trouble with file pointers...

Hi, all--this time I've got a program that's supposed to calculate passenger efficiency in passenger kilometers per liter, given an input file that lists number of passengers, total commuting distance per week, and gasoline consumed per week. I'm not sure what's going on, but it doesn't seem to be reading the input file properly. Here's the program:

Code:

---

#define SENTINEL 0
int main()
{
        FILE *inp, *outp;
        double min_efficiency, num_passengers, commute, gas_used, efficiency, subsidy;
        printf("Please enter the minimum efficiency> ");
        scanf("%lf", &min_efficiency);
        inp=fopen("carpool.txt", "r");
        outp=fopen("effic.txt", "w");
        fprintf(outp, "%87s\n", "CARPOOLS MEETING MINIMUM PASSENGER EFFICIENCY OF 25 PASSENGER KM / L");
        fprintf(outp, "%-16s%-21s%-20s%-19s%-10s\n", "Passengers", "Weekly Commute", "Gasoline", "Efficiency", "Weekly");
        fprintf(outp, "%-16s%-21s%-20s%-19s%-10s\n", " ", "(km)", "Consumption(L)", "(pass km / L)", "Subsidy($)");
        fscanf(inp, "%lf%lf%lf", &num_passengers, &commute, &gas_used);
        while (num_passengers!=SENTINEL)
        {
                efficiency=num_passengers*commute/gas_used;
                subsidy=.08*num_passengers*commute;
                if (efficiency>=min_efficiency)
                {
                        fprintf(outp, "%-16d%-21.0lf%-20.1lf%-19.1lf%-10.2lf\n", num_passengers, commute, gas_used, efficiency, subsidy);
                }//if efficiency>=min_efficiency
                fscanf(inp, "%lf%lf%lf", &num_passengers, &commute, &gas_used);
        }//while num_passengers != 0
        fclose(inp);
        fclose(outp);
        return 0;
}//main

---

And here's the output I get on running it:

Code:

---

teyla:~ quasigreat$ cat carpool.txt
4 75 11.0
2 50 9.0
5 95 15.5
2 60 4.5
4 35 4.3
6 70 20.7
0 0 0

teyla:~ quasigreat$ ~quasigreat/a.out
Please enter the minimum efficiency> 25
teyla:~ quasigreat$ cat effic.txt
                  CARPOOLS MEETING MINIMUM PASSENGER EFFICIENCY OF 25 PASSENGER KM / L
Passengers      Weekly Commute      Gasoline            Efficiency        Weekly   
                (km)                Consumption(L)      (pass km / L)      Subsidy($)
0              0                    0.0                (it puts a random huge number here, which I've
                                                            removed to save your browser window)     
0              0                    0.0                (here, too)   
0              0                    0.0                -0.0              0.00     
0              0                    0.0                0.0                (and here)

---

I'm probably making a really basic mistake somewhere, but I can't for the life of me figure it out. If someone could point out what I'm doing wrong, I'd be eternally grateful.

Your logic is shot, you use the uninitialized variables THEN you read their values from a file.

Code:

---

double min_efficiency, num_passengers, commute, gas_used, efficiency, subsidy;

---

Is where your huge number comes from.

The logic should be,

Code:

---

OPEN FILE

WHILE reading from file DO
    CALC stuff
    PRINT stuff
END WHILE

CLOSE FILE

---

ie

Code:

---

FILE * out, * in;

/* ... */

while(fscanf(in, ...) != EOF)
{
    if(...)
    {
        fprintf(out, ...);
    }
}

---

Hmm. I'm not sure I understand. Just before the while loop begins, there's this line:

Code:

---

fscanf(inp, "%lf%lf%lf", &num_passengers, &commute, &gas_used);

---

Is there some reason I'm not aware of that this wouldn't read from the file into those variables? And efficiency and subsidy are both assigned before the if statement that prints them. I did try your suggestion to use "while (fscanf(inp, "%lf%lf%lf", &num_passengers, &commute, &gas_used)!=EOF)", and it gave me one line of 0s. Could you perhaps clarify a little further?

OK, you declared num_passengers as a double and then print it as an int. Unless you need to account for the possibility of a serial killer carrying hacked-up bodies, or perhaps a hearse or medical examiner's van, you should consider changing num_passengers to an int. Don't forget to change the corresponding scanf formatters.

If you must keep it as a double, cast the argument in fprintf to (int).

Thanks so much, that's fixed it. Eternal gratitude, coming right up. :D