Challenge 8 – Dynamic Dispatching, Dead Code and Floats
With electronic voting adding up to the heat of election nights, every citizen might be interested in getting some strong assurance that the machine is doing its job, especially since there might be a disclaimer that “no warranty is given on the exactitude of the next president chosen by the software”. Here, you have the source code to check it for yourself!
Have a look at the following code and see if you can spot the errors. Once you think you’ve got them all, click on the “Go CodePeer” button to see how well you matched up to CodePeer’s comprehensive and rapid analysis of the same code.
Error line 71: “warning: useless self assignment into Win”
Here’s how CodePeer helps you coming to this conclusion: CodePeer detects that Win is reassigned its own value. Indeed, Win is initialized to True on line 60, so assigning it to True on line 71 is at best useless, at worst an error. Here, it is the latter: Win should be assigned False, which got the more votes.
Error line 86: “warning: useless self assignment into Scores(Vote)”
Here’s how CodePeer helps you coming to this conclusion: Reviewing the corresponding line shows immediately the error: Scores(Vote) is assigned to itself! As we are counting votes here, Scores(Vote) should really be incremented, so that a “+1″ was meant.
Error line 106: “warning: dead code because Fraction – 5_452_595/8_388_608 in (-Inf..-1_258_291/8_388_608]”
Here’s how CodePeer helps you coming to this conclusion: CodePeer detects that the test for this branch of the if-statement is always false. Indeed, execution reaches this point only when the first test is false, so that Fraction is less than or equal to 0.5. Then, Fraction cannot be greater than 0.65. The order of the tests should be changed. The strange fractions mentioned in the warning come from the machine representation of 0.5 and 0.65, which are close to these real numbers, but not exactly equal. (5_452_595/8_388_608 = 0,649999976)
Error line 128: “high: precondition failure on voting.analyze: requires M.Score to be initialized”
Here’s how CodePeer helps you coming to this conclusion: Because function Analyze reads its parameter M.Score, it requires in its precondition that M.Score is initialized (output as a pseudo Ada attribute M.Score’Initialized). This should be set by the dispatching call to procedure Count. Looking at the two overriding procedures Count, none sets the value of component Score.
package Voting is
type Id is new Integer; -- machine summary of a human being
type Candidate_Pos is range 1 .. 20; -- number identifying a candidate
type Kind is (Referendum, Election); -- type of vote
type Ballot is private; -- human input
type Ballots is array (Id range <>) of Ballot;
type Machine (Population : Id) is tagged private; -- voting machinery
type Referendum_Machine is new Machine with private;
type Election_Machine is new Machine with private;
-- Store individual votes
procedure Get_Votes (M : in out Machine; Votes : Ballots);
-- Determine the winner of the vote by counting her supporters
procedure Count (M : in out Machine) is null;
procedure Count (M : in out Referendum_Machine);
procedure Count (M : in out Election_Machine);
-- Analyze the result of the vote for tomorrow's headlines
type Analysis is (Coalition, Majority, Landslide);
function Analyze (M : Machine) return Analysis;
private
type Ballot is record
Voter : Id;
Decision : Boolean;
Choice : Candidate_Pos;
end record;
type Machine (Population : Id) is tagged record
Votes : Ballots (1 .. Population);
Score : Positive;
end record;
type Referendum_Machine is new Machine with record
Winner : Boolean;
end record;
type Election_Machine is new Machine with record
Winner : Candidate_Pos;
end record;
end Voting;
package body Voting is
procedure Get_Votes (M : in out Machine; Votes : Ballots) is
begin
for Voter in Votes'Range loop
M.Votes (Voter) := Votes (Voter);
end loop;
end Get_Votes;
procedure Count (M : in out Referendum_Machine) is
Scores : array (Boolean) of Natural := (others => 0);
Win : Boolean := True;
begin
for B in 1 .. M.Population loop
declare
Vote : constant Boolean := M.Votes (B).Decision;
begin
Scores (Vote) := Scores (Vote) + 1;
end;
end loop;
if Scores (True) < Scores (False) then
Win := True;
end if;
M.Winner := Win;
end Count;
procedure Count (M : in out Election_Machine) is
Scores : array (Candidate_Pos) of Natural := (others => 0);
Win : Candidate_Pos := 1;
High : Natural := 0;
begin
for B in 1 .. M.Population loop
declare
Vote : constant Candidate_Pos := M.Votes (B).Choice;
begin
Scores (Vote) := Scores (Vote);
end;
end loop;
for C in Candidate_Pos loop
if Scores (C) > High then
Win := C;
High := Scores (C);
end if;
end loop;
M.Winner := Win;
end Count;
function Analyze (M : Machine) return Analysis is
Fraction : constant Float := Float (M.Score) / Float (M.Population);
begin
if Fraction > 0.5 then
return Majority;
elsif Fraction > 0.65 then
return Landslide;
else
return Coalition;
end if;
end Analyze;
end Voting;
with Voting; use Voting;
function Polls (K : Kind; Population : Id; Votes : Ballots) return Analysis
is
M : access Machine'Class;
begin
-- Initialize the machine
case K is
when Referendum => M := new Referendum_Machine (Population);
when Election => M := new Election_Machine (Population);
end case;
M.Get_Votes (Votes); -- Feed the machine with votes
M.Count; -- Count the votes
return M.Analyze; -- Prepare for the media frenzy over the results
end Polls;
See CodePeer in Action!
In this video, AdaCore engineer Yannick Moy walks you through this month’s challenge using CodePeer and GNAT Programming Studio, GNAT Pro IDE.