at([])

X[index]

X can be a scalar/vector(including tuple and array vector)/matrix/table/dictionary/pair/function.

index can be a Boolean expression/scalar/vector/tuple/array vector/pair.

If index is a Boolean expression, returns the elements in X that satisfy the condition specified by index (i.e., index = true). If not, returns the elements in X with the corresponding index. If X is a function, index is used as an argument of X. It is equivalent to function at.

Note: When index is a tuple (e.g., index = (0, 1) ), and X is a tuple of vectors, X[(0, 1)] returns the value at position 1 of the first element in X.

The following table shows the data forms supported by X and index.

x=5 7 0 4 2 3;
x[x>3];
[5,7,4]
x at x>3;
[5,7,4]

shares=500 1000 1000 600 2000;
prices=25.5 97.5  19.2 38.4 101.5;
prices[shares>800];
[97.5,19.2,101.5]

// returns business days between 2017.01.01 and 2017.01.07.
dates=2017.01.01..2017.01.07;
dates[x->x.weekday() between 1:5];
[2017.01.02,2017.01.03,2017.01.04,2017.01.05,2017.01.06]

// when index is specified as a tuple 
tp = [2.3 2.1 2.2, 3.1 2.9 2.8, 5.7 6.9]
tp at [1 2 3, 0 1 2]
([3.1,2.9,2.8],[5.7,6.9,],)

(sum)[1..10]
 55

(corr)[1 3 -1, 2 3 4]
 -0.5000

Related functions: eachAt