Trouble using while loop

I am trying to get the number of times the value tempv is 50 or higher, as well as the average of tempv, both using a while loop. I entered the below

clear, clc
tempv = [50, 60, 65, 30, 40, 66, 30, 55, 72, 72, 75, 50, 60, 65, 30, 40, ... 
            66, 30, 55, 63, 72, 75, 50, 60, 65, 30, 40, 66, 30, 55, 53];

% Write your code here:

n=0;
x=length(tempv);
sum=0;
i=0;
while i<31
    if tempv(i) >= 50 && tempv(i) <= 75
        n=n+1;
        fprintf('It was a warm day on May %i\n', i)
    end
sum=sum+tempv(i);
end
fprintf('In May the number of days the T was over 50 was: %i\n', n)
y=sum/x;
fprintf('The average temperature in Fahrenheit was:  %f\n', y)

However this returns the following error in the command window:

Subscript indices must either be real positive integers or logicals.

What is the error referring to here? Where is my mistake?

[per isakson]

See Debug a MATLAB Program

Answer 1

[Nick]

You are setting i = 0 and then trying to access tempv[0]. In matlab indexing starts with 1 not 0.