5

I have a list of xi,yi points. I want to remove any points from the list where x^2+y^2<1. I probably have to write a for loop of some sorts, not sure how to begin. 

Subscript[x, i] = -2 + 0.2 m;
m = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 
19, 20};
Print[Subscript[x, i]];
Subscript[y, i] = .2 q;
q = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
Print[Subscript[y, i]]
a = {-1.8, -1.6, -1.4, -1.2, -1., -0.8, -0.6, -0.4, -0.2, 0., 0.2, 
0.4, 0.6, 0.8, 1., 1.2, 1.4, 1.6, 1.8, 2.};
b = {0.2, 0.4, 0.6, 0.8, 1., 1.2, 1.4, 1.6, 1.8, 2.};
Tuples[{a, b}];
pnts = {{-1.8, 0.2}, {-1.8, 0.4}, {-1.8, 0.6}, {-1.8, 0.8}, {-1.8, 
1.}, {-1.8, 1.2}, {-1.8, 1.4}, {-1.8, 1.6}, {-1.8, 1.8}, {-1.8, 
2.}, {-1.6, 0.2}, {-1.6, 0.4}, {-1.6, 0.6}, {-1.6, 0.8}, {-1.6, 
1.}, {-1.6, 1.2}, {-1.6, 1.4}, {-1.6, 1.6}, {-1.6, 1.8}, {-1.6, 
2.}, {-1.4, 0.2}, {-1.4, 0.4}, {-1.4, 0.6}, {-1.4, 0.8}, {-1.4, 
1.}, {-1.4, 1.2}, {-1.4, 1.4}, {-1.4, 1.6}, {-1.4, 1.8}, {-1.4, 
2.}, {-1.2, 0.2}, {-1.2, 0.4}, {-1.2, 0.6}, {-1.2, 0.8}, {-1.2, 
1.}, {-1.2, 1.2}, {-1.2, 1.4}, {-1.2, 1.6}, {-1.2, 1.8}, {-1.2, 
2.}, {-1., 0.2}, {-1., 0.4}, {-1., 0.6}, {-1., 0.8}, {-1., 
1.}, {-1., 1.2}, {-1., 1.4}, {-1., 1.6}, {-1., 1.8}, {-1., 
2.}, {-0.8, 0.2}, {-0.8, 0.4}, {-0.8, 0.6}, {-0.8, 0.8}, {-0.8, 
1.}, {-0.8, 1.2}, {-0.8, 1.4}, {-0.8, 1.6}, {-0.8, 1.8}, {-0.8, 
2.}, {-0.6, 0.2}, {-0.6, 0.4}, {-0.6, 0.6}, {-0.6, 0.8}, {-0.6, 
1.}, {-0.6, 1.2}, {-0.6, 1.4}, {-0.6, 1.6}, {-0.6, 1.8}, {-0.6, 
2.}, {-0.4, 0.2}, {-0.4, 0.4}, {-0.4, 0.6}, {-0.4, 0.8}, {-0.4, 
1.}, {-0.4, 1.2}, {-0.4, 1.4}, {-0.4, 1.6}, {-0.4, 1.8}, {-0.4, 
2.}, {-0.2, 0.2}, {-0.2, 0.4}, {-0.2, 0.6}, {-0.2, 0.8}, {-0.2, 
1.}, {-0.2, 1.2}, {-0.2, 1.4}, {-0.2, 1.6}, {-0.2, 1.8}, {-0.2, 
2.}, {0., 0.2}, {0., 0.4}, {0., 0.6}, {0., 0.8}, {0., 1.}, {0., 
1.2}, {0., 1.4}, {0., 1.6}, {0., 1.8}, {0., 2.}, {0.2, 0.2}, {0.2,
 0.4}, {0.2, 0.6}, {0.2, 0.8}, {0.2, 1.}, {0.2, 1.2}, {0.2, 
1.4}, {0.2, 1.6}, {0.2, 1.8}, {0.2, 2.}, {0.4, 0.2}, {0.4, 
0.4}, {0.4, 0.6}, {0.4, 0.8}, {0.4, 1.}, {0.4, 1.2}, {0.4, 
1.4}, {0.4, 1.6}, {0.4, 1.8}, {0.4, 2.}, {0.6, 0.2}, {0.6, 
0.4}, {0.6, 0.6}, {0.6, 0.8}, {0.6, 1.}, {0.6, 1.2}, {0.6, 
1.4}, {0.6, 1.6}, {0.6, 1.8}, {0.6, 2.}, {0.8, 0.2}, {0.8, 
0.4}, {0.8, 0.6}, {0.8, 0.8}, {0.8, 1.}, {0.8, 1.2}, {0.8, 
1.4}, {0.8, 1.6}, {0.8, 1.8}, {0.8, 2.}, {1., 0.2}, {1., 
0.4}, {1., 0.6}, {1., 0.8}, {1., 1.}, {1., 1.2}, {1., 1.4}, {1., 
1.6}, {1., 1.8}, {1., 2.}, {1.2, 0.2}, {1.2, 0.4}, {1.2, 
0.6}, {1.2, 0.8}, {1.2, 1.}, {1.2, 1.2}, {1.2, 1.4}, {1.2, 
1.6}, {1.2, 1.8}, {1.2, 2.}, {1.4, 0.2}, {1.4, 0.4}, {1.4, 
0.6}, {1.4, 0.8}, {1.4, 1.}, {1.4, 1.2}, {1.4, 1.4}, {1.4, 
1.6}, {1.4, 1.8}, {1.4, 2.}, {1.6, 0.2}, {1.6, 0.4}, {1.6, 
0.6}, {1.6, 0.8}, {1.6, 1.}, {1.6, 1.2}, {1.6, 1.4}, {1.6, 
1.6}, {1.6, 1.8}, {1.6, 2.}, {1.8, 0.2}, {1.8, 0.4}, {1.8, 
0.6}, {1.8, 0.8}, {1.8, 1.}, {1.8, 1.2}, {1.8, 1.4}, {1.8, 
1.6}, {1.8, 1.8}, {1.8, 2.}, {2., 0.2}, {2., 0.4}, {2., 0.6}, {2.,
 0.8}, {2., 1.}, {2., 1.2}, {2., 1.4}, {2., 1.6}, {2., 1.8}, {2., 
2.}};

I have included how I arrived to these set of points as a reference as well, incase it would be easier to include that directly into the loop as well, I took a more manual way to getting the specific list of points.

NonDairyNeutrino
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subrinarafiq
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3 Answers3

7

Use RegionMember:

Pick[pnts, RegionMember[Disk[{0, 0}, 1]][pnts], False] // Short

{{-1.8,0.2},{-1.8,0.4},{-1.8,0.6},<<159>>,{2.,1.6},{2.,1.8},{2.,2.}}

Carl Woll
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7

Use Select which will give you a list of points which satisfy your condition

pnts={...};
Select[pnts, Norm[#]>=1&]
Bill
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7

I would say the previous answers are better for your specific case of $x^2 + y^2 < 1$, but in the more general case I would use:

Select[pnts, #[[1]]^2 + #[[2]]^2 >= 1 &]

or 

Select[pnts, !#[[1]]^2 + #[[2]]^2 < 1 &]

where #[[1]] means the first element and #[[2]] means the second element in each pair of numbers (your x and y). I find that this syntax allows me to construct fairly complex selection criteria.

The two definitions I gave are equivalent, it just depends on which syntax makes the most sense to you - either take the opposite sign (>= instead of <) to get elements that are greater than or equal to, or simply ask for those elements that are "not lesser than".

MassDefect
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