0
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So, now that we've managed to safely convert whatever text is inside our displayLabel to a Double and
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store it inside this constant number, then the next step is to use that number and perform the calculation
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that triggered this calcButtonPressed, so we can figure out the calculation method by tapping into
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the sender.currentTitle.
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So this will be the string that corresponds to whichever of these buttons were pressed.
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And remember again, currentTitle is an optional string,
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so we're going to use optional binding, again, to say, if let calc Method = sender.current title,
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then we're going to say if the calcMethod is equal to the text of the button that is "+/-"
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then in that case, we want to flip the sign of the number.
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So we're going to set the displayLabel.text to equal on number that we've converted over here, and
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we're going to multiply it by minus 1.
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So that means that if the number that we entered was 8 then when we press this button it should turn
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it into minus 8 by multiplying it by minus 1.
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Now, the math's workout, but we have an error here that says, "Cannot assign value of type "Double to type
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'String?'
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So this right-hand side of the equation is going to be a Double because it's a Double multiplied by
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a Double,
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and the left-hand side, however, is expecting a string-- is expecting a string because it's the text for
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the displayLabel.
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So in this case, we're going to have to cast this Double into a String, and as we mentioned before,
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this never fails. You can always turn a number into a String.
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So, now if we run our app and test it, you can see that if I type a number, say, 89, and I press this button,
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then it becomes minus 89. And if I press it again, it becomes plus 89 again.
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So, now that we've got this working, here's a challenge for you.
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I want to make these other two buttons work.
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Remember that whenever I press the AC button, it should revert the display back to zero.
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And whenever I press the percentage button, it should take the current value and divide it by 100.
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So once you're ready, pause the video and try to complete that challenge.
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Okay. So, now that we've really got one of these methods established, it's not so hard to write out the code
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for the other ones. So let's add an "else if" and say if the calcMethod is, instead, equal to "AC" then that
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means that we actually want our displaLabel.text to simply just read "0."
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So we're going to add a string that is "0" and that already sorts out that particular case.
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Now, finally, if we have another calcMethod that was equal to the percentage sign, then in this case, we
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simply want to divide the number by 100, or alternatively, if you wish,
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you can also multiply it by  0.01, it does the same thing.
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So, in this case, we're going to set the displayLabel.text =  number * 0.01,
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and we, of course, have to turn this Double into a String.
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So, now let's run our app and let's say, 89, as a percentage is 0.89,
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and then if we turn it into a negative and we press the AC button to clear our screen, and you can see everything
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is working.
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So, now we sorted out these top three buttons.
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Now, there's just one slight problem.
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You see we've got this decimal point here.
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And if I wanted to write 2.3, then this becomes a double
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that is 2.3.
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But I can also press this button again and I've got 2.3.6 which is not a mathematical
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number.
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I don't even know what that is.
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So we have to ensure that this doesn't happen.
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And that is what we're going to address in the next lesson.