per_line; i++) {
       fscanf(fp, "%d", &arr1[i]);
    }

    return 0;
}

Or perhaps you'd be better served with an n-dimensional array (e.g. arr1[2][4])?

Again - without knowing more about the format, we can only guess...

PS: You should ALWAYS check for errors in your code!

Upvotes: 2

Carl Norum
Carl Norum

Reputation: 224864

The first thing your program does is try to read a decimal number from the file in ReadSize(). That gets 4, which then, as you describe, reads 4 numbers. You need to put a 10 at the top of your input file if you want it to match the format that program expects.

Upvotes: 1

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