问题
To me they are both same and that is why i am wondering why we have dictionary data structure when we can do everything with arrays/list? What is so fancy in dictionaries?
回答1:
Arrays provide random access of a sequential set of data. Dictionaries (or associative arrays) provide a map from a set of keys to a set of values.
I believe you are comparing apples and oranges - they serve two completely different purposes and are both useful data structures.
Most of the time a dictionary-like type is built as a hash table - this type is very useful as it provides very fast lookups on average (depending on the quality of the hashing algorithm).
回答2:
Arraylists just store a set of objects (that can be accessed randomly). Dictionaries store pairs of objects. This makes array/lists more suitable when you have a group of objects in a set (prime numbers, colors, students, etc.). Dictionaries are better suited for showing relationships between a pair of objects.
Why do we need dictionaries? lets say you have some data you need to convert from one form to another, like roman numeral characters to their values. Without dictionaries, you'd have to hack this association together with two arrays, where you first find the position the key is in the first list and access that position in the second. This is terribly error prone and inefficient, and dictionaries provide a more direct approach.
回答3:
To build on what Andrew said, in some languages such as PHP and Javascript, the array can also function as a dictionary (known as associative arrays). It also comes down to loose v strict typing in the language.
回答4:
You could in theory do everything with dictionaries. But do not forget that at some point the program runs on a real machine which has limitations due to the hardware: processor, memory, nature of the storage (disc/SSD) ...
Behind the scenes the dictionaries are often using a Hash table
In some languages you can choose between many different types of list/array and hash tables as there are many different implementations of those structures, each with advantages and disadvantaged.
Use an array when you work with a sequence of elements or need to randomly access an element at a given index (0, 1, 2, ...)
Use a dictionary when you have key/value format and need fast retrieval via key
If you want to understand more about these I recommend you learn more about data structures as they are fundamental
NOTE: depending on the language the name of those structures may vary and is a source of confusion.
回答5:
The confusion lies in the different naming conventions in different languages. In my understanding, what is called a "Dictionary" in Python is the same as "Associative Array" in PHP.
来源：https://stackoverflow.com/questions/2695024/array-list-vs-dictionary-why-we-have-them-at-first-place