lec4 Associative containers

Associative containers

Container Adaptors

Associative Containers

• map/set: keys in sorted order, faster to iterate through a range of elements
  • it’s object seems like need and only need less operator(<)
• unordered map/set: faster to access individual elements by key
• stl does not follow all the theory: overload all the operators
• use bool or integer to check whether the key is exit has no difference in performance

• use braket seems will initial the no exit key-value, get will throw a exception

• std::map<T>
• mymap.at[key] vs. mymap[key]
• Equivalent of Stanford .containsKey(key):
  • mymap.count(key)
  • And a (slightly faster) alternative that we’ll learn next lec
• std::set<T>
• A set is just a specific case of a map that doesn’t have a value!
  • or you can think of the value as being true(if present) or false
• Literally all the same functions are the c++ map, minus element access(except .get())

Iterators

• iterators allow iteration over any container, whether it is ordered or not
• iterators let us view a non-linear collection in a linear manner
  • we can get an iterator pointing to the ‘start’ of the sequence by calling mySet.begin()
  • we can get the value of an iterator by using the dereference * operator
  • we can advance the iterator one by using the ++ operator(prefix)
  • we can check if we have hit the end by comparing to mySet.end()
• for squential containers, iterators really are pointers under the hood(difference containers is different)
• why ++ in the left instead of right?
  • 前置++（如 ++iter）的操作是直接在原迭代器上进行加1操作，并返回更新后的迭代器。这种方式更高效，因为它不需要额外的拷贝操作来保存原始值。在大多数情况下，特别是当迭代器只是用来遍历容器时，前置++是首选，因为它可以避免不必要的开销。而后置++（如 iter++）则会在执行加1操作之前先返回迭代器的一个副本，这个副本是加1操作之前的原始值。这意味着后置++涉及到一次额外的临时对象创建和返回，因此在性能上相对较低效。这种形式主要用在需要先使用当前值再递增的场景中，尽管这种情况在迭代器遍历中并不常见。
• Summary
  • create iterator
  • Dereference iterator to read value currently pointed to
  • Advance iterator
  • Compare aginst another iterator(.end())
• why is powerful?
  • Many scenarios require looking at elements, regardless of what type of container is storing those elements
  • iterator let us go through sequences of elements in a standardised way