Variable Port Initialization - default values are not NULL

Variable Initialization 

The Integration Service does not set the initial value for variables to NULL. 
The Integration Service uses the following guidelines to set initial values for variables:

• Zero for numeric ports
• Empty strings for string ports
• 01/01/0001 for Date/Time ports

Therefore, use variables as counters, which need an initial value. For example, you can create a numeric variable with the following expression:
VAR1 + 1

This expression counts the number of rows in the VAR1 port. If the initial value of the variable were set to NULL, the expression would always evaluate to NULL. This is why the initial value is set to zero.

Execution Order of Ports in an Expression Transformation?

Port Order

All Ports in Expression Transformation are executed from Top to Bottom and the order of the ports execution is as follows.

1. Input ports. Informatica calculates all input ports first as they are not dependent on any other ports. So, we can create input ports in any order.
2. Variable ports. Variable ports can reference input ports and variable ports, but not output ports. As variable ports can reference input ports, the Informatica calculates variable ports after input ports. Likewise, since variable port can reference other variables, the display order for variable ports is the same as the order in which the PowerCenter Server evaluates each variable.

3. Output ports. As output ports can reference both input ports and variable ports, the Informatica evaluates output ports last. The display order for output ports does not matter, as output ports cannot reference other output ports. Output ports should appear at the bottom of the list of ports