Why are these interations not working?

Hi there,

I am trying to perform simple iterations to solve a pair of simultaneous equations. However, I am not getting the correct results when using a for loop. My code looks like this:

syms x y

eq1 = 2*x + y == 16

eq1 = 

eq2 = 5*y - x == 25

eq2 = 

x1 = 16/2;
y1 = 25/5;

fyx = -1/2;
fxy = 1/5;

x2 = fyx*y1

x2 = -2.5000

y2 = fxy*x1

y2 = 1.6000

x3 = fyx*y2

x3 = -0.8000

y3 = fxy*x2

y3 = -0.5000

x4 = fyx*y3

x4 = 0.2500

y4 = fxy*x3

y4 = -0.1600

for i = 1:4
    x2 = fyx*y1
    y2 = fxy*x1;
    y1 = x2;
    x1 = y2;
end

x2 = -2.5000

x2 = 1.2500

x2 = -0.6250

x2 = 0.3125

Outside the loop the iterations are giving the correct answer. That is x2, x3, x4 = -2.50, -0.80, 0.250 respectively, and y2, y3, y4, 1.69, -0.50, -0.160 repectively.

But in the loop for x2, x3, x4 is 2.50, 1.250, -0.6250...

Am I doing something incorrect in my for loop to get these incorrect results?

Many thanks in adavance!

Answer 1

[Torsten]

x1 = 16/2;
y1 = 25/5;

fyx = -1/2;
fxy = 1/5;

x2 = fyx*y1

x2 = -2.5000

y2 = fxy*x1;
x3 = fyx*y2

x3 = -0.8000

y3 = fxy*x2;
x4 = fyx*y3

x4 = 0.2500

y4 = fxy*x3;

for i = 1:3
    x2 = fyx*y1
    y2 = fxy*x1;
    y1 = y2;
    x1 = x2;
end

x2 = -2.5000

x2 = -0.8000

x2 = 0.2500

[Scott Banks]

Thanks for that, Torsten!