For example:
mapping x; int y = 200; x = allocate_mapping(y);where y is the initial size of the mapping. Using allocate_mapping is the preferred way to initalize the mapping if you have some idea of how many elements the map will contain (200 in this case). The reason is that allocating storage all at once is slightly more memory efficient. Thus if you are using mappings to store a soul with 200 entries, the above initialization would be quite appropriate. Note, that the above initialization does not restrict you to 200 entries. It just that the first 200 entries will be stored more efficiently. Note: if you will be deleting many elements from the mapping, you should use x = ([]) to initialize the mapping rather than using allocate_mapping().