How to add the iteration code in MATLAB

I wanna ask how to add the iteration code, because now the code I do is keep using the old value of xr.

for iteration = 1:3

iteration = 0;
xl = 4
xu = 6
xr =(xl+xu)/2
Xr = xr

syms a b xl fxl
E2 = sym('fxl = a*xl + b')
fxl = solve(E2,'fxl')
fXl = subs(fxl,{xl,a,b},{4,2,3})

syms a b X_r fxr
E3=sym('fxr = a*X_r+ b')
fxr = solve(E3,'fxr')
fXr = subs(fxr,{X_r,a,b},{Xr,2,3})

PreviousApprox = Xr
Test = fXl * fXr

              if Test <  0
                  Xu = xr
                  Xl = xl
                  Xr_new =(Xl +Xu)/2 
                  CurrentApprox = Xr_new
                   error=(CurrentApprox-Xr)/CurrentApprox*100

              elseif Test > 0
                  Xu = xu 
                  Xl = Xr
                  Xr_new=(Xl +Xu)/2 
                  CurrentApprox = Xr_new
                   error=(CurrentApprox-Xr)/CurrentApprox*100

              else
                  Test = 0
                   CurrentApprox = Xr_new
                   error=(CurrentApprox-Xr)/CurrentApprox*100
                  disp('we got the root value');
                  break

              end
              ApproxErrorPercentage(iterations)=abs((CurrentApprox-PreviousApprox)/(CurrentApprox)*100)

              end

[Alper Olca]

x1=5;
x2=5;
x3=5;
x4=5;
td=10^-2;
a=0;

for i= 1:10
    a=0+i;
    if( (abs(x1-x1)<td && abs(x2-x2)<td) && (abs(x3-x3)<td)&& abs(x4-x4)<td)
        x1=(-23+x2-x3+2*x4)/4;
        x2=(-21-2*x1+x3-3*x4)/6;
        x3=(-11+x1+2*x2-x4)/5;
        x4=(22+x1-2*x2+3*x3)/6;
    end
    
     k=(4*x1-x2+x3-2*x4);
     l=(2*x1+6*x2-x3+3*x4);
     m=(-x1-2*x2+5*x3+x4);
     n=(-x1+2*x2-3*x3+6*x4);
    end
rslt=[k l m n ; x1 x2 x3 x4]

Answer 1

[Cam Salzberger]

Hello Mavis,

In each loop, it seems like you use a previous value of xr to create a new value of Xr_new. Generally, users making use of this design pattern will either update xr to the value of Xr_new at the beginning or end of the loop, or simply only ever use xr. For example:

Xr_new = 1;
for k = 1:10
    xr = Xr_new;
    seeHowGoodXrIs(xr)
    Xr_new = updateValue(xr);
end

If you don't need to use the previous value more than once, it's generally easier to just do "in-place" updates to the value:

xr = 1;
for k = 1:10
   seeHowGoodXrIs(xr)
   xr = updateValue(xr);
end

Hope this helps!

-Cam