Replacing the variable that are generated through loops ?

I have a code that generate the equation through loops :

Eqn(1)=4*K_T*P(1, 0)*P(4, 3) - 4*K_L*P(1, 0)*P(5, 4) + 4*K_T*P(2, 1)*P(3, 2) - 4*K_T*P(1, 0)*P(5, 4)

Eqn(2)= 2*K_T*P(3, 2)^2 + 4*K_T*P(1, 0)*P(5, 4) + 4*K_T*P(2, 1)*P(4, 3)

Please Note - In P(i,j) - i and j are not specifying any MATRIX position. They are numbers which are generated through some equation.P(i,j) are variables. There are many such variables. Now I want to replace them with some other variables.

So, my question is - Is it possible that the variables get changed to my desired variables, which i would specify ?

like P(1,0)==C(1)

P(3,2)==C(4)

thanks !!

[madhan ravi]

subs()?

[Walter Roberson]

To confirm, P(1,0) designates not P indexed at row 1 column 0, but rather a distinquished scalar variable P_1_0 and P(5,4) is a distinguished scalar variable P_5_4 rather than P indexed at row 5 column 4?

[P K]

Yes Walter.

Answer 1

[Walter Roberson]

B = 1;
P = sym('P', [B+5 B+4]);

Eqn(1) = 4*K_T*P(B+1, B+0)*P(B+4, B+3) - 4*K_L*P(B+1, B+0)*P(B+5, B+4) + 4*K_T*P(B+2, B+1)*P(B+3, B+2) - 4*K_T*P(B+1, B+0)*P(B+5, B+4);
Eqn(2) = 2*K_T*P(B+3, B+2)^2 + 4*K_T*P(B+1, B+0)*P(B+5, B+4) + 4*K_T*P(B+2, B+1)*P(B+4, B+3);

subs(Eqn, [P(B+1, B+0), P(B+3,B+2)], [C(1), C(4)])

[P K]

Walter

Can you explain it ?

In the Eqn(1) - your code has P(B+1, B+0), which would result P(2,1) as output instead of P(1,0).

[Walter Roberson]

It does not replace P(2,1) as output: it adjusts indexing bases from 0 to 1.

It would, however, replace P(1,0) with P_2_1 because P = sym('P', [B+5 B+4]); is one based. You can get around that and have P10 appear instead if you use

temp_nn = sym('P%d%d', [5, 4]);
temp_0n = sym({'P00', 'P01', 'P02', 'P03', 'P04'});
temp_n0 = sym({'P10'; 'P20'; 'P30'; 'P40'; 'P50'});
P = [temp_0n; [temp_n0, temp_nn]];