# istriu

istriu(A) -> Bool

Test whether a matrix is upper triangular.

## Examples

1. Check if a matrix is upper triangular:

``````julia> A = [1 2 3; 0 4 5; 0 0 6];
julia> istriu(A)
true``````

This example checks if the matrix `A` is upper triangular and returns `true`.

2. Handle non-upper triangular matrix:

``````julia> B = [1 2 3; 4 5 6; 7 8 9];
julia> istriu(B)
false``````

It returns `false` since the matrix `B` is not upper triangular.

3. Identify upper triangular property in a sparse matrix:
``````julia> using SparseArrays
julia> C = sparse([1 0 0; 4 5 6; 0 0 9]);
julia> istriu(C)
true``````

In this example, `istriu` correctly identifies the upper triangular property in the sparse matrix `C`.

Common mistake example:

``````julia> D = [1 2 3; 0 4 5; 6 7 8];
julia> istriu(D)
ERROR: MethodError: no method matching istriu(::Array{Int64,2})``````

In this example, the input matrix `D` is not a square matrix. The `istriu` function expects a square matrix as input. Make sure to provide a square matrix to avoid such errors.