c++ - 为什么我的 vector 会丢弃它的数据？

Question

我正在编写一个基本的债务计算器来练习我的 C++，但我的一个 vector 不断降低它的值。

包含 vector 的结构:

struct debt 
{
std::string institution, debtType;
double creditLimit, balance, intrest, debtRatio, minMonthly;
int monthsToPayOff;
std::vector <double> monthlyBalance; //this vector is giving me trouble
};

我调用了一个函数，将每个月的余额减去 vector 中的每月付款。

for (auto i : vectDebt) 
{
    double tempBalance = i.balance;

    while (tempBalance > 0) 
    {
        i.monthlyBalance.push_back(tempBalance); //this seems to work fine
        tempBalance -= i.minMonthly;
    }

    if (i.monthlyBalance.size() > vectMonths.size())
        vectMonths.resize(i.monthlyBalance.size());
}

Visual Studio 调试器显示值已正确存储在上面的代码中。然后我尝试将值打印到 .csv 文件，但此时 vector 已经为空。

//print the payoff balances in debt.csv
for (auto i: vectDebt) 
{
    for (int j = 0; j != i.monthlyBalance.size(); j++) 
    {
        fileout << i.monthlyBalance[j] << ","; //the debugger shows monthlyBalance has size = 0
    }
    fileout << "\n";
}

这是我的完整代码，任何关于改进程序的额外评论也适用:

债务.h

#pragma warning(disable : 4996) //diable localtime warning
#ifndef DEBT_H
#define DEBT_H
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <time.h>
#include <vector>

struct debt 
{
std::string institution, debtType;
double creditLimit, balance, intrest, debtRatio, minMonthly;
int monthsToPayOff;
std::vector <double> monthlyBalance;
};

class Debt 
{
  private:
time_t t = time(NULL);
tm *timeinfo = localtime(&t);
const std::string months[12] = { "January","February","March","April","May","June","July",
    "August","September","October","November","December" };
std::ofstream fileout;
std::vector <debt> vectDebt;
std::vector <std::string> vectMonths;

public:
Debt() {    }
void addDebt(debt temp) 
{
    vectDebt.push_back(temp);
}
std::string returnMonth(int month) 
{
    std::string currentMonth = months[month - 1];
    return currentMonth;
}
void calcPayoff();
void printDebt();
void printPayoff();
~Debt() {   }
};

#endif //DEBT_H

债务.cpp

#include "Debt.h"

void Debt::calcPayoff()
{
for (auto i : vectDebt) 
{
    double tempBalance = i.balance;

    while (tempBalance > 0) 
    {
        i.monthlyBalance.push_back(tempBalance);
        tempBalance -= i.minMonthly;
    }

    if (i.monthlyBalance.size() > vectMonths.size())
        vectMonths.resize(i.monthlyBalance.size());
}
}

void Debt::printDebt()
{
fileout.open("debt.csv");
fileout << "Institution(type),Current Balance,Intrest,Min Monthly Payment,Credit Limit\n";

for (auto i : vectDebt) 
{
    fileout << std::fixed << std::setprecision(2) << i.institution << "("
        << i.debtType << "),"
        << i.balance << ","
        << i.intrest << ","
        << i.minMonthly << ","
        << i.creditLimit << "\n";
}
printPayoff();
fileout.close();
}

void Debt::printPayoff()
{
int monthCounter = timeinfo->tm_mon;
fileout << "\nBalance per Month until paid off:\n";

for (int i = 0; i != vectMonths.size(); i++) 
{
    vectMonths.at(i) = months[monthCounter % 12];
    monthCounter++;
}

for (auto i : vectMonths)
    fileout << i << ",";

fileout << "\n";

//print the payoff balances in debt.csv
for (auto i: vectDebt) 
{
    for (int j = 0; j != i.monthlyBalance.size(); j++) 
    {
        fileout << i.monthlyBalance[j] << ",";
    }
    fileout << "\n";
}
 }

源文件:

#include "Debt.h"

void main() {
Debt newList;
int check = 1;
std::string institution = "",
    debtType = "";
double balance = 0,
    intrest = 0,
    creditLimit = 0,
    minMonthly = 0;

std::cout << "This program will ask you to enter each of your debts institution(bank name), "
        << "debt type(loan, credit, car), current balance, interest rate, credit limit, and minimum monthly payment.\n";

while (check == 1) {
    debt temp;
    if (check == 1) {
        std::cout << "Enter name of institution: ";
        std::cin >> temp.institution;

        std::cout << "Enter debt type (loan, credit, car...): ";
        std::cin >> temp.debtType;

        std::cout << "Enter current balance: ";
        std::cin >> temp.balance;

        std::cout << "Enter inrest rate for this debt: ";
        std::cin >> temp.intrest;

        std::cout << "Enter credit limit for this debt: ";
        std::cin >> temp.creditLimit;

        std::cout << "Enter the minimum monthly payment for this debt: ";
        std::cin >> temp.minMonthly;

        temp.debtRatio = 100*(temp.balance/temp.creditLimit);

        newList.addDebt(temp);

        std::cout << "Add another debt? \n"
            << "1) Yes\n"
            << "2) No\n";
        std::cin >> check;
    }
}
newList.calcPayoff();
newList.printDebt();

Answer 1

for (auto i : vectDebt) {
    //...
}

在此循环中，i 将是 vectDebt 中每个项目的拷贝。 i 的任何更改都不会反射(reflect)在 vectDebt 中的项目中。您需要的是对 vectDebt 中每个项目的引用。为此，将循环更改为

for (auto& i : vectDebt) {
    //...
}