Problem with sscanf reading multiple strings from a input file

Question

So I'm slowly continuing learning C. And now, I have a task, to read data from a file and sort it.

File data:

House naming 1 30 300
House naming 2 45 450
.......
House naming 10 5 120

So, first value: House naming, can be any naming like Empire state building

The second value is: House address (I chose only integer values)

The third value is: Age of a building

The fourth value is: Kilowatt-hour/year

Programm must take data from a file -> Print it out -> Sort(how? see below) -> print out again, sorted.

Sorting:

• kwh < 200 - sustainable building,
• kwh < 300 && age < 40 - needs renovation,
• kwh > 300 && age > 40 - set for demolition.

Here's the code:

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

int main(void) {
    int kwh;
    int age;
    char building[SIZE];
    int addr;
    char buff[SIZE];
    FILE *fi;

    // opening the files and checking if it succeeded
    fi = fopen(F_INPUT, "r");
    if (fi == NULL) {
        printf("Error opening input file \"%s\"", F_INPUT);
        exit(EXIT_INPUT_FAIL);
    }
    while (fgets(buff, sizeof(buff), fi) != NULL) {
        sscanf(buff, "%s %d %d %d", building, &addr,&age,&kwh);
        if (kwh < 200) {
            puts(buff);
            printf("Sustainable\n");
        } else
        if (kwh < 300 && age < 40) {
            puts(buff);
            printf("Needs renovation\n");
        } else
        if (kwh > 300 && age > 40) {
            puts(buff);
            printf("IN DEMOLITION LIST\n");
        }
    }
    /* close the files when they're not needed anymore */
    fclose(fi);
    return 0;
}

I've combined a few steps to make it a bit easier, reads data -> outputs already marked 1) Sustainable, 2) Needs renovation, 3) set for demolition.

The problem is somewhere in the while loop and I think it's in sscanf function. In my logic, if I am not mistaken, it must read a string from a file, using logic(look at sscanf and input file): char value, integer, integer, integer. Programm reads the file, outputs the data, but marks all buildings as sustainable.

What you suggest to read more carefully or what logic is better to choose for reading multiple strings.

Output:

House naming 1 30 300
Sustainable
House naming 2 45 450
Sustainable
........
House naming 10 5 120
Sustainable

Answer 1

Your problem is tricky to solve with sscanf() because there is no explicit separator between the house name and the 3 numeric fields. %s is inappropriate: it parses a single word. In your program, sscanf() actually fails to convert the numbers and returns 1 for all lines, leading to undefined behavior when you compare the numeric values that are actually uninitialized.

Here is a modified version using the %[ conversion specification:

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

#define F_INPUT  "input.txt"
#define EXIT_INPUT_FAIL  1

int main(void) {
    char buff[256];
    char building[100];
    int addr, age, kwh;
    FILE *fi;

    // opening the files and checking if it succeeded
    fi = fopen(F_INPUT, "r");
    if (fi == NULL) {
        printf("Error opening input file \"%s\"", F_INPUT);
        exit(EXIT_INPUT_FAIL);
    }
    while (fgets(buff, sizeof(buff), fi) != NULL) {
        /* parse the building name upto and excluding any digit,
           then accept 3 integral numbers for the address, age and power */
        if (sscanf(buff, "%99[^0-9]%d%d%d", building, &addr, &age, &kwh) != 4) {
            printf("parsing error: %s", buff);
            continue;
        }
        if (kwh < 200) {
            puts(buff);
            printf("Sustainable\n");
        } else
        if (kwh < 300 && age < 40) {
            puts(buff);
            printf("Needs renovation\n");
        } else
        if (kwh > 300 && age > 40) {
            puts(buff);
            printf("IN DEMOLITION LIST\n");
        }
        // allocate structure with building details and append it to the list or array of buildings
    }
    /* close the files when they're not needed anymore */
    fclose(fi);
    // sort the list or array and print it
    // free the list or array
    return 0;
}

Answer 2

Reading the line from the file into a string via fgets() is a good first step as OP has done.

Can "House naming" include digits? like "Stdio 54"?
Yea, it can include digits and can not. If I am right, the task says nothing about namings.

The next part is tricky as there is not a unique separator between the house name and the following 3 integers.

One approach would be to find the 3 trailing integers and then let the remaining beginning text as the house name.

  while (fgets(buff, sizeof(buff), fi) != NULL) {
    int address, age, power;
    char *p = buff + strlen(buff);  // start as buff end
    p = reverse_scan_int(buff, p, &power);
    p = reverse_scan_int(buff, p, &age);
    p = reverse_scan_int(buff, p, &address);
    if (p) {
      *p = '\0';
      trim(buff);  // remove leading trailing white-space
      printf("house_name:'%s', address:%d age:%d power:%d\n", buff, address,
          age, power);
    } else {
      printf("Failed to parse '%s'\n", buff);
    }
  }

Now all we need is reverse_scan_int(). Sample untested code idea:

#include <ctype.h>
#include <stdbool.h>
char *reverse_scan_int(const char *begin, char *one_past, int *i) {
  if (one_past == NULL) {
    return NULL;
  }
  // is...() functions work best with unsigned char
  unsigned char *p = (unsigned char *) one_past;
  // Skip trailing whitespace;
  while (p > begin && isspace(p[-1])) {
    p--;
  }
  // Skip digits;
  bool digit_found = false;
  while (p > begin && isdigit(p[-1])) {
    p--;
    digit_found = true;
  }
  if (!digit_found)
    return NULL;
  if (p > begin && (p[-1] == '-' || p[-1] == '+')) {
    p--;
  }
  *i = atoi(p); // More roubust code would use `strtol()`, leave for OP.
  return (char *) p;
}

There are lots of ways to trim a string including this one.