CAS command question

[Han]

(01-04-2017 12:56 PM)compsystems Wrote:  Hello Mr. Parisse

c:=a-b; [enter] Internally is a+neg(b) // ok

It's new to my, extracting parts of the expression using []
c[1]; -> +
c[2]; -> a
c[3]; -> b

Please explain, the correspondence of the position or numeral to extract the parts. thanks

Parisse might have more to add, but the expression[index] format is just a "list" starting with an operator followed by the arguments. For example:

eq1:=t^2+2*t-cos(t)/sin(2*t);

eq1[1]; -> '+' (operator)
eq1[2]; -> 't^2' (argument 1)
eq1[3]; -> '2*t' (argument 2)
eq1[4]; -> '-cos(t)/sin(2*t)' (argument 3)

Each argument can also be an expression that is stored similarly.

eq1[2,1]; -> '^' (seems the Prime doesn't display this; but copy it to the command line and you see an exponent)
eq1[2,2]; -> 't'
eq1[2,3]; -> 2