More BefunGen desc

www/data/programs/desc/BefunGen/03_Examples/02_Sieve of Eratosthenes.markdown

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+###Sieve of Erasthothenes
+
+Calculates the primes between 0 and 19200 with the sieve of Erasthothenes
+
+```textfunge
+program SieveOfEratosthenes : display[240, 80]
+	begin
+		// Init all Fields with '?'
+		init(); 
+		
+		// Set an 'X' to every Primenumberfield
+		calculate();
+		
+		// Output the primes
+		output();
+	end
+	
+	void init()
+	var 
+		int i;
+	begin
+		for(i = 0; i < DISPLAY_SIZE; i++) do
+			display[i % DISPLAY_WIDTH, i / DISPLAY_WIDTH] = '?';
+		end
+		
+		display[0, 0] = ' ';
+		display[1, 0] = ' ';
+	end
+	
+	void calculate()
+	var 
+		int i;
+	begin
+		for(i = 2; i < DISPLAY_SIZE; i++) do
+			doNumber(i);
+		end
+	end
+	
+	void doNumber(int i) 
+	var
+		char c;
+	begin
+		c = display[i % DISPLAY_WIDTH, i / DISPLAY_WIDTH];
+		
+		if (c == 'X' || c == ' ') then
+			return;
+		elsif (c == '?') then
+			display[i % DISPLAY_WIDTH, i / DISPLAY_WIDTH] = 'X';
+			
+			clear(i);
+		end
+	end
+	
+	void clear(int n)
+	var
+		int p;
+	begin
+		for(p = 2*n; p < DISPLAY_SIZE; p += n) do
+			display[p % DISPLAY_WIDTH, p / DISPLAY_WIDTH] = ' ';
+		end
+	end
+	
+	void output()
+	var
+		int i;
+	begin
+		out "Prime Numbers:\r\n";
+		for(i = 2; i < DISPLAY_SIZE; i++) do
+			if (display[i % DISPLAY_WIDTH, i / DISPLAY_WIDTH] == 'X') then
+				outf i, "\r\n";
+			end
+		end
+	end
+end
+```
+
+> **Note:** *This and other examples are included in the BefunGen download*