Prime number list checker

Ok so just for fun and practice I have been having a go at creating a prime number checker. I have been successful in making it work for a specific number. The code is here.

#To test if number is prime

prompt = input("number to test if prime: ");
n = prompt;
i = 2; #start of mod test
t = floor(sqrt(n));
counter = 0;
tic
for i = 2:t
  if mod(n,i) == 0 
    disp('n is not prime')
    break
  else
    counter = (counter + 1);
  end
end

if counter == t-1
  disp('n is prime')
end
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I then tried to make a program which would test a range of numbers. It's been successful for n=10, but when I go higher than that it doesn't seem to pick up the primes. I'm not sure where I'm going wrong.

#Want to test numbers 2:n if they're prime

prompt = input("max number to test: ");
n = prompt;
l = 2; #start of mod test
counter = 0;
tic

for i = 2:n #cycle to test 2 up to n
  t = floor(sqrt(i)) #Only need to test up to root of number
  for l = 2:t
    if mod(i,l) == 0 
      break
    else
      counter = (counter + 1);
      end
  end
  if counter == t-1 # if tested up to the root of the number, it must be prime
    prime = sprintf('%d is prime', round(i));
    disp(prime)
    counter = 0;
  end
end
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Any help in getting it to work for larger values would be greatly appreciated and also any ways to make the code more efficient. The top program can test 982451653 in 0.268 seconds on my laptop.

Answer 1

[James Tursa]

Your code for resetting "counter" is inside an if-test. You need to move it so that it always executes. E.g.,

%#Want to test numbers 2:n if they're prime

prompt = input('max number to test: ');
n = prompt;
l = 2; %#start of mod test
counter = 0;
tic

for i = 2:n %#cycle to test 2 up to n
  t = floor(sqrt(i)); %#Only need to test up to root of number
  counter = 0;  % <-- Moved from below
  for l = 2:t
    if mod(i,l) == 0 
      break
    else
      counter = (counter + 1);
    end
  end
  if counter == t-1 %# if tested up to the root of the number, it must be prime
    prime = sprintf('%d is prime', round(i));
    disp(prime)
%    counter = 0;   % <-- Moved to up above
  end
end
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[James Blackwell]

Thank you so much James, It has been bugging me all day. Can't believe it was something so simple!