Attempted to access sym(67); index out of bounds because numel(sym)=2 - matlab

Question

I am receiving the following error message:
Attempted to access sym(67); index out of bounds because numel(sym)=2.
I have been working on this for three days. I looked for similar error, but it didn't help. My code is below:
filename='DriveCyclesCP.xlsx';
V=xlsread('DriveCyclesCP.xlsx',2,'C9:C774'); % Get the velocity values, they are in an array V.
N=length(V); % Find out how many readings
mass = 1700 ; % Vehicle mass+ two 70 kg passengers.
area_Cd = 0.75; % Frontal area in square metres
Crr=0.009; %rolling resistance
g=9.8; % gravity acceleration
T=774; %UDDS cycle time duration
V_ave = 21.5; % UDDS avearage speed im m/s
rd=0.3; % Effective tire radius
Qhv =12.22; % E85 low Heating value in kWh/kg
Vd = 2.189; % engine size in L
md=0.801; % mass density of Ethanol
mf =Vd*md; % mf is the fuel mass consumed per cycle
Per = zeros(1,N); % engine power for each point of the drive cycle
a = zeros(1,N); % acceleration
SFC = zeros(1,N); % specific fuel consumption
Wc = zeros (1,N); % mass flow rate
nf = zeros (1,N); %fuel efficiency
Pm = zeros (1,N); % motor power
Pt = zeros (1,N);
Te =zeros (1,N); % Engine Troque
Tt = zeros (1,N);
Tm =zeros (1,N);
we =zeros (1,N); % Engine rot speed
wt = zeros (1,N);
wm =zeros (1,N);
S =zeros (1,8);
int (sym ('C'));
for C=1:N
a(C)=V(C+1)-V(C);
Pt(C)= V(C)*(mass*g*Crr + (0.5*area_Cd*1.202*(V(C))^2) + mass*a(C))/1000;
Per(C)=(mass*g*Crr +0.5*area_Cd*1.202*(V(C))^2 +mass*g*0.03)/1000*0.85;% e
syms Te(C) Tt(C) Tm(C) wt(C) we(C) wm(C) k1 k2
S = solve( Pm(C)==Pt(C) - Per(C), Tt(C)*wt(C)== Pt(C), Tt(C)*wt(C)== Te(C)*we(C) + Tm(C)*wm(C), wt(C)==we(C)/k1, wt(C)==wm(C)/k2, Pm(C)==wm(C) *Tm(C), Per(C)==we(C) *Te(C), Tt == k1*Te + k2*Tm );
end

Answer 1

The problem is on the line
int (sym ('C'));
You have defined sym to be a matrix with 2 entries somewhere (either earlier in the code or in a previous mfile), thus it treats sym as a matrix instead of a function. Thus when Matlab gets to the statement sym('C') it first converts the character 'C' to its ASCII integer representation (this just happens to be the number 67), then it tries to calculate sym(67) which is impossible as sym only has 2 elements.
Thus you have to stop sym from being a matrix (variable) and let it be a function again. There are two ways to solve this, either you can start you file with the statement clear;, this will remove all variables in memory, which might not be what you want; or you can use a function instead of script, as this hides all variables that have been defined previously and prevents this sort of error.
Note the line numel(X) is a way to measure how many elements are in X. Thus numel(sym)=2 means that sym has 2 elements.
P.S. There is an error in the lines (notice that I only taken some of the lines of you code)
N=length(V); % Find out how many readings
for C=1:N
a(C)=V(C+1)-V(C);
end
When C becomes equal to N, then V(C+1) will generate an error.