to read a txt file and find its minima from second column for a given value of first column

Question

I have a data file in which first column varies from -180 to +180 with a width of 5, while the second column is some corresponding value. i need to find minima from second column for one corresponding value of first say -180,print that,then subsequently find minima for -175 from second column print that and so...I have code for finding minima when i have only one column. How do I include this loop

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

using namespace std;

int main()
{
    ifstream myfile("ma3.txt");
    if(myfile.is_open())
    {       
      int arrSize=0.0;
      double arr[2000];
      double min=0;

      while(true)
      {
         double x,y;
         myfile>>x;
         if(myfile.eof()) break;
         arr[arrSize++]=x;        
      }
      //for(int i=0; i<arrSize; ++i)
      //   cout<<arr[i]<<"";
      //cout<<endl;

      min=arr[0];
      for(int i=0;i<arrSize;i++)
      {
         {          
             if(arr[i]<min)
             {
                min=arr[i];
             }
         }
      }

      cout<<"Smallest element:";
      cout<<min;
      cout<<endl;
    }
    else
    {
       cout<<"Unable to open file";
    }

    return 0;
} 

The input data:

-180 431.845 
-180 434.337 
-180 436.819 
-180 439.289 
-180 469.936 
-180 472.152 
-180 474.343 
-180 476.509 
-180 478.649 
-180 480.761 
-180 482.846 
-180 484.902 
-180 486.929 
-180 488.926
-175 387.566 
-175 384.891 
-175 382.216 
-175 379.541 
-175 376.868 
-175 374.197 
-175 371.53 
-175 368.867 
-175 366.209 
-175 363.557 
-175 360.912 
-175 358.275 
-175 355.648 
-175 353.03 
-175 350.422 
-175 347.826 
-175 345.243 
-175 342.673 
-175 340.117 
-175 337.576 
-175 335.05 
-175 332.541 
-175 330.05 
-175 327.576

Answer 1

Use an associative container to track multiple minima. For example an ordered std::map<int, double> seems convenient for this task:

• http://en.cppreference.com/w/cpp/container/map

The general structure of the solution could look like this:

#include <iostream>
#include <map>
int main() {
    std::map<int, double> mins;
    int x1;
    double x2;
    // TODO: read from file instead
    // TODO: read lines using `std::getline()` 
    //       and parse values using `std::stringstream` instead
    while (std::cin >> x1 >> x2) {
        // check if x1 is already being tracked
        if (mins.find(x1) != mins.end()) {
            // unknown key: just insert current value
            mins[x1] = x2;
        }
        else {
            // TODO: update value as needed
        }
    }
    // TODO: print key-value pairs in map
}

For reference:

• https://en.wikipedia.org/wiki/Associative_arrays
• https://en.wikipedia.org/wiki/Associative_containers
• Read file line by line
• http://en.cppreference.com/w/cpp/container/map
• How to find if a given key exists in a C++ std::map
• http://de.cppreference.com/w/cpp/container/map/find
• C++ printing out map values

Answer 2

This can also be done simply and sequentially while reading the data, and without storing it in any container. Because the values of x are grouped together just print out the lowest value when a change occurs in the first column. Something like this (untested):

bool first = true;
int x = 0;
double y = 0;
double minY = 0;
int prevX = 0;


while (myfile >> x >> y)
{
    if (x == prevX)
    {
        if (y < minY)
            minY = y;
    }
    else
    {
        if (!first)
            cout << prevX << " smallest element: " << minY << "\n";

        minY = y;
        prevX = x;
        first = false;
    }
}

if (!first)
    cout << prevX << " smallest element: " << minY << "\n";