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Task/Successive-prime-differences/FreeBASIC/successive-prime-differences.basic

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+#include "isprime.bas"
+
+function nextprime( n as uinteger ) as uinteger
+    'finds the next prime after n
+    if n = 0 then return 2
+    if n < 3 then return n + 1
+    dim as integer q = n + 2
+    while not isprime(q)
+        q+=2
+    wend
+    return q
+end function
+
+function spd( byval n as integer, d() as integer ) as boolean
+    if not isprime(n) then return false
+    for i as integer = lbound(d) to ubound(d)
+        if not nextprime(n) = n + d(i) then return false
+        n+=d(i)
+    next i
+    return true
+end function
+
+sub print_set( byval n as uinteger, d() as uinteger )
+    print "( ";n;" ";
+    for i as integer = lbound(d) to ubound(d)
+        print n+d(i);" ";
+        n+=d(i)
+    next i
+    print ")"
+end sub
+
+function count_below( max as uinteger, d() as uinteger ) as uinteger
+    dim as uinteger c = 0, last = 0
+    for n as uinteger = 2 to max-d(ubound(d))
+        if spd(n, d()) then
+            c+=1
+            if c=1 then print_set( n, d() )
+            last = n
+        end if
+    next n
+    print_set(last, d())
+    return c
+end function
+
+dim as integer n, c
+
+'example 1, differences of 2
+redim as uinteger d(0)
+d(0) = 2
+print "Differences of 2 (the twin primes)"
+c = count_below(1000000, d())
+print "Number of occurrences: ", c
+
+'example 2, difference of 1
+d(0) = 1
+print
+print "Differences of 1"
+c = count_below(1000000, d())
+print "Number of occurrences: ", c
+
+'example 3, differences of 2,2
+redim as uinteger d(1)
+d(0) = 2 : d(1) = 2
+print
+print "Differences of 2, 2"
+c = count_below(1000000, d())
+print "Number of occurrences: ", c
+
+'example 4, differences of 2,4
+d(1) = 4
+print
+print "Differences of 2, 4"
+c = count_below(1000000, d())
+print "Number of occurrences: ", c
+
+'example 5, differences of 2,2
+d(0) = 4 : d(1) = 2
+print
+print "Differences of 4, 2"
+c = count_below(1000000, d())
+print "Number of occurrences: ", c
+
+'example 6, differences of 6,4,2
+redim as uinteger d(2)
+d(0) = 6 : d(1) = 4 : d(2) = 2
+print
+print "Differences of 6, 4, 2"
+c = count_below(1000000, d())
+print "Number of occurrences: ", c