There must be Error using fprintf Unable to convert \'sym\' value to \'double\'.

Question

There must be Error using fprintf
Unable to convert 'sym' value to 'double'.

Error in Quiz2pt2 (line 65)
fprintf('There must be %4.2f moles of NH3 produced to raise the blood pH back to a normal
7.4. ',Z(1));

Why am I receiving this error??

Code:

%% Analyze:

% Assume: Open, Reactive, Dynamic

% Basis/Given:

Ka=5.6*10^-10; %M

Vblood= 5000; %mL

pHi=7.2;

pHf=7.4;

mAo=(60/100)*Vblood; %ug

MWnh4=18039000 %um/mol

nAo=mAo/MWnh4

nHo=10^(-pHi)*Vblood;

nHf=10^(-pHf)*Vblood;

% HA: nHAq=nHAf-nHAo

% H: -nHc=nHf-nHo

% A: nAi-nAc=nAf

%% Calculate:

syms nHAo Kap nHo nAo

E1='Kap=Vblood*Ka';

E1a='Kap =((nHo*nAo)/nHAo)';

E1b='nAo=(60/100)*Vblood';

E1c='nHo=10^(-pHo)*Vblood';

X=solve(E1,E1a,E1b,E1c,nHAo,Kap,nHo,nAo);

nHAo=X.nHAo;

%n.Hi-n.Hc=n.Hf-n.Ho

%-n.Ac=n.Af-n.A0

%n.HAg=n.HAf-n.HAo

%Kap=(n.hf*n.Af)/n.HAf

%n.Hc=n.Ac

%n.Ac=n.HAg

Kap=Vblood*Ka;

nAo=(60/100)*Vblood;

nHo=10^(-pHi)*Vblood;

M=[1,-1,0,0,0,0;

   0,0,-1,-1,0,0;

   0,0,0,0,1,-1;

   0,0,0,-nHf,0,Kap;

   0,1,-1,0,0,0;

   0,0,1,0,-1,0];

b=[nHf-nHo;

-nAo;

-nHAo;

0;

0;

0];

Z=M;

Z(1)

%% Finalize:

fprintf('There must be %4.2f moles of NH3 produced to raise the blood pH back to a normal 7.4. ',Z(1));

Explanation / Answer

Following reasons for the error and it's solution
1) Your input variable contains lots of symbolic variables and -ve exponent values.
To solve this, you can use VPA function(variable precision arithmetic)
for example-
R=[a/c a^-3]
M=[cos(c) sin(c/9); exp(c*a) (a*c)/log(c)]
Z=(vpa(R)/vpa(M))
2) To solve a high-degree polynomial expression using "solve".
In your case-
nHo=solve(10^(-pHi)*Vblood);
nHf=solve(10^(-pHf)*Vblood);
After that you should use VPA function to print final result.

3) In your program you are trying to convert high-degree polynomial expression into doule type array. And in matlab you can not convert symbolic expression into double tyoe array.
Solution to this proble is- Either you convert all the values of variables into non-symbolic expression using VPA function first and, then calculate value of Z in your final solution. or first use "eval" function to calculate the expression for each variables and then use vpa function. For example- R=[eval(a/c a^-3)]
M=[cos(c) sin(c/9); exp(c*a) (a*c)/log(c)]
Z=(vpa(R)/vpa(M))

4) If all above solution is not working, you can try to print value of Z by directly converting into double value (You will get result, but in this method you will loose some decimal data)
for example-
R=[a/c a^-3]
M=[cos(c) sin(c/9); exp(c*a) (a*c)/log(c)]
Z=double(vpa(R)/vpa(M))
fprintf('%4.2f',Z);

Please let me know if you are still facing this getting this error.

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