Manhattan Distance between two vectors

**serge** · 03-25-2014

Hi

I can't figure out the error in this code; it compiles but returns rubbish. Can you help? Thank a lot!

serge

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>

Code:

int main(int argc, char* argv[]) {
	std::vector<double> VEC1;
	std::vector<double> VEC2;
	
	VEC1.push_back(1.2);
	VEC1.push_back(3.4);
	VEC1.push_back(5.1);
	VEC2.push_back(4.5);
	VEC2.push_back(1.0);
	VEC2.push_back(1.9);
	for ( std::vector<double>::iterator it1 = VEC1.begin(); it1 != VEC1.end(); ++it1 ) std::cout << " " << *it1;
	std::cout << std::endl;	
	for ( std::vector<double>::iterator it1 = VEC2.begin(); it1 != VEC2.end(); ++it1 ) std::cout << " " << *it1;
	std::cout << std::endl;
	double DIST = 0.0;
	
	for ( std::vector<double>::iterator it1 = VEC1.begin(); it1 != VEC1.end(); ++it1 ) {
		for ( std::vector<double>::iterator it2 = VEC2.begin(); it2 != VEC2.end(); ++it2 ) {
			DIST += abs( (*it1) - (*it2) );
		}
	}
	
	std::cout << DIST << std::endl;

  return 0;
}

**laserlight** · 03-25-2014

How does it not work? Like, what is your expected output and actual output? How did you arrive at your expected output?

**serge** · 03-25-2014

Originally Posted by laserlight

How does it not work? Like, what is your expected output and actual output? How did you arrive at your expected output?

The result should be:
abs(1.2-4.5)+abs(3.4-1.0)+abs(5.1-1.9)=3.3+2.4+3.2=8.9
whereas the program returns -14

**laserlight** · 03-25-2014

Originally Posted by serge

The result should be:
abs(1.2-4.5)+abs(3.4-1.0)+abs(5.1-1.9)=3.3+2.4+3.2=8.9
whereas the program returns -14

Right. Now look at your loops:

Code:

    for ( std::vector<double>::iterator it1 = VEC1.begin(); it1 != VEC1.end(); ++it1 ) {
        for ( std::vector<double>::iterator it2 = VEC2.begin(); it2 != VEC2.end(); ++it2 ) {
            DIST += abs( (*it1) - (*it2) );
        }
    }

VEC1 has 3 elements. VEC2 has 3 elements. Therefore, the inner loop runs 3 * 3 = 9 times, i.e., DIST is updated 9 times. According to you, however, DIST should only be updated 3 times, i.e., once for each of the three terms in your manual computation. Clearly, the use of nested loops here is a logic error.

**serge** · 03-25-2014

Ok, a stupid error. But then one cannot use one iterator on two arrays. So, what is the best way of doing it

**laserlight** · 03-25-2014

Use two iterators, but compare them with their respective end iterators and increment them in the same loop.

**manasij7479** · 03-25-2014

Or just use the [] operator, maybe?
I don't think being verbose with iterators has any advantage here.

Code:

for(int i=0;i<std::min(vec1.size(),vec2.size());++i)
    dist+=std::fabs(vec1[i]-vec2[i]);

**Elysia** · 03-25-2014

>>for(int i=0;i<std::min(vec1.size(),vec2.size());++i)
You should put an assert or a check before. If the vectors are not of the same size, it is an error (you cannot subtract two mathematical vectors that are not of equal length). You are just ignoring this error.
Furthermore, .size() does not return an int.

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