Find the position of all the prime numbers up to a given number

Question

I want Mathematica code the does what the following pseudocode does.

s = 0
for i = 1 to x
  if i is a prime number then 
    s = s + 1, print s 
  else  
    print "unspecified"

I tried

numPrime1[n_] :=  
 Block[{i, j, g, s, f}, s = 0; f = 0; g = 0; 
  For[i = 1, i <= n, i++,               
    For[j = 1, j <= i, j++,
      If[OddQ[i/j], f = f + 1]
      ]
     If[f == 2, s = s + 1, break]               
     If[OddQ[n/i], g = g + 1;]
    ]
   If[g == 2, Print[s], Print["Ungültig"];]]
numPrime1[17]

Now that just returns Null^3

Edit 1

Now it returns 1, but I don't know why it shouldn't be 1. Can someone give me advice to make that work?

Edit 2

Solved it now this way

numPrime1[n_] := Block[{i, j, g, s, f}, s = 0; f = 0; g = 0;
  For[i = 1, i <= n, i++,
   For[j = 1, j <= i, j++,
     If[IntegerQ[i/j], f = f + 1];] 
    If[f == 2, s = s + 1]; 
   If[IntegerQ[n/i], g = g + 1];
   f = 0]; 
  If[g == 2, Print[s], Print["Ungültig"]];]
numPrime1[17]

All my tests are now correct.

Answer 1

Mathematica has a lot of prime functionality built in. In particular PrimeQ[x] gives True if x is prime and false otherwise, and PrimePi[x] counts the number of primes less than or equal to x.

So, it seems like you want something like

numPrime[n_] := If[PrimeQ[n], PrimePi[n], Null]

so that numPrime[17]=7 and numPrime[21] is Null (or you could print something, throw an error, etc.)