Program to calculate the nth Prime Number

I am attempting to write a program that accepts a user input for the value of n, and returns the nth prime number. I.e. if n = 20, the output is 71. I have currently written the following code, however it currently returns 0 when n is greater than 2, but I'm not sure why this is?

number = input('Enter a value for n: ');
if (number<0) 
    error('Only positive integer values are accepted!');

primes = zeros(1,number);
primes(1) = 2;
primes(2) = 3;
checkThis = 5;
i=3;

if (i<=number)
    x=round(sqrt(checkThis));
    checkRange=1:2:x;
    disp(checkRange);

      for j=1
          if(mod(checkThis,checkRange(j))==0)
              itIsPrime=0;
          else
              itIsPrime=1;
              disp(itIsPrime);
              j=length(checkRange);
          end
      end

      if (itIsPrime==1)
          primes(i)=checkThis;
          i=i+1;
      else
          checkThis = checkThis + 2; 
      end
  end
  disp(primes(number));

[madhan ravi]

 _I.e. if n = 20, the output is 71_

mind explaining the pattern because the idea is not discernible

[Walter Roberson]

madhan: for example,

t = primes(100);    %all primes up to and including 100
t(20)               %will be 71 -- so 71 is the 20th prime.

[madhan ravi]

Thank you sir Walter , understood now

[Liam Su]

please show how you output the nth prime

[Walter Roberson]

  disp(primes(number));

is the line of code that is outputing the nth prime to the display.

Answer 1

[Walter Roberson]

      checkRange=1:2:x;

That always starts with 1

      for j=1

That is a loop with j only equal to 1.

          if(mod(checkThis,checkRange(j))==0)

When j = 1, checkRange(j) is checkRange(1) which is 1, so you are taking mod(checkThis, 1) which is going to equal 0 for all integers checkThis . So you mark them all as non-prime

              itIsPrime=0;
          else
              itIsPrime=1;
              disp(itIsPrime);
              j=length(checkRange);

Caution: j is the for loop control variable. If you modify a for loop control variable within the for loop, then if there were any iterations remaining, then the next iteration, the loop control variable will be set to the next loop value, as if the variable had not been modified. Setting the loop control variable to the last value it would normally have looped to, does not have the effect of exiting the loop. If you want to exit the loop early, use break

Answer 2

[madhan ravi]

Do you even realise that there is an inbuilt function in Matlab to do that? as sir Walter already gave the answer?? Also don't use the variable name as primes because it's an inbuilt function of Matlab

clear all
t = primes(100);   
n = input('nth value? ')
t(20)