2

Newcomer to C here and struggling a bit.

I'm reading input that looks like this:

9, 344, 100
10, 0, 469
...

I am trying to group each line so that I can send each 3 numbers as parameters in a function.

I have been trying to use scanf but because it maps to memory addresses, I am having a problem with retaining the numbers after each line. I don't know how many lines of data I will have so I cannot make a certain number of arrays. Also, I am limited to functions in <stdio.h>.

If I use scanf is there anyway for me to avoid using malloc?

I have attached an idea below. Looking for suggestions and maybe some clarification on how scanf works.

Apologies if I'm missing something obvious here.

int main() {
  int i = 0;
  int arr[9]; //9 is just a test number to see if I can get 3 lines of input
  char c;
  while ((c = getchar()) != EOF) {
    scanf("%d", &arr[i]);
    scanf("%d", &arr[i + 1]);
    scanf("%d", &arr[i + 2]);
    printf("%d, %d, %d\n", arr[i],
      arr[i + 1], arr[i + 2]); //serves only to check the input at this point
      //at this point I want to send arr 1 to 3 to a function
    i += 3;
  }
}

The output of this code is a bunch of memory addresses and some of the correct values interspersed. Something like this:

0, 73896, 0
0, 100, -473670944

When it should read:

0, 200, 0
0, 100, 54

int main(){
    char c;
    while ((c=getchar()) != EOF){
        if (c != '\n'){
            int a;
            scanf("%d", &a);
            printf("%d ", a);
        }
        printf("\n");
    }
}

This code prints out the input correctly, but doesn't allow me to use scanf more than once in the while block without a memory problem.

Improve this question
2
  • 1
    You don't know what you are getting since you aren't checking the return value of scanf. Commented Oct 1, 2021 at 11:23
  • the function: getchar() returns an int, not a char. The code (depending on the signness of char on your compiler) will not recognize EOF. Strongly suggest replacing: char c; with int c; Commented Oct 2, 2021 at 18:22

2 Answers 2

Reset to default
3

One option is to scan all three at the same time. You also need to match the commas (,) in the input.

Example:

#include <stdio.h>

int main() {
    int arr[9];

    int i = 0;
    for(; i + 3 <= sizeof arr / sizeof *arr // check that there is room in "arr"
          &&                               // and only then, scan:
          scanf(" %d, %d, %d", &arr[i], &arr[i+1], &arr[i+2]) == 3;
          i += 3)
    {
        printf("%d, %d, %d\n", arr[i], arr[i+1], arr[i+2] );
    }
}
Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

1

I would rather use fgets and sscanf for that.


char buff[64];

/* .......... */

if(fgets(buff, 63, stdin) != NULL)
{
    if(sscanf(buff, "%d,%d,%d", &arr[i], &arr[i + 1], &arr[i + 2]) != 3)
    {
        /* handle scanf error */
    }
}
else
{
    /* handle I/O error / EOF */
}
Improve this answer

3 Comments

OP was also asking about avoid malloc. Number of lines is not known in advance
OP wants to solve X-Y problem. malloc or fixed size array. There is no other way
UV for proper fgets() / sscanf(). With scanf() alone you are one mal-formatted character or extra-character away from corrupting all input from that point forward...

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.