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A student wants to swap a and b, and writes…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student wants to swap a and b, and writes
After the program, both variables hold 3. Explain what went wrong, line by line, and write a correct swap in two different ways.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Follow the four lines and look for the moment when a value is lost.
a = 7andb = 3. Nowais7andbis3.a = bputs the value ofb, which is3, intoa. The old value7is gone.b = aputs the value ofa, which is now3, intob. Sobgets3again.
Both hold 3. The 7 was lost in step 2, before we had saved it.
Two correct ways.
- With a third variable:
temp = a, thena = b, thenb = temp. - In one line:
a, b = b, a.
Check. After either way, a is 3 and b is 7.
Answer: The first value of a was overwritten before it was saved. Save it in temp first, or use a, b = b, a.
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