Setting the calculator to add or subtract

- When the user taps the plus or the minus button,…there are a few things that we want to happen.…First, we want the calculator to change modes.…So, it's either going to be in addition mode…or subtraction mode, when the plus…or the minus button is tapped.…But we also want to save the number…that was entered into the calculator…before that button was pressed.…For example, the user taps a one on the calculator,…and they're going to add something to the number one.…So after that they tap the plus button.…We're going to keep that one on the screen…and save it in the background.…

When the user taps a two to add two to one,…a two then shows up on the calculator,…not a 12, just a two, and that one…is then stored in the background.…So when the user taps the equals button,…two is then added to the number…that's saved in the background…through the savedNum variable.…So let's look at how that works.…I'll go inside a tappedPlus, and all we're…going to do in here is call changeMode,…passing in modes.ADDITION.…

I'll do the same thing for tappedMinus,…