The statements are executed if the real part of the expression
has all non-zero elements. The ELSE and ELSEIF parts are optional.
Zero or more ELSEIF parts can be used as well as nested IF’s.
The expression is usually of the form expr rop expr where
rop is ==, <, >, <=, >=, or ~=.
if I == J
A(I,J) = 2;
elseif abs(I-J) == 1
A(I,J) = -1;
A(I,J) = 0;
See also relop, else, elseif, end, for, while, switch.