How to sort a 2D array by column

[monarch_dodra via Digitalmars-d-learn]

On Saturday, 7 June 2014 at 19:06:10 UTC, katuday wrote:
> 	the_table.sort; // I believe this sort uses all the columns 
> inside the_row. What I want is use specific column(s)

Supposing you use the *function* "std.algorithm.sort", then that 
is going to sort you rows according to the (default) predicate 
"<".

In this case, "<" means lexicographical comparison for arrays. 
EG: compares the first element, then the second, then the third, 
until 2 are different.

I don't have your input, but here is an example program:

import std.stdio, std.algorithm, std.random;

//----
void main()
{
     int[][] arr = new int[][](10, 10);
     foreach(i; 0 .. 10)
         foreach(j; 0 .. 10) {
             arr[i][j] = rndGen().front % 5;
             rndGen().popFront();
         }
     writefln("%(%s\n%)", arr.sort()); //Note the "()": Very 
important.
}
//----
[0, 1, 0, 0, 0, 0, 2, 3, 1, 1]
[0, 4, 4, 1, 2, 3, 4, 2, 1, 0] </--
[0, 4, 4, 1, 3, 4, 0, 2, 3, 3] </--
[2, 0, 0, 4, 1, 3, 3, 4, 2, 4]
[2, 0, 4, 3, 3, 0, 2, 4, 2, 1]
[3, 1, 2, 1, 1, 4, 1, 4, 1, 2]
[3, 3, 2, 1, 0, 4, 0, 2, 3, 2]
[4, 0, 0, 4, 3, 0, 4, 3, 4, 2]
[4, 2, 1, 1, 1, 2, 0, 1, 2, 2]
[4, 4, 3, 2, 2, 1, 1, 0, 4, 2]
//----
As you can see here, this was ordered by looking up to 5 elements 
deep.

If you need something more customize, you can do it by specifying 
your own function. For example, according to each row's Euclidean 
norm:

//----
bool compare(int[] lhs, int[] rhs) {
     auto a = reduce!"a += b^2"(0, lhs);
     auto b = reduce!"a += b^2"(0, rhs);
     return a < b;
}

void main()
{
     int[][] arr = new int[][](10, 4);
     foreach(i; 0 .. 10)
         foreach(j; 0 .. 4) {
             arr[i][j] = rndGen().front % 9 - 4;
             rndGen().popFront();
         }
     writefln("%(%2s\n%)", arr.sort!compare);
}
//----
[-2, -3, -1,  3]
[ 4, -2, -2, -2]
[-2,  0,  2, -4]
[ 3, -2, -4,  1]
[ 3, -2,  2,  3]
[-2,  3,  3,  0]
[ 1,  2,  0, -2]
[ 4,  2, -1,  0]
[ 2,  1,  1,  3]
[ 3,  3,  3,  4]
//----

In the above examples, I used numbers, but the language really 
doesn't care, as long as your elements have strict total ordering 
according to the predicate you have used.