Further reading Related articles at NadeauSoftware. Nobody should ever release unoptimized code. The thinking is that programmers forget how big the destination array is and accidentally copy beyond the end of it. For mid-sized arrays, fast copying runs near the L1 cache speed and performance is high.
This is a very common code construct and should have been optimized.
See Algorithmic Complexity for details. To insert a key, value pair into the map, you can use operator: Benchmark results — compiled without optimizations The plot below shows performance for code compiled with GCC without optimizations.
QMap and QHash provide very similar functionality. For example, the following code snippet will create items in memory: For very small arrays, loop overhead dominates and performance is low. If you want to check whether the map contains a certain key, use contains: The ICC code hits an amazing If you call insert with a key that already exists in the QMapthe previous value will be erased.
QHash provides average faster lookups than QMap. Another way is to use QMutableMapIterator:: Additional, while the pointer loop green was optimized, its performance drops off suddenly at the Kbyte array test, which is odd.
If you want to navigate through all the key, value pairs stored in a QMapyou can use an iterator. In addition, you can clear the entire map using clear. Another approach is to call find to get the STL-style iterator for the first item with a key and iterate from there: However, these results are for code without any compiler optimizations.
Unoptimized code performance for the looping methods is extremely poor at 1. All tests were run on Linux using one core, and all tests were run multiple times and their results averaged.
A great compiler, like ICC, can optimize loops to do as well. One way is to call remove ; this will remove any item with the given key. Normally, a QMap allows only one value per key. The L1 cache supports two simultaneous CPU loads per clock cycle and an 8-byte bus for each load.
The reason is that operator silently inserts an item into the map if no item exists with the same key unless the map is const. Another way to insert items into the map is to use insert:This article benchmarks five common methods and four common compilers to find the fastest method to copy an array quickly.
QMap:: QMap (std::initializer_list > list) Constructs a map with a copy of each of the elements in the initializer list list. This function is only available if the program is being compiled in C++11 mode. I was just reading a Wikipedia article on Copy-on-write (curious if there are any filesystems that support it), and was surprised by the following passage: COW is also used outside the kernel, in.
Copy On Write ArrayList * @return an array containing all the elements in this list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws A Map implemented with ArrayLists: A variant of killarney10mile.comist in which all mutative.
From what I know, array is a hash map internally, which is excessive and inefficient for storing a simple array of elements.
If PHP had a class or programming construct similar to C++'s std::vector, it would be just great. This array never changes during the lifetime of the iterator, actions in a thread prior to placing an object into a CopyOnWriteArrayList happen-before actions subsequent to the access or removal of that element from the CopyOnWriteArrayList in another thread.
This class is a member of the Java Collections Framework. Since:Download