Indexing a value from a vector

I have a for loop where I need to index the vector element when the value exceeds 120, and display the number of iterations it took to reach that value. Can anyone help me? My code:

n=100;
p(1)=14.7;
for i=2:n-1
  p(i)=((patm*v1+p(i-1)*vtire)/(v2+vtire));
end
pg=p-patm

and of course all these variable are defined earlier in the script

Answer 1

[Wayne King]

If you don't know the number of iterations in advance, why use a for loop?

Here, I'll test when any element of p goes negative and the looking at the length of the vector, you'll know it occured in the length-1.

 n = 2;
 p(1) = 14.7;
 while all(p>0)
   p(n) = p(n-1)-0.01;
   n = n+1;
 end
 length(p)

[Nick]

The question asks to find how many pumps of the bike pump it takes to reach 120 psi. My loops goes simulates 101 pumps, I just need to index the pump number that makes it to 120 psi. It should be around the 24th pump

[Wayne King]

Then what Azzi suggests should work

[Nick]

It did'nt work, it just give me the values that are greater than 120. I just need the first value greater than 120, and the number of strokes

Answer 2

[Azzi Abdelmalek]

n=100;
p(1)=14.7;
for i=2:n-1
p(i)=((patm*v1+p(i-1)*vtire)/(v2+vtire));
end
pg=p-patm
idx=find(p>=120)