variable in if statement is undefined on some execution paths

function RA = fcn(density, V, L, Cb, Tf, Abt, B, T, hB, S)

if (Tf/L) <= 0.04

    c4=Tf/L;

elseif Tf/L>0.04

 c4=0.04;
end

c3 = 0.56*(Abt^1.5)/(B*T*(0.31*sqrt(Abt)+Tf-hB));

c2 = exp(-1.89*sqrt(c3));

CA=0.006*(L+100)^-0.16 - 0.00205+0.003*sqrt(L/7.5)*Cb^4*c2*(0.04-c4); RA = 0.5*density*V^2*S*CA;

Why i receive an error message about c4 is undefined on some execution paths ?? what is wrong with my If statement ??

Answer 1

[David Young]

You can change

elseif Tf/L>0.04

to

else

since if the first test fails the second must succeed. That should remove the warning. It's also a little faster, since it avoids an unnecessary division and comparison.

Alternatively, you could write

c4 = min(0.04, Tf/L);

instead of the whole if-statement.

[ameen]

What about this If statement :

if (B/L)<0.11

    c7=0.229577*((B/L)^0.33333);
elseif (B/L)>0.11 && (B/L)<0.25
    c7=(B/L);
elseif (B/L)>0.25
    c7=0.5-0.0625*L/B;
end

c1 = 2223105*(c7^3.78616)*((T/B)^1.07961)*((90-iE)^-1.37565);

I receive the same error message ??

[David Young]

Same solution - you can replace the final elseif with else. The first elseif should have the test (B/L) >= 0.11 otherwise the case that B/L is exactly equal to 0.11 is not captured.

[ameen]

Thank you but why my original code gives this error ؟

Answer 2

[John D'Errico]

Of course, the simple answer is just to use

c4 = min(Tf./L,0.04);

No if statements are needed at all in this simple case, and the result is efficiently vectorized.

As for the message that c4 is undefined on some paths, I'd need to see more information. You have not provided the complete message, nor have you given us sufficient context to even be able to replicate that warning.

What is telling you this fact? Is it mlint? Is it in the command window? Is it an error message of some form? What version of MATLAB are you using?

Lacking any of that information, I might conjecture that in SOME version of MATLAB, that mlint tells you this fact, due to the lack of an else statement as the last clause in your ifs. mlint will not be intelligent enough to know that for real numbers a, that

if A <= 1
  stuff
elseif a > 1
  stuff
end

actually resolves all REAL cases for a. Whereas this simple form must resolve all cases:

if A <= 1
  stuff
else
  stuff
end