c++ - fseek() 将指针设置到错误的位置

Question

我有一个具有以下结构的 .dat 文件:

Object name | Area | Length |Form Factor

我正在尝试查找哪个对象具有最小面积。我读取了整个文件并使用 ftell() 跟踪具有最小面积的对象及其位置。当我读完文件后，我将流位置指示器设置为我从 ftell() 获得的任何值。然后我将该行作为最终结果读取，但现在，我读取的是错误的行。

对象.dat:

toy1 12785.00 550.70 23.22
toy2 11506.50 495.63 0.48
Pen 35008.65 1450.98 0.17
Pencil 42788.35 1773.42 0.209
card7 128433.00 1552.67 0.67

代码:

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

using namespace std;

int main()
{
    FILE *cfPtr;
    errno_t err;

    char name[15];
    double area, length, fF;
    double minArea = 100000.0;
    long address, minAreaAdd;

    char string[100];

    if ((err = fopen_s(&cfPtr, "objects.dat", "r")) != 0)   // Check if we can reach the file
        printf("The file 'objecs.dat' was not opened to read.\n");
    else
    {
        while (fgets(string, 100, cfPtr)) 
        {
            address = ftell(cfPtr);
            sscanf_s(string,"%s %lf %lf %lf", &name, sizeof name, &area, &length, &fF);

            if (area < minArea)
            {
                minArea = area;
                minAreaAdd = address;
            }
        }

        cout << "Min Area: " << minArea << " Address: " << minAreaAdd << "\n\n";

        fseek(cfPtr, minAreaAdd, SEEK_SET);
        fgets(string, 100, cfPtr);
        sscanf_s(string, "%s %lf %lf %lf", &name, sizeof name, &area, &length, &fF);

        cout << "Name: " << name << " Area : " << area << " Length: " << length << " Form factor: " << fF << "\n";

        fclose(cfPtr);
    }
    return 0;
}

输出:

Min Area: 11506.5 Address: 55

Name: Pen Area : 35008.7 Length: 1450.98 Form factor: 0.17

Answer 1

当您获得文件位置并将其存储到 address 时，那是在您读入该行之后，因此当您寻求它以获取答案时，您将获得该行 在正确的之后。