is_tril.hhs

/**
 * @author Jason Reynolds
 * 
 * @param input - the matrix to be check if its lower triangular or not
 * @returns - boolean value true or false depending on if its determined to be lower triangular or not 
 * 
 * Function : is_tril - a boolean function to determine if a square matrix is lower triangular or not. Counter part to is_triu (upper triangular)
 * 
 * 
 */



function is_tril(input) {

    *import math: ndim
    *import math: deep_copy

    if (arguments.length === 0) {
        throw new Error('Exception occurred in is_tril - no argument given');
    }
    else if (arguments.length > 1) {
        throw new Error('Exception occurred in is_tril - wrong argument number');
    }

    if (!(Array.isArray(input)) && !(input instanceof Mat) && !(input instanceof Tensor)) {
        throw new Error('Exception occurred in is_tril - input must be a 2-dimensional JS array, matrix or tensor');
    }

    //declare raw_in to be input.clone() if Mat otherwise input itself
    let in_type = (input instanceof Mat) || (input instanceof Tensor);
    let raw_in = (in_type) ? input.clone() : deep_copy(input);

    //degrade raw_in to JS array for generalized code
    if (in_type) {
        raw_in = raw_in.val;
    }
    //check: any dimension 0? square matrix? 2d? if so, throw error 
    if (!(ndim(raw_in) === 2) || raw_in.length === 0 || raw_in[0].length === 0 || !(raw_in.length === raw_in[0].length)) {
        throw new Error('Exception occurred in is_tril - wrong dimensions, need a square 2d, non empty matrix as input');
    }

    //iterate through array and if you spot a non-zero entry in a spot in the lower triangular region, return false
    //cut a sub matrix in the lower triangular region
    for (let k = 1; k < raw_in.length; k++) {
        for (let h = 0; h < raw_in.length - 1; h++) {
            //when in the tringaular zone exactly
            if (h < k) {
                if (!(raw_in[h][k] === 0)) {
                    return false;
                }
            }
        }
    }
    //if it goes through the whole loop just fine then all the lower triangular entries are zero
    return true;
}