# Part 4: Sum and Element Constraints¶

We ask you not to publish your solutions on a public repository. The instructors interested to get the source-code of the solutions can contact us.

## Element¶

Implement Element1D.java

An element constraint is to index an array T by an index variable x and link the result with a variable z. More exactly the relation T[x]=z must hold.

Assuming T=[1,3,5,7,3], the constraint holds for

```x = 1, z = 3
x = 3, z = 7
```

but is violated for

```x = 0, z = 2
x = 3, z = 3
```

Check that your implementation passes the tests Element1DTest.java

Two possibilities:

1. extends Element2D and reformulate Element1D as an Element2D constraint in super call of the constructor.
2. implement a dedicated algo (propagate, etc) for Element1D by taking inspiration from Element2D.

Does your filtering achieve domain-consistency on D(Z)? Implement a domain-consistent version, write tests to make sure it is domain consistent.

## Element constraint with array of variables¶

Implement Element1DVar.java

We have already seen the element constraint to index an array of integers T by an index variable x and link the result with a variable z: T[x]=z. This time the constraint more general since T is an array of variables.

We ask you to imagine and implement the filtering algorithm for Element1DVar constraint. This filtering algorithm is not trivial, at least if you want to do it efficiently. Two directions of implementation are

1. The domain consistent version
2. The hybrid domain-bound consistent one, assuming the domain of z is a full range but not the domain of x in which you can create holes (you can start with this one, easier than the full domain consistent one).

Check that your implementation passes the tests Element1DVarTest.java Those tests are not checking that the filtering is domain-consistent. Write additional tests to check the domain consistency.

## The stable mariage problem¶

Complete the partial model StableMariage.java This model makes use of the Element1DVar constraint you have just implemented and is also a good example of manipulation of logical and reified constraints. Check that you discover the 6 solutions.