Add characters to field values if missing

[ZoeM]

hi!

I have a field that I would like to add characters to at the beginning if they are missing. Please see simple example below, the two missing entries should also have "SI-" added to the beginning. 

I would like to complete this using a Formula Tool.

Any help rendered truly appreciated. 

State	ID
MI	SI-1234
FL	5678
CA	SI-2345
NM	SI-7890
OK	8520
TX	SI-7412
MO	SI-6542
IL	SI-6548
WI	SI-9510

[DataNath]

Hey @ZoeM, here's how I'd go about this, leveraging the StartsWith() function:

The exclamation mark is uses as NOT and so in this expression we're essentially saying: if the ID doesn't begin with SI then add 'SI-' to the ID number. The else tells Alteryx to leave the ID as is if it doesn't satisfy the if condition i.e. does begin with SI.

[MilindG · Accepted solution]

See if this fixes your problem.

[binu_acs]

@ZoeM One way of doing this

[Emmanuel_G]

@ZoeM 

Find in attachement one way of doing that

[ZoeM]

While all solutions provided the same answer, I had to choose one :)