Algol (1958)


  real procedure SquareRoot(x)l
    value x; real x;
    begin
      SquareRoot := 0.0;
      if x > 0.0 then
        begin real x, z;
          y := 1.0;
    loop: z := y;
          y := (x/z+z)/2.0
          if abs(y*y-x) >= 0.00001 then
            go to loop;
          SquareRoot := y
        end
      else if x < 0.0 then
        outstring(6, 'argument error.')
    end
  end SquareRoot