MaplePrimes - answers and comments on Question, if statement: Error, cannot determine if this expression is true or false

if condition

Markiyan Hirnyk — Tue, 04 Dec 2012 10:45:07 Z

It works after replacing U(tk) <= 4 by is (U(tk) - 4 <= 0).

PS. See http://www.mapleprimes.com/maplesoftblog/102311-More-Gems-From-The-Little-Red-Book-Of-Maple-Magic .

U not known

Preben Alsholm — Tue, 04 Dec 2012 20:13:52 Z

U as such is not known ever, but V is defined to be the value of U at tk.
Change 'if U(tk) <=4 to 'V <= 4 ´.

restart;
k := 0; B[1] := 0.001/365; B[2] :=0.002/365:
ode := diff(U(t), t) = -(A[1]+A[2]*U(t))*U(t);
ic[0] := U(365*k) = 1000;
sol[0] := dsolve({ic[0], subs(A[1] = B[1], A[2] = B[2], ode)}, U(t), numeric);
sigma := .5;
for k to 4 do
tk := 365*k;
V := rhs(sol[k-1](tk)[2]);
if V <= 4 then ic[k] := U(tk) =0
else ic[k] := U(tk) = sigma*V
end if;
sol[k] := dsolve({ic[k], subs(A[1] = B[1], A[2] = B[2], ode)}, U(t), numeric)
end do;

#Plot:

for k to 5 do p[k-1]:=plots:-odeplot(sol[k-1],[t,U(t)],365*(k-1)..365*k) end do:
plots:-display(seq(p[k],k=0..4),thickness=2);

To see the effect of the 'if' clause you can replace the 4 by e.g. 40.

FAIL

Preben Alsholm — Tue, 04 Dec 2012 20:17:59 Z

U is not known. The output from is(U(tk)-4<=0) is FAIL (try inserting a print(is(U(tk)-4<=0)); ).

Notice the result of

if FAIL then F else G end if;

You are right

Markiyan Hirnyk — Tue, 04 Dec 2012 21:07:09 Z

@Preben Alsholm Many thanks from me to you for the indication.

@Markiyan Hirnyk Thank you

reemeaaaah — Wed, 05 Dec 2012 11:45:16 Z

@Markiyan Hirnyk

Thank you

Don't

Markiyan Hirnyk — Wed, 05 Dec 2012 12:54:11 Z

@reemeaaaah mention it.