c++ - 读取文件时打印出正确的值，但在读取文件后打印出垃圾

Question

问题:为什么它在 while 循环内(读取/输入文件时)打印出正确的值，而在 while 循环外却不打印？我不明白。非常感谢您的帮助。

输入文件:

1
2
3
4
5

---

#include <iostream>
#include <string>
#include <fstream>
#include <string>

using namespace std;

int sumNumbers(int sum, int* numbers, int numElements, int count) 
{
    if (count == numElements)   return sum;

    sumNumbers(sum + numbers[count], numbers, numElements, count + 1);

    return 0;
}

int main(int argc, char* argv[]) 
{
    int* numbers;
    int numElements = 0;; 
    int sum = 0;

    string fileName = argv[2];

    ifstream ifile(fileName);

    if( ifile.fail() ) {    
        cout << "The file could not be opened. The program is terminated." << endl;
        return 0;
    }

    while ( !ifile.eof() ) {
        numbers = new int[++numElements];
        ifile >> numbers[numElements - 1];
        cout << "Position " << numElements - 1 << ": " << numbers[numElements - 1] << endl;
    }

    cout << numbers[0] << endl;
    cout << numbers[1] << endl;
    cout << numbers[2] << endl;
    cout << numbers[3] << endl;
    cout << numbers[4] << endl;

    cout << "--------------\n";

    for(int i = 0; i < numElements; i++) {
        cout << "Position " << i << ": " << numbers[i] << endl;
    }

    sumNumbers(sum, numbers, numElements, 0);

    cout << "The sum of the numbers in the file is: " << sum << endl;

    return 0;
}

---

输出:

Position 0: 1
Position 1: 2
Position 2: 3
Position 3: 4
Position 4: 5
0
-805306368
0
-805306368
5
--------------
Position 0: 0
Position 1: -805306368
Position 2: 0
Position 3: -805306368
Position 4: 5
The sum of the numbers in the file is: 0

Answer 1

您在每次循环迭代中实例化(并泄漏)了一个新数组。而且您只填充该数组的一个元素。循环结束后，您将得到最终数组，其中只有最后一个元素集。

SO 上有很多题都是处理从文件中读取数字到数组或容器中的问题。在这里，数字被读入 std::vector。 .

#include <fstream>
#include <vector>
#include <iterator>
#include <iostream>
#include <algorithm>

int main()
{
    std::vector<int> numbers;
    ifstream ifile(fileName);
    std::istream_iterator<int> eof;
    std::istream_iterator<int> it(ifile);
    std::copy(it, eof, std::back_inserter(numbers));

    for(int i = 0; i < numbers.size(); ++i) 
    {
        cout << "Position " << i << ": " << numbers[i] << endl;
    }

}

或者，您可以用 while 循环替换 istream_iterators 和对 std::copy 的调用:

int n=0;
while (ifile >> n) {
  numbers.push_back(n);
}