# Expressions of decision variables

The keyword dexpr allows you to create
reusable decision expressions. Indeed, if an expression has a particular
meaning in terms of the original problem, writing it as a decision
expression (`dexpr`

) makes the model more
readable.

For example, the `scalableWarehouse.mod`

example
expresses the total fixed costs as a decision expression:

```
dexpr int TotalFixedCost = sum( w in Warehouses ) Fixed * Open[w];
dexpr float TotalSupplyCost = sum( w in Warehouses, s in Stores ) SupplyCost[s][w] * Supply[s][w];
```

This way, the two total cost expressions defined are shown in the OPL IDE along with their values.

You can also use arrays of decision expressions. For example:

```
dexpr int slack[i in r] = x[i] - y[i];
```

This array is handled efficiently as only the “definition”
is kept. Not all the expressions for each value of the indices are
created. As a consequence, you cannot change the definition of the `dexpr`

for a particular element of the array.

Using decision expressions is particularly useful and recommended if you plan to write objectives to be used with ODM Enterprise. Please refer to the ODM Enterprise documentation.