fgets() reads and stores a char form a line twice - c

Question

So I have a file data.txt, with random integers written in it, for example "17 12 5 4 16".
So, I used fgets() to read this line of integers and store it in a char str []. As expected str [0] == 17, but str [1] == 7, an so on... every 2 digits the integer is stored in an element in the array, but the second digit is also stored in the following element.
Example: str [3] = 12, str [4] = 2, str[5] = ' '.
What could be done to fix this issue?
Here is my very typical code:
FILE* fp = fopen ("data.txt", "r");
int i = 0, j = 0;
int size = fileSize(fp) + 1; // Enumerate the char numbers in file
char str [size];
rewind (fp);
fgets (str, size, fp);
Here is my entire code
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
typedef struct RATIONAL
{
int top;
int bottom;
} RATIONAL;
void printArray (int arr [], int count)
{
for (int i = 0; i < count; ++i)
{
printf ("%d\n", arr[i]);
}
}
int fileSize (FILE* fp)
{
char c;
int i = 0;
while ((c = getc (fp)) != EOF)
{
i++;
}
return (i);
}
int main ()
{
RATIONAL ratArray [23]; //array to hold rational numbers
int intArray [45]; //array to hold integer values
FILE* fp = fopen ("data.txt", "r");
int i = 0, j = 0;
int size = fileSize(fp) + 1; // Enumerate the char numbers in file
char str [size];
rewind (fp);
fgets (str, size, fp);
//Convert string to array of ints
while (i < size -1)
{
if (isdigit (str [i]))
{
intArray[j++] = atoi(&str[i]);
i++;
}
else
i++;
}
printArray (intArray, intArray[0]);
Input: data.txt

Answer 1

What you experience is absolutely expected. The key misunderstanding is that you expect arr[N] to hold the (N+1)th integer as it was an array of integers... but it's not. It is an array of characters.
If you store integers within a char array, every digit will occupy an array location. So arr[0] will contain the most significant digit of the first number and arr[a] the least significant digit.
So, in your example, the string "17 12 5 4 16" will be stored in this way:
--------------------------------------------------------------------------------------------
| 0x31 | 0x37 | 0x20 | 0x31 | 0x32 | 0x20 | 0x35 | 0x20 | 0x34 | 0x20 | 0x31 | 0x36 | 0x00 |
--------------------------------------------------------------------------------------------
| '1' | '7' | ' ' | '1' | '2' | ' ' | '5' | ' ' | '4' | ' ' | '1' | '7' | '\0' |
--------------------------------------------------------------------------------------------
All digits are in the same array and separated by spaces. The string is closed by a string terminator '\0'.
Interesting enough, if you print the whole string with printf("%s\n", arr) you get
17 12 5 4 16
but if you try to print passing the address of the second character, with printf("%s\n", &arr[1]) you get
7 12 5 4 16
And so on. This means that C has no idea (not yet) that your intention is to store 5 integers. The string is one until the string terminator is found (the space doesn't terminate the string), so starting from the second digit will just make C print all the string except the first digit.
Since you have a string formatted in a well known way and you want an array of integers, you have to parse it to get the integers:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char arr[] = "7 12 5 4 16";
int main()
{
char * token = strtok(arr, " ");
while(token != NULL)
{
printf("Next int: %d %s\n", atoi(token)); // Here you can assign the value to your array
token = strtok(NULL, " ");
}
return 0;
}
In my demonstrative example, in which the input string was a global array instead of something read from a file like in your scenario, I used strtok() (strtok_r can be used in case a re-entrant function is required) and I got the integers using the simple atoi() (just because the input format was _under control - I recommend using strtol() for a better control of the input).