How to read input character by character/number by number in C?

Question

I have a text file that might contain thousands and thousands of numbers(0-9 --> single digit)/characters, like: 13612371029301276312357829031029352131265309182765236728726355263789120938728...(goes on like this)

In C, how do I read them into an array such that each number gets stored separately? I mean after storing, array[0]=1 array[1]=3 array[2]=6... and so on [each number is read individually, this is not a big number but a collection of numbers, entered without any kind of spaces]

I think you get my point by now... How do I store them, if the numbers have no separators??

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Here is a rewording:

I have a file that has a very large number of digits in it: ~10^8 digits which do not have any seperators:

the file would look like this: 127389472397413417398410274812371972398748263718238421389410923409234109329413413413241341... and goes on and on

I would like to read the file sequentially - digit by digit. How do I do that in C??

Answer 1

Just read it as string. Strings in C are basically arrays of characters.

• More info about reading a text file in C
• More info about strings in C

Answer 2

I'm assuming you don't want the characters but the real values in which case I would do it like this:

1. Decide on how many numbers you need to read (if the file is all numbers it is just the size of the file).
2. Create a char array of that size.
3. read file content into char array.
4. use a for loop to adjust all values ot their numerical counterpart (i.e. do array[i] = array[i] - '0' in the loop)

Enjoy your new array with all numbers stored in an array as numerical values.

Answer 3

If you want to get the value of the the first number, you just do

int firstNumber = myString[0] - '0' ;

To get the 5th one you do

int number5 = myString[4] - '0' ;

Answer 4

To get one character at a time, see fgetc. To put a lot of digits together into a single huge integer, see e.g. GMP. What is it exactly, that you DO want to accomplish?!

Answer 5

Edit: output array of digits ((char)0 to char(9)).

#include <sys/stat.h>
#include <stdlib.h>
#include <stdio.h>

char* read_file_into_array(char* filename, size_t *array_size)
{
  char *array;
  struct stat st;
  FILE *f;
  size_t i;

  if (stat(filename, &st) != 0) {
    printf("Error reading %s\n", filename);
    return NULL;
  }
  array = malloc(st.st_size+1);
  if (!array) {
    printf("Error allocating memory\n");
    return NULL;
  }
  f = fopen(filename, "rb");
  if (!f) {
    printf("Error opening file\n");
    return NULL;
  }
  if (fread(array, 1, st.st_size, f) != st.st_size) {
    printf("Error reading file\n");
    return NULL;
  }
  fclose(f);
  /* Put numeric value into each field */
  for(i=0; i<st.st_size; i++)
    if (array[i] >= '0' && array[i] <= '9')
      array[i] = array[i]-'0';
    else /* end of digits */
      break;

  /* Provide size to caller */
  *array_size = i;

  return array;
}

Answer 6

EDIT: What do you mean, "what functions do I use for such large inputs?" The same ones you use for any inputs. Several answers have given you some very nice functions. fgetc() reads characters one-at-a-time from a filehandle - the common trick to convert a digit (stored as a char) to a numeric value is x - '0', where x is the digit character. malloc() can make a dynamically-allocated array of whatever size you want for you, but you'll have to free() it when you're done. To get the file size, use stat() on most Unix-like systems, or for a more portable approach use fseek() and ftell() to find it. These are all standard and fairly common functions, and I don't know what your trouble is if you know C and know these functions.

Answer 7

According to This post, you can allocate really big memories by malloc.

But if the file is really huge and you cannot allocate such a big memory, you can just simply use File Mapping APIs if the OS is Windows.

With File Mapping you can just map a file to memory. After it, you just have a pointer (a char* for example) that points to file data.