ARM > Introduction to ARM > Task One - Answers
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Task One
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The routine turns an ASCII character lower-case. In C it would be written like this:
int mystery(int c)
{
unsigned int t;
t = c - 'A';
if (t <= 'Z' - 'A')
c += 'a' - 'A';
return c;
}
The tricky thing here is the coercion to unsigned int
which allows us to replace two comparisons with a single one. This trick, the unsigned range optimisation, is discussed in the Efficient C for ARM course.
We can write it in a more expected way like this:
int mystery2(int c)
{
if (c >= 'A' && c <= 'Z')
c += 'a' - 'A';
return c;
}