You should use dynamic memory when:
- If you want your object to persist beyond the scope in which it was created.
- Usually, stack sizes are limited and hence if your object occupies a lot of memory then you might run out of stack space in such cases one would usually go for dynamic memory allocation.
Note that c99 standard introduces Variable Length Arrays(VLA) in C so you need not use dynamic memory allocation just because you do not know the array dimensions before hand(unless ofcourse #2 mentioned above is the case)
It is best to avoid dynamic memory allocations as much as you can because it means explicitly managing the memory instead of the automatic mechanism provided by the language.Explicit memory management means that you are prone to make more errors, which might lead to catastrophic effects.
Having said that dynamic memory allocations cannot be avoided always and must be used when the use is imperative(two cases mentioned above).
