How to avoid violating the DRY principle when the boolean expression of a while loop is reassigned every loop pass?

问题

Both

while MyFunction(1, 2, 3) > 0 do
  begin
    Temp := MyFunction(1, 2, 3);
    ShowMessage(IntToStr(Temp));
  end;

and

Temp := MyFunction(1, 2, 3);
while Temp > 0 do
  begin
    ShowMessage(IntToStr(Temp));
    Temp := MyFunction(1, 2, 3);
  end;

violate the DRY principle because there are two calls to MyFunction when only one is necessary.

回答1:

Easy,

  function dotemp(a,b,c:integer;var value : integer):boolean;
  begin
    value:a*b+c;
    result:=value>0; 
  end;

  begin
    while dotemp(a,b,c,temp) do
        begin
           Operateontemp(temp);
         end;
  end;

Pascal has no lvalue assignments, but it DOES have var parameters.

回答2:

I don't quite see the problem. Why don't you invert the logic and forget about Break:

repeat
  Temp := MyFunction(1, 2, 3);
  if Temp <= 0 then
  begin
    //code which uses Temp, for example:
    ShowMessage(IntToStr(Temp));
  end;
until Temp > 0;

That does exactly the same as you first sample, i.e. the while True do code snippet, but neither calls the function twice, nor does it need to use an assignment as an expression (not possible in Delphi), nor does it need to exit early with Break.

In C, you would do this with

do
{
    ...
} while (temp <= 0);

Note that every

if (condition) then 
  Break;

can be replaced by

if not (condition) then
  // rest of original loop.
end;

for the rest of the loop.

回答3:

There is the possibility to use an infinite loop and Break:

while True do
  begin
    Temp := MyFunction(1, 2, 3);
    if Temp <= 0 then Break;
    ShowMessage(IntToStr(Temp));
  end;

回答4:

Temp := 1;
while Temp > 0 do
begin
   //...
end;

Or repeat-until

来源：https://stackoverflow.com/questions/37928523/how-to-avoid-violating-the-dry-principle-when-the-boolean-expression-of-a-while

标签

while-loop

coding-style

pascal

lazarus

freepascal