Undefined Function or Variable

In my code I have a nested if loop. However, I define a variable and then call it in a separate if statement and it says it is undefined when it is clearly defined above. Plese help me. My code is attached below along with the error code.

clc;
clear;
close all;

tic

% Initial Conditions

%%%%%%%%%%%%%% SOUNDING ROCKETS %%%%%%%%%%%%%%%%%

ML = 1                    ; % kg     - Payload Mass
% hmax = [10 20 30]*10^3    ; % ft     - Maximum Altitude
% amax = [5 10 20]          ; % NonDim - Normalized Max Acceleration
% SM = [1 2 3]              ; % NonDim - Static Margin
hmax = 20*10^3;
amax = 10;
SM = 2;
rhos = 2700               ; % kg/m^3 - Shell Density (Aluminum)
rhop = 1772               ; % kg/m^3 - Propellant Density
sigmas = 60*10^6          ; % Pa    - Shell Working Stress
N = 3                     ; % NonDim - Number of Fins
g = 9.81                  ; % m/s^2  - Acceleration due to Gravity

% Vertical Takeoff so we get;
theta = 0;
ct = cosd(theta);

% AT STP
T = 298                      ; % Kelvin        - Temperature
Pa = 101.325*10^3            ; % Pa            - Pressure

gamma = 1.4                  ;
R_gc = 287                   ; % J/kg*K        - Universal Gas Constant

a_ss = sqrt(gamma*R_gc*T)    ; % Meters/Second - Speed of Sound

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Part I
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Rmax = 1+amax                               ; % ND            - Mass Ratio
Weq = sqrt((hmax*g)./((log(Rmax)/2).* ...
      (log(Rmax)-2)+((Rmax-1)./Rmax)))      ; % Meters/Second - Equiv Vel.
tb = (Rmax-1)*Weq/(g*Rmax)                  ; % Seconds       - Burn Time

Meq = Weq/a_ss                              ; % ND            - Equiv Mach

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 2 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

P0_Pa = (1+Meq^2*((gamma-1)/2))^(gamma/(gamma-1)) ; % ND - Pressure Ratio
P0 = P0_Pa*Pa                                       ; % Pa - Pressure

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 3 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Guessing Value for D based on Diagram

D = 0.0922; % Meters

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 4 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

delta = D*P0/(2*sigmas) ; % Meters

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 5 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Guessing a lambda value:
lambdamax = 0;

% Guessing Value for L based on Diagram
while true
 syms Li X_cp X_cg     
 
 t=D*P0/(2*sigmas);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 6 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding the structural mass as a function of D, L, delta, rhos

Mn = delta*rhos*pi*D*(D+(sqrt(D^2+(D/2)^2))) ; % kg
Vf = 0.5*D*D*delta                           ; % Meters^3
Mf = 3*(rhos*Vf)                             ; % kg
Mfb = pi*D*rhos*D*delta                      ; % kg

Mcyl = (2*pi*(D/2)*(Li+D)+2*pi*(D^2/4))*rhos*delta      ; % kg
Mfin = (3/2)*D*rhos*delta                               ; % kg
Mcone = pi*(D/2)*((D/2)+sqrt(D^2 + (D^2/4)))*rhos*delta ; % kg

Ms = Mcyl+Mfin+Mcone                                    ;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 7 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding the Propellent Mass

R = Rmax                  ; % ND
Mp = ((R - 1)*(Ms + ML))  ; % kg

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 8 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding Lp

Lp = Mp/(pi*D^2*rhop/4) ; % Meters

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 9 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding the Center of Pressure 

Ckn = (4*N*(4/3))/(1+sqrt(6))                            ; % ND
Xcp = (1.33*D + Ckn*(D+Li+(D/3)))/(2+Ckn)                ; % Meters

% Finding the Center of Gravity

Xcgnum = Mcone*(2/3)*D + Mcyl*((Li+D)/2)+Mp*(2*D+Li-Lp+(Lp/2))...
         + Mfin*(D+Li+(1/3)*D);
Xcgden = Mcone+Mcyl+Mfin+Mp                              ;

Xcg = Xcgnum/Xcgden                                      ; % Meters

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 10 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

eqn1 = Xcp-Xcg-D*SM == 0                                 ; % Meters


% Finding the value for L
L1 =vpasolve(eqn1,Li);
L=L1(real(L1)>0);






