how-to increment by 1 in informatica ?

Should be damn simple heh ?!
Well it is not , if you have not done it 100 times it WILL always behave differently ...
So ,

1. Define in the Designer a new mapping variable - obs the variable type
2. In the Transformaton , add a new variable port -
   with name: SET_VARIABLE_NAME
   and value: SETVARIABLE($$VARIABLE_NAME , PORT_OR_FUNCTION)

3. You could get the current value of the variable in the expression with the
SETVARIABLE($$VARIABLE_NAME , NULL ) call