How to sort a 2d array in cpp by values of two columns?

Question

I have a std::vector< std::vector<double> > array, the entries of which are

1 80 -0.15 -0.9 -0.15 0.6 0 -1.5 
1 81 -0.15 -0.9 -0.15 0.7 0 -1.6 
1 82 -0.15 -0.9 -0.15 0.8 0 -1.7 
1 83 -0.15 -0.9 -0.15 0.9 0 -1.8 
.
.
.
79 155 0.15 0.9 0.15 -0.9 0 1.8 
79 156 0.15 0.9 0.15 -0.8 0 1.7 
79 157 0.15 0.9 0.15 -0.7 0 1.6 
79 158 0.15 0.9 0.15 -0.6 0 1.5

Each row has 8 elements. I want to sort the array by the 7th and 8th element using the std::sort function as

auto sortfunc = [](vector<double> va, vector<double> vb){ return (va[7] < vb[7] ) && (va[6]< vb[6] ); };

sort(array.begin(),array.end(), sortfunc );

The result is not a completely sorted array

3 153 -0.15 -0.7 0.1 -0.1 -0.25 -0.6 
2 154 -0.15 -0.8 0.1 0 -0.25 -0.8 
2 153 -0.15 -0.8 0.1 -0.1 -0.25 -0.7 
2 152 -0.15 -0.8 0.1 -0.2 -0.25 -0.6 
7 153 -0.1 -0.7 0.1 -0.1 -0.2 -0.6 
7 154 -0.1 -0.7 0.1 0 -0.2 -0.7 
.
.
.
74 94 0.1 0.8 -0.05 -0.5 0.15 1.3 
74 95 0.1 0.8 -0.05 -0.4 0.15 1.2 
74 96 0.1 0.8 -0.05 -0.3 0.15 1.1 
74 97 0.1 0.8 -0.05 -0.2 0.15 1 
77 100 0.15 0.7 -0.05 0.1 0.2 0.6 
77 99 0.15 0.7 -0.05 0 0.2 0.7

This doesn't give me an array that is sorted by the given condition as the elements in 7th and 8th column doesn't appear in a particular order.

What am I doing wrong here?

Github Gist for the arrays is here

Your sort function only returns true if both values from the two columns are less. That's clearly not standard sorting rules. Surely you want to check if the first is less, and if it's not then maybe if it's equal then you check if the second is less. Think about how you would do this if you were looking at two values, and translate that logic onto paper. Then translate that into code. — paddy
– paddy, Commented Feb 11, 2021 at 9:53
(va[7] < vb[7] ) && (va[6]< vb[6] ) is not a strict weak ordering. You need to modify your condition to get a reliable result. — super
– super, Commented Feb 11, 2021 at 9:55

Galik · Accepted Answer · 2021-02-11 10:00:28Z

3

Your sort criteria looks off. I think you need something more like this:

auto sortfunc = [](std::vector<double> const& va, std::vector<double> const& vb)
{
    if(va[7] == vb[7])
        return va[6] < vb[6];

    return va[7] < vb[7];
};

Sort by the first column unless the first column is equal in which case sort according to the second column.

answered Feb 11, 2021 at 10:00

Galik

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Alan Birtles · Accepted Answer · 2021-02-11 10:21:43Z

sortfunc does not implement the requirements of Compare as it does not have a strict weak ordering. It therefore causes undefined behaviour when used with std::sort.

If you want to compare multiple values the easiest way is to use std::tuple which automatically compares the first value then only compares the second value if the first matches:

auto sortfunc = [](vector<double> va, vector<double> vb){ return std::tie(va[7], va[8]) < std::tie(vb[7], vb[8]); };

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