检查精度到小数点后第n位？ [英] Check precision to nth decimal place?

问题描述

我必须计算热板的值，并且仅将其精确到小数点后一位。我很想尝试找出所有数组值更改后如何检查它们。我发现724次运行之后，到第4个小数点（打印了多少个）没有任何改变。

I have to calculate the values of a hot plate and have it accurate only to the first decimal place. I am stumped on trying to figure out how to check all the array values if they changed. I found out that 724 runs made no change after that to the 4th decimal (how many were being printed).

有没有一种方法可以只比较double变量直到

Is there a way to compare doubles variables only up to the n-th decimal place?

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

using namespace std;

const int ARRAY_SIZE = 20;
const int NEIGHBORS = 4;

void initialize(double hot_plate[][ARRAY_SIZE]);
bool writeFile(const double HOT_PLATE[][ARRAY_SIZE],
               const string FILE_NAME);

double sum_cell(const double HOT_PLATE[][ARRAY_SIZE],
                const int CELL_X, const int CELL_Y);

int main()
{
    double hot_plate[ARRAY_SIZE][ARRAY_SIZE];

    initialize(hot_plate);

    string file_name = "hot_plate.csv";

    //accuracy up to 4 decmials
    int runs = 724;
    while ( runs > 0)
    {
        for (int i = 0; i < ARRAY_SIZE; i++)
        {
            for (int j = 0; j < ARRAY_SIZE; j++)
            {
                if (i > 0 && i < ARRAY_SIZE - 1 && j > 0 && j < ARRAY_SIZE - 1)
                {
                    hot_plate[i][j] = sum_cell(hot_plate, j, i);
                }
            }
        }
        runs--;
    }

    if (writeFile(hot_plate, file_name))
    {
        cout << "File wrote correctly\n";
    }
    else
    {
        cout << "The file did not write!\n";
    }

    //system("pause");

    return 0;
}

////////////////////////////////////////////////////////////////////////////////
//////////////////////////// Completed Code ////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

double sum_cell(const double HOT_PLATE[][ARRAY_SIZE],
                const int CELL_X, const int CELL_Y)
{
    /* This code should never go out of bounds as it's in an if statement
       if (i > 0 && i < ARRAY_SIZE - 1 && j > 0 && j < ARRAY_SIZE - 1)
    */
    double cell_num = HOT_PLATE[CELL_X - 1][CELL_Y]; // Top
    cell_num += HOT_PLATE[CELL_X][CELL_Y - 1]; // Left
    cell_num += HOT_PLATE[CELL_X][CELL_Y + 1]; // Right
    cell_num += HOT_PLATE[CELL_X + 1][CELL_Y]; // Bottom

    cell_num /= NEIGHBORS;

    return cell_num;
}

// setup the Array so all values are defined when starting
void initialize(double hot_plate[][ARRAY_SIZE])
{
    for (int i = 0; i < ARRAY_SIZE; i++)
    {
        for (int j = 0; j < ARRAY_SIZE; j++)
        {
            if (i == 0 || i == ARRAY_SIZE - 1)
            {
                if (j == 0 || j == ARRAY_SIZE - 1)
                {
                    hot_plate[i][j] = 0.0;
                }
                else
                {
                    hot_plate[i][j] = 100.0;
                }
            }
            else
            {
                hot_plate[i][j] = 0.0;
            }
        }
    }
}

// Write the data to the CSV file
bool writeFile(const double HOT_PLATE[][ARRAY_SIZE],
               const string FILE_NAME)
{
   // open the file
   ofstream fout(FILE_NAME);
   if (fout.fail())
      return false;

   for (int i = 0; i < ARRAY_SIZE; i++)
   {
       for (int j = 0; j < ARRAY_SIZE; j++)
       {
           fout << HOT_PLATE[i][j];
           if ( j < ARRAY_SIZE - 1)
           {
               fout << ", ";
           }
           else if (i != ARRAY_SIZE - 1)
           {
               fout << endl;
           }
       }
   }

   // close the input stream from the file.
   fout.close();
   return true;
}

推荐答案

是否有办法只将双精度变量比较到小数点后第n位？

Is there a way to compare doubles variables only up to the n-th decimal place?

是，请检查绝对值它们之间的差值小于10 ^ -n。

Yes there is, check whether the absolute value of the difference between them is less than 10^-n.

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