The Good Old Variable Swap, Redux
3rd
September 2022, 18:36
Some time back, I wrote a short piece, a how-to on the classic variable swap. Today is a good time to revisit that. As with the last time, code samples are in QBasic.
I was demonstrating this in a class, and one of the students had this bright idea: would this method work with multiplication and division, instead of addition and subtraction?
His idea was as follows.
And my answer was an unequivocal no. This is why.
In his example, a was 3 and b was 2. That would work. But consider this: what if either a or b was 0? Let's try both times.
Example 1: a = 0, b = 2
Example 2: a = 3, b = 0
In either case, at some point we are going to end up in a scenario where a division by zero would be attempted. And as we all know, once a division by zero is attempted, an exception occurs.
We could get around this limitation by doing a Try-catch, and assuming the value of 0 once the exception is thrown. But that would add a level of complexity to what is supposed to be a very simple formula.
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I was demonstrating this in a class, and one of the students had this bright idea: would this method work with multiplication and division, instead of addition and subtraction?
His idea was as follows.
DIM a = 3
DIM b = 2
a = a * b
b = a / b
a = a / b
DIM b = 2
a = a * b
b = a / b
a = a / b
And my answer was an unequivocal no. This is why.
In his example, a was 3 and b was 2. That would work. But consider this: what if either a or b was 0? Let's try both times.
Example 1: a = 0, b = 2
DIM a = 0
DIM b = 2
'a = 0 * 2 = 0
a = a * b
'b = 0 / 2 = 0
b = a / b
'a = 0 / 0
a = a / b
DIM b = 2
'a = 0 * 2 = 0
a = a * b
'b = 0 / 2 = 0
b = a / b
'a = 0 / 0
a = a / b
Example 2: a = 3, b = 0
DIM a = 3
DIM b = 0
'a = 3 * 0 = 0
a = a * b
'b = 0 / 0
b = a / b
a = a / b
DIM b = 0
'a = 3 * 0 = 0
a = a * b
'b = 0 / 0
b = a / b
a = a / b
In either case, at some point we are going to end up in a scenario where a division by zero would be attempted. And as we all know, once a division by zero is attempted, an exception occurs.
The dreaded zero.
We could get around this limitation by doing a Try-catch, and assuming the value of 0 once the exception is thrown. But that would add a level of complexity to what is supposed to be a very simple formula.
The takeaway
It was a nice try, and I like to encourage that. An experimental and curious mindset is always valuable to a programmer. What's important is that we always bear certain basics in mind.Happy pr0gramming,
