iPhone UX for entering 2 values and calculating 3rd?

Question

I'm trying to present a view on the iPhone which asks a user for any 2 of 3 values and using them to calculate the third value. I'm struggling a bit with the usability.

One option is to have three rows each with a label, textfield and button. Whichever row the tapped button is on is the value that will be calculated. I'm not terribly fond of that approach as it clutters the view with two many buttons.

I'd like to be able to use one button which would be doable if the user only ever entered 2 values. I could calc the empty value.

Another scenario would be to automatically clear out the third text field so it's always empty. (for example, enter Value 1, enter Value 2, clear Value 3, tap button and value 3 would be calculated) I think that users would struggle with having text fields cleared out like that.

Can any usability experts provide a nicer solution?

Answer 1

How about this:

1. Start out with all fields blank. User enters something in field A, it becomes "highlighted" (green border?).

2. User enters something in field B, it too becomes highlighted. At this point, field C is grayed out, and the OK/Cancel buttons become enabled.

3. Tap OK and field C value is calculated, then all fields enabled. (Maybe field C is highlighted in a different color to show what just happened.)

4. Tap cancel and all fields enabled. If you now start editing field C, field B becomes grayed out, OK/Cancel enabled.

Anyway, at any point you have last-two-fields-edited + 3rd grayed-out field.

Otherwise, take your initial solution but on each row, instead of a full-sized button, use a small icon to indicate "calculate." More clutter, but easier to understand.

Answer 2

Maybe get rid of the button?

Just use the last two entered values to determine the third continuously.

Would that work?

Answer 3

Make a custom text field with a little gray icon on the left-hand side which indicates that this is the calculated value.

Tap the left of any edit box to 'move' the calculated property.

Answer 4

UIPicker with two rows to choose, then the third field is calculated from whatever two are chosen whenever the picker changes?

I think it might help to have a little more detail on what kinds of things you are choosing and the possible number of items to choose from.