# Creating a grid of coloured points

I wish to create a grid of 31x31 points with coordinates of the form $(k/30,n/30)$ within the unit square, and give each point a colour based on the 2-adic value of both its coördinates.

I plan to use Wolfram Mathematica, as this program has a simple function to calculate the 2-adic value, but I do not know how to generate a grid in which I can assign colours to the points. If we write $v(x)$ as the 2-adic value of $x$, the colouring should be as follows:

$(x,y)$ is coloured:

• Blue if $v(x) \geq v(y)$ and $v(x) \geq v(1)=1$
• Green if $v(x) < v(y)$ and $v(y) \geq v(1)=1$
• Red if $v(x) < v(1)=1$ and $v(y) < v(1)=1$

Can someone help me?

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• Please add any code you have tried. – bbgodfrey Sep 26 '15 at 14:01
• Take a look at ArrayPlot. – gwr Sep 26 '15 at 14:02

I don't know much about 2-adic values, hence I took the code for it from this mathworld article. Here are a couple of solutions to do it:

## Using Graphics

PadicNorm[x_Integer, p_Integer?PrimeQ] := p^(-IntegerExponent[x, p])
Graphics[{
Table[{PointSize@0.016,
Which[
v[k/30] >= v[n/30] && v[k/30] >= 1, Blue,
v[k/30] < v[n/30] && v[n/30] >= 1, Green,
v[k/30] < 1 && v[n/30] < 1, Red
],
Point[{k/30, n/30}]
},
{k, 0, 30}, {n, 0, 30}
]
}
]


Which produces the following: ## Using squares instead of points

Using appropriate substitutions on a ConstantArray and selectively changing the background color:

PadicNorm[x_Integer, p_Integer?PrimeQ] := p^(-IntegerExponent[x, p])
vx[j_] := PadicNorm[(j - 1)/30, 2]
vy[i_] := PadicNorm[(31 - i)/30, 2]
ReplacePart[
ConstantArray[0, {31, 31}],
{{i_, j_} :>
Item[" ", Background -> Blue] /; vx[j] >= vy[i] && vx[j] >= 1,
{i_, j_} :>
Item[" ", Background -> Green] /; vx[j] < vy[i] && vy[i] >= 1,
{i_, j_} :> Item[" ", Background -> Red] /; vx[j] < 1 && vy[i] < 1}
] // Grid


The functions vx and vy serve to easily convert between the numbering of the elements of a matrix and the required cartesian way of ordering coordinates. Here is the output: ## Output squares using Graphics

Slight modification of the previously used code with Graphics:

PadicNorm[x_Integer, p_Integer?PrimeQ] := p^(-IntegerExponent[x, p])
Graphics[{
Table[
{
Which[
v[k/30] >= v[n/30] && v[k/30] >= 1, Blue,
v[k/30] < v[n/30] && v[n/30] >= 1, Green,
v[k/30] < 1 && v[n/30] < 1, Red
],
Rectangle[{k/30, n/30}, {(k + 1)/30, (n + 1)/30}]
},
{k, 0, 30}, {n, 0, 30}
]
}
]


And this is the output: • Thank you very much, this is what I needed! Yet, is there a way to, instead of colouring adjacent squares, colour loose points in the unit square? – Pepijn Sep 26 '15 at 15:20
• @Pepijn of course. Sorry I didn't understand that that's what you needed. See the edit. – glS Sep 26 '15 at 15:26
• Thank you very much, this is exactly what I needed! – Pepijn Sep 26 '15 at 15:38