无法在 for 循环中显示正确的百分比或数组总数

Question

我的程序遇到的问题是，首先，当我计算百分比时，它不是将数组中的所有元素加到一个总数中并从中提取它们。我试着把总数 += percents[i];在嵌套的 for 循环中，但它只给了我负的 %。

另外，我最后的总数不会显示任何内容。起初，我拥有它和 main() 中定义的所有函数，但它什么也没做。即使更改后，它也不起作用。另外，最后一件事，我的文件有 20 项，但循环仅读取 19 项。如果我更改为 20，它会崩溃。

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

using namespace std;

void inputValues(string names[], int votes[])
{

    ifstream inputfile;

    inputfile.open("votedata.txt");

    if(inputfile.is_open())
    {
         for(int i = 0; i < 19; i++)
         {
             inputfile >> names[i] >> votes[i];
         }
    }
}

double *calcPercents( double percents[], int votes[], double total)
{
    for(int i = 0; i < 19; i++)
    {
        percents[i] = votes[i];

        total += percents[i];

        percents[i] = (percents[i]/total)*100;
    }

    return percents;
}

string determineWinner(string names[], double percents[])
{
    double temp = 0;
    string winner;
    for(int i = 0; i < 19; i++)
    {
        if(percents[i] > temp)
        {
            temp = percents[i];
            winner = names[i];
        }
    }
    return winner;
}

void displayResults(string names[], int votes[], double percents[])
{
    int total = 0;
    calcPercents(percents, votes, total);
    cout << "Candidate  Votes Received  % of Total Votes " << endl; 
    for(int i = 0; i < 19; i++)
    {
        cout << names[i] << "       " << votes[i] << "      " << percents[i] << "%" << endl;
    }
    cout << "Total " << total << endl;

    cout << " The winner of the election is " << determineWinner(names, percents) << endl;

}

int main()
{
  string names[19], winner;
  int votes[19];
  double percents[19];

  inputValues(names, votes);
  displayResults(names, votes, percents);
}

我的文件是这样的：

bob (tab) 1254

joe (tab) 768

等等

Answer 1

如果你必须使用数组而不是std::vectors，你应该将它们的大小传递给使用它们的函数。一种方法是使用常量设置大小，如下所示：

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

using namespace std;  // bad practice

const int size = 20;

void inputValues(string names[], int votes[], int n);
// add the size as a parameter of the function ^^^^

int calcPercents( double percents[], int votes[], int n );
//^^ I'll explain later why I changed your signature ^^^

string determineWinner(string names[], double percents[], int n );
void displayResults(string names[], int votes[], double percents[], int n);

int main()
{
    // It's always better to initialize the variables to avoid undefined behavior
    // like the negative percentages you have noticed
    string names[size] ="";
    int votes[size] = {0};
    double percents[size] = {0.0};

    inputValues(names, votes, size);
    displayResults(names, votes, percents, size);
}

要计算百分比，您可以使用两个循环，一个用于求和，另一个用于获取百分比。在您的函数中，您通过值将总计作为参数传递，因此它将被复制并且更改将永远不会在函数之外可见。我选择返回该值，即使这样做会导致函数名有点误导：

int calcPercents( double percents[], int votes[], int n )
{
    int total = 0;
    for(int i = 0; i < n; i++)
    // note the bound ^^^
    {
        total += votes[i];
    }
    double factor = 100.0 / total;
    for(int i = 0; i < n; i++)
    {
        percents[i] = factor * votes[i];
    }
    return total;
}

你还应该对输入函数添加一些检查，我只添加了大小参数。请注意，初始化数组后，即使读取文件失败，数组也不包含随机值：

void inputValues(string names[], int votes[], int n)
{

    ifstream inputfile;

    inputfile.open("votedata.txt");

    if(inputfile.is_open())
    {
        for(int i = 0; i < n; i++)
        {
            inputfile >> names[i] >> votes[i];
        }
    }
}

我也会更改决定获胜者的函数：

string determineWinner(string names[], double percents[], int n )
{
    if ( !n ) 
    {
        return "";
    }
    double max = percents[0];
    // update an index instead of a string
    int winner = 0;              
    for( int i = 1; i < n; i++ )
    {
        if( percents[i] > max )
        {
            max = percents[i];
            winner = i;
        }
    }
    return names[winner];
}

最后一个函数，只记得加上大小：

void displayResults(string names[], int votes[], double percents[], int n)
{
    int total = calcPercents(percents, votes, n);
    cout << "Candidate  Votes Received  % of Total Votes " << endl; 
    for( int i = 0; i < n; i++ )
    {
        cout << names[i] << "       " << votes[i] << "      "
             << percents[i] << "%" << endl;
    }
    cout << "Total " << total << endl;

    cout << " The winner of the election is "
         << determineWinner(names, percents,n) << endl;    
}