%***************** Re-going through all non-symbolically *****************%








%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 6 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding the structural mass as a function of D, L, delta, rhos

Mn = delta*rhos*pi*D*(D+(sqrt(D^2+(D/2)^2)))   ; % kg
Vf = 0.5*D*D*delta                             ; % Meters^3
Mf = 3*(rhos*Vf)                               ; % kg
Mfb = pi*D*rhos*D*delta                        ; % kg

Mcyl = (2*pi*(D/2)*(L+D)+2*pi*(D^2/4))*rhos*delta       ; % kg
Mfin = (3/2)*D*rhos*delta                               ; % kg
Mcone = pi*(D/2)*((D/2)+sqrt(D^2 + (D^2/4)))*rhos*delta ; % kg

Ms = Mcyl+Mfin+Mcone                                    ;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 7 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding the Propellent Mass

R = Rmax                        ; % ND
Mp = vpa(((R - 1)*(Ms + ML)))   ; % kg

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 8 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding Lp

Lp = Mp/(pi*D^2*rhop/4)         ; % Meters

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 9 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Finding the Center of Pressure 

Ckn = (4*N*(4/3))/(1+sqrt(6))                            ; % ND
        
Xcpi = vpasolve((1.3333*D + (Ckn*(D+L+D./2)))/(2+Ckn) - X_cp==0);
Xcp = Xcpi(real(Xcpi)>0)               ; % Meters

% Finding the Center of Gravity

Xcgnum = Mcone*(2/3)*D + Mcyl*((L+D)/2)+Mp*(2*D+L-Lp+(Lp/2))...
         + Mfin*(D+L+(1/3)*D);
Xcgden = Mcone+Mcyl+Mfin+Mp            ;

Xcgi = vpasolve((Xcgnum/Xcgden)-X_cg)  ; % Meters
Xcg = Xcgi(real(Xcgi)>0)               ; % Meters

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Block 10 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Lpf = L+D                 ; % Meters
lambda = ML/(Ms+Mp)       ;
M0 = ML+Ms+Mp             ; % kg

if (L>D && Xcp>Xcg && Lp<Lpf)
    if (lambda >= lambdamax)
                Lnew = L                  ;
                Lpnew = Lp                ;
                Dnew = D                  ;
                tnew = t                  ;
                Xcpnew = Xcp              ;
                Xcgnew = Xcg              ;
                Msnew = Ms                ;
                M0new = M0                ;
                Mpnew = Mp                ;
                lambdamax_new = lambdamax ;
                lambdamax = lambda        ; 
                D = D+0.001               ;
    else
                Lp = Lpnew                ;
                t = tnew                  ; 
                Xcp = Xcpnew              ; 
                Xcg = Xcgnew              ; 
                L = Lnew                  ; 
                D = Dnew                  ; 
                lambdamax = lambdamax_new ; 
                Ms = Msnew                ;
                M0 = M0new                ;
                Mp = Mpnew                ;
        break                             ;
    end
else 
    
                Lp = Lpnew                ;
                t = tnew                  ; 
                Xcp = Xcpnew              ; 
                Xcg = Xcgnew              ; 
                L = Lnew                  ;
                D = Dnew                  ; 
                lambdamax = lambdamax_new ; 
                Ms = Msnew                ;
                M0 = M0new                ;
                Mp = Mpnew                ;
    break                                 ;
end

end

% Problem 1

 % Creating the Table 
        Impluse = Weq/g;
        m_dot = Mp/tb;
        Thrust = m_dot*Impluse*g

        pressure_R=P0/Pa
        tD_r= t/D
        t
        D
        L
        Lp
        ld_r =L/D
        Xcg
        Xcp
        Mp
        Ms
        M0
        lambdamax
        Eplison = lambda_m*M_s/(M_l)

toc

Below is the error:

Undefined function or variable 'Lpnew'.

Error in Project1Code (line 227)
                Lp = Lpnew                ;

Answer 1

[Walter Roberson]

if (L>D && Xcp>Xcg && Lp<Lpf)

That is false the first time through (because Lp > Lpf). Your code expects that it will be true the first time through, as you only assign to Lpnew when the condition is true.