提供变量的地址而不是其值

Question

我想在 C++ 中测试fopen和fclose的示例，其中代码读取文件中的 integer 值，但如果文件为空（无整数），则检索到的值不是 0 而是-858993460 。 我认为这是fscanf()中变量的地址，但我怎么能得到null或/0值以下代码有问题

#include <stdio.h>

int main()
{

    {
        int a, sum = 0 ,num, n = 0;
        FILE* pFile;
        pFile = fopen("input.txt", "r");
        fscanf(pFile, "%d", &num);
        while (n != 5) {
            n = n + 1;
            sum = sum + num;
            fscanf(pFile, "%d", &num);
            
        }
        fclose(pFile);
        printf("I have read: %d numbers and sum is %d \n", n, sum);
        scanf("Hi %d", &a);
        return 0;
    }
}

input.txt：（空）或：

12 32 43 56 78

注意：代码是有问题的，因为num永远不会被读取为0 ，因此循环会继续进行。

我将添加有关该问题的更多详细信息：如果输入文件包含五个整数：

12 32 43 56 78

代码如下：

#include <stdio.h>

int main()
{

    {
        char str[80];
        int a, sum = 0 ,num, n = 0;
        FILE* pFile;
        pFile = fopen("input.txt", "r");
        fscanf(pFile, "%d", &num);
        while (n != 5) {
            n = n + 1;
            sum = sum + num;
            fscanf(pFile, "%d", &num);
            
        }
        fclose(pFile);
        printf("I have read: %d numbers and sum is %d \n", n, sum);
        scanf("Hi %d", &a);
        return 0;
    }
}

output：

I have read: 5 numbers and sum is 221

Answer 1

执行此操作的更多 C++ 方法如下所示。 您可以将此作为参考。

#include <iostream>
#include <fstream>
#include <sstream>
int main()
{
    std::ifstream inputFile("input.txt");
    
    std::string line;
    int num = 0, sum = 0; //always initialize built in types in local/block scope 
    
    if(inputFile)
    {
        //go line by line
        while(std::getline(inputFile, line))
        {
            std::istringstream ss(line);
            
            //go through individual numbers
            while(ss >> num)
            {
                sum += num;
            }
        }
        
    }
    
    else 
    {
        std::cout<<"Input file cannot be read"<<std::endl;
    }
    inputFile.close();
    
    std::cout << "The sum of all the intergers from the file is: "<<sum<<std::endl;
    return 0;
}

以上程序的output可以看这里。

Answer 2

修复很简单，永远不会错过 IO 函数的返回值。

FILE* pFile;
// pFile = fopen("input.txt", "r");
// fscanf(pFile, "%d", &num);
// while (n != 5) {
if ((pFile = fopen("input.txt", "r")) == NULL)  // If unable to open file
    return 1;
while (n != 5 && fscanf(pFile, "%d", &num) == 1) {  // And if num is successfully assigned
    n = n + 1;
    sum = sum + num;
    // Odd: fscanf(pFile, "%d", &num);
}
fclose(pFile);