Re: English Dictionary App
Katja Gains
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Remove the item at the given position in the list, and return it. If no indexis specified, a.pop() removes and returns the last item in the list.It raises an IndexError if the list is empty or the index isoutside the list range.
The optional arguments start and end are interpreted as in the slicenotation and are used to limit the search to a particular subsequence ofthe list. The returned index is computed relative to the beginning of the fullsequence rather than the start argument.
List comprehensions provide a concise way to create lists.Common applications are to make new lists where each element is the result ofsome operations applied to each member of another sequence or iterable, or tocreate a subsequence of those elements that satisfy a certain condition.
A list comprehension consists of brackets containing an expression followedby a for clause, then zero or more for or ifclauses. The result will be a new list resulting from evaluating the expressionin the context of the for and if clauses which follow it.For example, this listcomp combines the elements of two lists if they are notequal:
There is a way to remove an item from a list given its index instead of itsvalue: the del statement. This differs from the pop() methodwhich returns a value. The del statement can also be used to removeslices from a list or clear the entire list (which we did earlier by assignmentof an empty list to the slice). For example:
As you see, on output tuples are always enclosed in parentheses, so that nestedtuples are interpreted correctly; they may be input with or without surroundingparentheses, although often parentheses are necessary anyway (if the tuple ispart of a larger expression). It is not possible to assign to the individualitems of a tuple, however it is possible to create tuples which contain mutableobjects, such as lists.
Though tuples may seem similar to lists, they are often used in differentsituations and for different purposes.Tuples are immutable, and usually contain a heterogeneous sequence ofelements that are accessed via unpacking (see later in this section) or indexing(or even by attribute in the case of namedtuples).Lists are mutable, and their elements are usually homogeneous and areaccessed by iterating over the list.
A special problem is the construction of tuples containing 0 or 1 items: thesyntax has some extra quirks to accommodate these. Empty tuples are constructedby an empty pair of parentheses; a tuple with one item is constructed byfollowing a value with a comma (it is not sufficient to enclose a single valuein parentheses). Ugly, but effective. For example:
This is called, appropriately enough, sequence unpacking and works for anysequence on the right-hand side. Sequence unpacking requires that there are asmany variables on the left side of the equals sign as there are elements in thesequence. Note that multiple assignment is really just a combination of tuplepacking and sequence unpacking.
Python also includes a data type for sets. A set is an unordered collectionwith no duplicate elements. Basic uses include membership testing andeliminating duplicate entries. Set objects also support mathematical operationslike union, intersection, difference, and symmetric difference.
Curly braces or the set() function can be used to create sets. Note: tocreate an empty set you have to use set(), not ; the latter creates anempty dictionary, a data structure that we discuss in the next section.
It is best to think of a dictionary as a set of key: value pairs,with the requirement that the keys are unique (within one dictionary). A pair ofbraces creates an empty dictionary: . Placing a comma-separated list ofkey:value pairs within the braces adds initial key:value pairs to thedictionary; this is also the way dictionaries are written on output.
The main operations on a dictionary are storing a value with some key andextracting the value given the key. It is also possible to delete a key:valuepair with del. If you store using a key that is already in use, the oldvalue associated with that key is forgotten. It is an error to extract a valueusing a non-existent key.
Performing list(d) on a dictionary returns a list of all the keysused in the dictionary, in insertion order (if you want it sorted, just usesorted(d) instead). To check whether a single key is in thedictionary, use the in keyword.
The comparison operators in and not in are membership tests thatdetermine whether a value is in (or not in) a container. The operators isand is not compare whether two objects are really the same object. Allcomparison operators have the same priority, which is lower than that of allnumerical operators.
Comparisons may be combined using the Boolean operators and and or, andthe outcome of a comparison (or of any other Boolean expression) may be negatedwith not. These have lower priorities than comparison operators; betweenthem, not has the highest priority and or the lowest, so that A andnot B or C is equivalent to (A and (not B)) or C. As always, parenthesescan be used to express the desired composition.
The Boolean operators and and or are so-called short-circuitoperators: their arguments are evaluated from left to right, and evaluationstops as soon as the outcome is determined. For example, if A and C aretrue but B is false, A and B and C does not evaluate the expressionC. When used as a general value and not as a Boolean, the return value of ashort-circuit operator is the last evaluated argument.
Note that in Python, unlike C, assignment inside expressions must be doneexplicitly with thewalrus operator :=.This avoids a common class of problems encountered in C programs: typing =in an expression when == was intended.
Sequence objects typically may be compared to other objects with the same sequencetype. The comparison uses lexicographical ordering: first the first twoitems are compared, and if they differ this determines the outcome of thecomparison; if they are equal, the next two items are compared, and so on, untileither sequence is exhausted. If two items to be compared are themselvessequences of the same type, the lexicographical comparison is carried outrecursively. If all items of two sequences compare equal, the sequences areconsidered equal. If one sequence is an initial sub-sequence of the other, theshorter sequence is the smaller (lesser) one. Lexicographical ordering forstrings uses the Unicode code point number to order individual characters.Some examples of comparisons between sequences of the same type:
Note that comparing objects of different types with is legalprovided that the objects have appropriate comparison methods. For example,mixed numeric types are compared according to their numeric value, so 0 equals0.0, etc. Otherwise, rather than providing an arbitrary ordering, theinterpreter will raise a TypeError exception.
The following code example creates an empty Dictionary of strings with string keys and uses the Add method to add some elements. The example demonstrates that the Add method throws an ArgumentException when attempting to add a duplicate key.
The example uses the Item[] property (the indexer in C#) to retrieve values, demonstrating that a KeyNotFoundException is thrown when a requested key is not present, and showing that the value associated with a key can be replaced.
The example shows how to use the TryGetValue method as a more efficient way to retrieve values if a program often must try key values that are not in the dictionary, and it shows how to use the ContainsKey method to test whether a key exists before calling the Add method.
The Dictionary generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary class is implemented as a hash table.
As long as an object is used as a key in the Dictionary, it must not change in any way that affects its hash value. Every key in a Dictionary must be unique according to the dictionary's equality comparer. A key cannot be null, but a value can be, if its type TValue is a reference type.
Dictionary requires an equality implementation to determine whether keys are equal. You can specify an implementation of the IEqualityComparer generic interface by using a constructor that accepts a comparer parameter; if you do not specify an implementation, the default generic equality comparer EqualityComparer.Default is used. If type TKey implements the System.IEquatable generic interface, the default equality comparer uses that implementation.
The capacity of a Dictionary is the number of elements the Dictionary can hold. As elements are added to a Dictionary, the capacity is automatically increased as required by reallocating the internal array.
.NET Framework only: For very large Dictionary objects, you can increase the maximum capacity to 2 billion elements on a 64-bit system by setting the enabled attribute of the configuration element to true in the run-time environment.
The foreach statement of the C# language (for each in C++, For Each in Visual Basic) returns an object of the type of the elements in the collection. Since the Dictionary is a collection of keys and values, the element type is not the type of the key or the type of the value. Instead, the element type is a KeyValuePair of the key type and the value type. For example:
Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.
Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.
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