fgets() 从一行读取并存储一个字符两次

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).