Undefined function or variable

Hi, I was running a matlab script where I need to import a file and then make some computations. The problem is that when I import the first file, everything works properly, but when I import another file of the same format but with other values, I got the error messagge "undefined function or variable". Really, I don't why, because I haven't changed anything except the imported data.

[Ameer Hamza]

Can you give an example of a code that causes such an error?

[michela elia]

for i=loc1(1,1):1:loc(1,1)

if(O1P(1,i)>=(v(j)*pk(1,1)+pk1(1,1)))

a=i;

break

end

end

for i=loc(1,1):1:loc1(1,2)

if(O1P(1,i)<=(v(j)*pk(1,1)+ pk1(1,1)))

b=i;

break

end

end

the error is related to the variable b