## Example 3: Selection views based on conditions

Using condition functions (predicates), we can filter away uninteresting data and keep only interesting data. In physics codes, this process is often called cutting on a predicate.

## Conditions on 1-d matrices (vectors)

    // the naming shortcut (alias) saves some keystrokes:
    cern.jet.math.Functions F = cern.jet.math.Functions.functions;

    double[] v1 = {0, 1, 2, 3};
    DoubleMatrix1D matrix = new DenseDoubleMatrix1D(v1);
    // 0 1 2 3

    // view all cells for which holds: lower <= value <= upper
    final double lower = 0.2;
    final double upper = 2.5
    matrix.viewSelection(F.isBetween(lower, upper));
    // --> 1 2

    // equivalent, but less concise:
    matrix.viewSelection(
    	new DoubleProcedure() {
    		public final boolean apply(double a) { return lower <= a && a <= upper; }
    	}
    );
    // --> 1 2

    // view all cells with even value
    matrix.viewSelection(
    	new DoubleProcedure() {
    		public final boolean apply(double a) { return a % 2 == 0; }
    	}
    );
    // --> 0 2

    // sum of all cells for which holds: lower <= value <= upper
    double sum = matrix.viewSelection(F.isBetween(lower, upper)).zSum();
    // --> 3

    // equivalent:
    double sum = matrix.viewSelection(F.isBetween(lower, upper)).aggregate(F.plus,F.identity);

## Conditions on 2-d matrices

    // view all rows which have a value < threshold in the first column (representing "age")
    final double threshold = 16;
    matrix.viewSelection(
    	new DoubleMatrix1DProcedure() {
    		public final boolean apply(DoubleMatrix1D m) { return m.get(0) < threshold; }
    	}
    );

    // view all rows with RMS < threshold.
    // the RMS (Root-Mean-Square) is a measure of the average "size" of the elements of a data sequence.
    final double threshold = 0.5;
    matrix.viewSelection(
    	new DoubleMatrix1DProcedure() {
    		public final boolean apply(DoubleMatrix1D m) { return Math.sqrt(m.aggregate(F.plus,F.square) / m.size()) < threshold; }
    	}
    );

## Conditions on 3-d matrices

    // view all slices which have an aggregate sum > 1000
    matrix.viewSelection(
    	new DoubleMatrix2DProcedure() {
    		public final boolean apply(DoubleMatrix2D m) { return m.zSum() > 1000; }
    	}
    );