I am new in Dafny and getting some errors that I can not figure it out.
- in my Dafny program for insertionSort (the code is here), I do not understand why I get an
invalid logical expression
on While loop over variable i
. while (i < |input|)
- in the same code at the swapping part (
input[j := b]; input[j-1 := a];
) also I get expected method call, found expression
. According to the tutorial input[j:=b]
is replacing index j of seq input with the value of b
The first error is because you are declared a function
rather than a method
. In Dafny the body of a function
is expected to be an expression, not a sequence of statements. So when the parser sees the keyword "while", it realizes something is wrong (since "while" can't be part of a statement) and gives an error message. I'm not sure why the error message refers to a "logical" expression.
Anyway, you can fix this problem by declaring a method
rather than a function
.
You need a method because you are using an imperative algorithm and not a functional algorithm. It's true that you want a subroutine that computes its output as a function of its input with no side effects. But, in Dafny, you still use a method
for this when the way you want to do it involves imperative constructs like assignments and while loops.
The second problem is that input[j := b]
is an expression whereas the parser exepected a statement. You can fix this by rewriting input[j := b]; input[j-1 := a];
as input := input[j:=b]; input := input[j-1];
.
Unfortunately, that will lead to another problem, which is that, in Dafny, input parameters can't be assigned to. So you are better off making another variable. See below, for how I did that.
method insertionSort(input:seq<int>)
// The next line declares a variable you can assign to.
// It also declares that the final value of this variable is the result
// of the method.
returns( output : seq<int> )
// No reads clause is needed.
requires |input|>0
// In the following I changed "input" to "output" a few places
ensures perm(output,old(input))
ensures sortedBetween(output, 0, |output|) // 0 to input.Length = whole input
{
output := input ;
// From here on I changed your "input" to "output" most places
var i := 1;
while (i < |output|)
invariant perm(output,old(input))
invariant 1 <= i <= |output|
invariant sortedBetween(output, 0, i)
decreases |output|-i
{
...
output := output[j := b];
output := output[j-1 := a];
j := j-1;
...
}
}
By the way, since input parameters can't be changed, wherever you have old(input)
, you could just use input
. They mean the same thing.