在 FILE，c 语言中查找最低和最高温度

Question

I am trying to make this code to work but my ability in programming is not good enough, so i thought i should ask you after so many search i did.

So, I've made a program which makes a binary .dat file which includes 2 integers (day and month) and 2 floats (min,max temperatures of the day). My homework says that it needs from ALL the inputs i did, the minimum temperature and the maximum temperature and print them with the day included. I am stuck at "searching" from all the inputs, the minimum and the maximum seperately.

I hope you understand, my english is not good enough.

I can't remember what i've tried so far. But i am stuck here.

EDIT: I've edited the code to make things more clearly.

#include <stdio.h>

typedef struct {
    int day, month;
    float max_temp, min_temp;
} Date;

Date D;

int main() {
    FILE *f, *t;
    float min = D.min_temp, max = D.max_temp;
    f = fopen("Metriseis_2012.dat", "rb");

    while (!feof(f)) {
        fread(&D, sizeof(Date), 1, f);
        if ((!feof(f)) && (D.min_temp < min)) {
            fseek(f, sizeof(Date), SEEK_SET);
            printf("\nDay %d\nMonth %d\nMin_Temp %.2f\nMax_Temp %.2f\n\n", D.day, D.month, D.min_temp, D.max_temp);
        }
    }
    fclose(f);
    return 0;
}

Answer 1

Generate data to be used to test the main algorithm

Here's some code to write a binary file containing date and temperature values (a necessary precursor to writing code to read such data):

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

typedef struct
{
    int   day;
    int   month;
    float max_temp;
    float min_temp;
} Date;

int main(void)
{
    const char filename[] = "Metriseis_2012.dat";
    FILE *fp = fopen(filename, "wb");
    if (fp == NULL)
    {
        fprintf(stderr, "Failed to open file '%s' for writing\n", filename);
        exit(EXIT_FAILURE);
    }

    for (int i = 0; i < 30; i++)
    {
        Date D =
        {
            .day = rand() % 28 + 1, .month = rand() % 12 + 1,
            .min_temp = rand() % 20,
        };

        D.max_temp = rand() % 50 + D.min_temp;
        printf("Day temperature: %6.2f - %6.2f on %.2d-%.2d\n", D.min_temp, D.max_temp, D.month, D.day);
        if (fwrite(&D, sizeof(D), 1, fp) != 1)
        {
            fprintf(stderr, "failed to write to '%s'\n", filename);
            exit(EXIT_FAILURE);
        }
    }

    fclose(fp);
    return 0;
}

The data generation code does not preclude repeats in the date (and indeed, the dates 02-08 and 08-01 both repeat in the sample data, with different temperature ranges). Fixing that is moderately hard. The generated data is not sorted into date order.

The separate assignment to D.max_temp avoids running foul of the indeterminate evaluation order specified in C11 §6.7.9 Initialization ¶23 :

The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and thus the order in which any side effects occur is unspecified. ¹⁵²⁾

^{152) In particular, the evaluation order need not be the same as the order of subobject initialization.}

You could add:

#include <time.h>

and (at the top of main() ):

    srand(time(0));

so that it generates different sequences of numbers when it is called no more frequently than once a second. There are other, better ways to seed the random number generator, but they involve more code (and often some platform specific code too).

Determine minimum and maximum temperatures and corresponding dates

Here's code that reads the data and determines the date on which the minimum temperature was recorded (and the minimum value), and the date on which the maximum temperature was recorded (and the maximum value).

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

typedef struct
{
    int day, month;
    float max_temp, min_temp;
} Date;

int main(void)
{
    const char filename[] = "Metriseis_2012.dat";
    FILE *fp = fopen(filename, "rb");

    if (fp == NULL)
    {
        fprintf(stderr, "Failed to open file '%s' for reading\n", filename);
        exit(EXIT_FAILURE);
    }

