The purpose of Evaluate Expression Tool in IWS is to provide users the ability to preview the functions output without having to actually execute the solution.
To demonstrate this feature, we will walk through the creation of a generic transform solution using the Transform and Generic Load template.
From New Solution, select Templates, select the Transform and Generic Load and enter a unique name for the solution. Click OK.
In this example, the Source is RAW format and the Destination is Delimited File (by comma separator). Click Apply. 
The worksheet describing the t-stream is created. 
If your worksheet view differs from the above screenshot, you can access this view by double clicking the translator file in the solution explorer: 
Choose a field using functions and click the 
icon: 
This will bring up the Evaluate expression pop-up window: 
You can now enter your functions manually or double-click their names in the bottom-right list of functions to create required expression.
The top-right box contains variables used in current solution which can be also added to the expression via double click. The window also contains several tooltips to guide you.
When the expression is composed, click the 
button to preview:
When you are satisfied with the result, click OK to save changes to the field.
You may also choose to copy the expression for further use elsewhere.
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