Piprimes: Difference between revisions

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 :* &nbsp; the OEIS entry: &nbsp; [http://oeis.org/A000720 A0000720 pi(n), the number of primes <= n. Sometimes called PrimePi(n)...].
 <br><br>
+=={{header|ALGOL 68}}==
+<lang algol68>BEGIN # Show some values of pi(n) - the number of priems <= n  #
+    # reurns a sieve of primes up to n #
+    PROC prime sieve = ( INT n )[]BOOL:
+         BEGIN
+            [ 1 : n ]BOOL p;
+            p[ 1 ] := FALSE; p[ 2 ] := TRUE;
+            FOR i FROM 3 BY 2 TO n DO p[ i ] := TRUE  OD;
+            FOR i FROM 4 BY 2 TO n DO p[ i ] := FALSE OD;
+            FOR i FROM 3 BY 2 TO ENTIER sqrt( n ) DO
+                IF p[ i ] THEN FOR s FROM i * i BY i + i TO n DO p[ s ] := FALSE OD FI
+            OD;
+            p
+         END # prime sieve # ;
+    # show pi(n) for n up to 21 #
+    INT max number = 100; # guess of how large the primes we need are #
+    INT max pi     = 21;
+    []BOOL prime   = prime sieve( max number );
+    INT pi        := 0;
+    FOR i TO max number
+    WHILE IF prime[ i ] THEN pi +:= 1 FI;
+          pi <= max pi
+    DO
+        print( ( " ", whole( pi, -2 ) ) );
+        IF i MOD 10 = 0 THEN print( ( newline ) ) FI
+    OD
+END</lang>
+{{out}}
+<pre>
+1  2  2  3  3  4  4  4  4
+5  6  6  6  6  7  7  8  8
+8  9  9  9  9  9  9 10 10
+11 11 11 11 11 12 12 12 12
+13 14 14 14 14 15 15 15 15
+15 16 16 16 16 16 16 17 17
+18 18 18 18 18 19 19 19 19
+20 21 21 21 21 21 21
+</pre>
 =={{header|REXX}}==