    Date Dmin = { 0, 0, 100.0, 100.0 };
    Date Dmax = { 0, 0,   0.0,   0.0 };
    Date D;
    while (fread(&D, sizeof(Date), 1, fp) == 1)
    {
        printf("Day temperature: %6.2f - %6.2f on %.2d-%.2d\n", D.min_temp, D.max_temp, D.month, D.day);
        if (D.max_temp > Dmax.max_temp)
            Dmax = D;
        if (D.min_temp < Dmin.min_temp)
            Dmin = D;
    }
    fclose(fp);

    printf("Min temperature: %6.2f on %.2d-%.2d\n", Dmin.min_temp, Dmin.month, Dmin.day);
    printf("Max temperature: %6.2f on %.2d-%.2d\n", Dmax.max_temp, Dmax.month, Dmax.day);

    return 0;
}

On my Mac, the output I got from the reading code was:

Day temperature:  13.00 -  21.00 on 02-08
Day temperature:   4.00 -  32.00 on 09-07
Day temperature:   0.00 -  15.00 on 02-16
Day temperature:   7.00 -  10.00 on 03-01
Day temperature:   0.00 -  12.00 on 02-24
Day temperature:   9.00 -  16.00 on 02-12
Day temperature:  19.00 -  47.00 on 10-09
Day temperature:  17.00 -  43.00 on 08-01
Day temperature:  10.00 -  43.00 on 08-01
Day temperature:  19.00 -  40.00 on 02-08
Day temperature:  13.00 -  49.00 on 09-28
Day temperature:   8.00 -  49.00 on 06-10
Day temperature:   1.00 -   4.00 on 02-07
Day temperature:   8.00 -  48.00 on 05-01
Day temperature:  10.00 -  13.00 on 09-17
Day temperature:   9.00 -  33.00 on 11-27
Day temperature:  17.00 -  25.00 on 02-02
Day temperature:  18.00 -  49.00 on 02-17
Day temperature:   5.00 -  19.00 on 02-20
Day temperature:   5.00 -  24.00 on 01-08
Day temperature:   9.00 -  10.00 on 11-28
Day temperature:   5.00 -   9.00 on 12-02
Day temperature:   8.00 -  31.00 on 07-04
Day temperature:  12.00 -  28.00 on 11-26
Day temperature:  18.00 -  62.00 on 10-21
Day temperature:  11.00 -  39.00 on 10-06
Day temperature:  17.00 -  64.00 on 03-14
Day temperature:   9.00 -  40.00 on 01-27
Day temperature:  15.00 -  63.00 on 04-18
Day temperature:   8.00 -  20.00 on 08-03
Min temperature:   0.00 on 02-16
Max temperature:  64.00 on 03-14

There are two entries for the minimum temperature — the first is selected. A more complex test could choose the earliest date or latest date.

It would be sensible to have the generator program accept command line arguments that specify the output file name and a number of output records to generate. The analyzer program should accept command line arguments that specify the file name(s) to be processed.

Answer 2

Hopefully the following code can help you. If you know the number of lines beforehand, it will be easier. In this example I used 4 lines in the .dat file.

#include <stdio.h>
#include <malloc.h>
#include <float.h>

typedef struct {
    int day, month;
    float max_temp, min_temp;
} Date;

Date D;

int main(void) {
    int LINE_NUMBERS = 4;
    FILE *fp;
    float min = FLT_MAX;
    float max = FLT_MIN ;
    fp = fopen("Metriseis_2012.dat", "r");
    Date *t_date = malloc(sizeof(Date)*LINE_NUMBERS);
    int i = 0;
    if (fp != NULL) {
        while (i < LINE_NUMBERS) {
            fscanf(fp, "%d %d %f %f",
                   &t_date[i].day,
                   &t_date[i].month,
                   &t_date[i].max_temp,
                   &t_date[i].min_temp);
            i++;
        }
    } else {
        perror("FP ERROR: ");
    }
    for (i = 0; i < LINE_NUMBERS; i++) {
        printf("%d %d  %f %f\n",
               t_date[i].day,
               t_date[i].month,
               t_date[i].max_temp,
               t_date[i].min_temp);
        if (min > t_date[i].min_temp)
            min = t_date[i].min_temp;
        if (max < t_date[i].max_temp)
            max = t_date[i].max_temp;
    }
    fclose(fp);
    printf("max %f min %f\n", max, min);
    return 0;
}

Example .dat file

1 2 3 4 
5 6 7 8
12 23 34 45
12 3 34 45

Output

1 2  3.000000 4.000000
5 6  7.000000 8.000000
12 23  34.000000 45.000000
12 3  34.000000 45.000000
max 34.000000 min 4.000000