MATLAB: Does Matlab add digits to a code generated vector

MATLABvector

I have a simple command that is creating a vector. Test = (51.7:.1:130)' I then iterate through this vector and use the values to search in a list of data that I have. What I find, is that Matlab is actually adding very small numbers to the end of some of my values. For instance 65.5 truly equals 65.500000… However, 65.6 (when you double click) equals 65.600000000000010

I can not think of a logical reason for this behavior. Now, I certainly can go ahead and round the vector before I loop, but that shouldn't be necessary. I'd like to understand why it can't create the vector that I specified.

Thanks for your help, Brian

Best Answer

There are two separate processes involved: floating-point approximation error, and the way MATLAB calculated colon-operator generated vectors. If you only want the vector elements to be defined at one decimal place, multiply the vector by 10, use round, floor, or fix, and then divide it by 10:

Test = round(Test*10)/10;

That should provide you with reasonably precise values for your comparison.

Related Solutions

MATLAB: Matlab ROUND function not rounding small decimals away

It is just a display issue and the fact that IEEE double cannot represent the numbers exactly. E.g.,

>> num2strexact(66.6)
ans =
6.6599999999999994315658113919198513031005859375e1

So the closest number to 66.6 that is representable in IEEE double, when converted to an exact decimal equivalent, in fact has all of those 9's and the 4 at the end. Rounding the number to the nearest tenth or hundredth etc isn't going to change anything because there isn't anything closer to 66.6 in the IEEE double set that you can round to.

Similarly,

>> num2strexact(59.4)
ans =
5.939999999999999857891452847979962825775146484375e1

MATLAB: Can I create a dynamic array with the mwArray class of the MATLAB Compiler 4.1 (R14SP1) C++ Utility Library

It is possible to create dynamic arrays with the mwArray class in MATLAB Compiler 4.1 (R14SP1). Below is an example (testme.cpp) that will let the user define the size of 2 arrays:

#include "mclcppclass.h"
int main(){
      if (!mclInitializeApplication(NULL,0))
      {   std::cerr << "could not initialize the library properly"<<std::endl;
           return -1;
      }  
      try
      {   int x,y;
          int* a = NULL;
          int* b = NULL;              
          std::cout << "Enter length of array 1:" <<std::endl;
          std::cin >> x;          // Create input data
          std::cout << "Enter length of array 2:" <<std::endl;
          std::cin >> y;
          a = new int[x]; // Allocate x ints and save ptr in a.
          b = new int[y]; // Allocate y ints and save ptr in b.     
          for (int i=0; i<x; i++) {
              a[i] = i;    // Initialize all elements in increments of 1.
          }    
          for (int j=0; j<y; j++) {
              b[j] = 1;    // Initialize all elements to one.
          }    
          mwArray in1(1, x, mxDOUBLE_CLASS, mxREAL);
          mwArray in2(1, y, mxDOUBLE_CLASS, mxREAL);         
          in1.SetData(a, x);
          in2.SetData(b, y);        
          std::cout << "The 1st array is:" << std::endl;
          std::cout << in1 << std::endl;      
          std::cout << "The 2nd array is:" << std::endl;
          std::cout << in2 << std::endl;        
          delete [] a;  //When done free memory pointed to by a.
          a = NULL; 
          delete [] b;
          b = NULL;     //When done, free memory pointed to by b.
      }
      catch (const mwException& e)
      {   std::cerr << e.what() << std::endl;
          return -1;        
      }
      catch (...)
      {  std::cerr << "Unexpected error thrown" << std::endl;
          return -1;
      }   
      mclTerminateApplication();
      return 0;
   }

You can compile this example using the following command:

mbuild -v testme.cpp

Best Answer

Related Solutions

MATLAB: Matlab ROUND function not rounding small decimals away

MATLAB: Can I create a dynamic array with the mwArray class of the MATLAB Compiler 4.1 (R14SP1) C++ Utility Library

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