# Strings Lists, Tuples and Dictionaries
## Strings
> A string is a sequence

**len** - built in funciton that returns the number of characters in a string

**object:** Something a variable can refer to. For now, you can use “object” and “value”
interchangeably.

**sequence:** An ordered collection of values where each value is identified by an integer
index.

**item:** One of the values in a sequence.

**index:** An integer value used to select an item in a sequence, such as a character in a string.
In Python indices start from 0.

**slice:** A part of a string specified by a range of indices.

**empty string:** A string with no characters and length 0, represented by two quotation
marks.

**immutable:** The property of a sequence whose items cannot be changed.

**traverse:** To iterate through the items in a sequence, performing a similar operation on
each.

**search:** A pattern of traversal that stops when it finds what it is looking for.

**counter:** A variable used to count something, usually initialized to zero and then incremented.

**invocation:** A statement that calls a method.

**optional argument:** A function or method argument that is not required.

**file object:** A value that represents an open file.

**reduction to a previously solved problem:** A way of solving a problem by expressing it as an instance of a previously solved problem.

**special case:** A test case that is atypical or non-obvious (and less likely to be handled correctly).

## Lists

**list:** A sequence of values.

**element:** One of the values in a list (or other sequence), also called items.
**nested list:** A list that is an element of another list.

**accumulator:** A variable used in a loop to add up or accumulate a result.

**augmented assignment:** A statement that updates the value of a variable using an operator like +=.

**reduce:** A processing pattern that traverses a sequence and accumulates the elements into
a single result.

**map:** A processing pattern that traverses a sequence and performs an operation on each
element.

**filter:** A processing pattern that traverses a list and selects the elements that satisfy some
criterion.
**object:** Something a variable can refer to. An object has a type and a value.

**equivalent:** Having the same value.

**identical:** Being the same object (which implies equivalence).

**reference:** The association between a variable and its value.

**aliasing:** A circumstance where two or more variables refer to the same object.

**delimiter:** A character or string used to indicate where a string should be split.

## Dictionaries

**mapping:** A relationship in which each element of one set corresponds to an element of another set.

**dictionary:** A mapping from keys to their corresponding values.

**key-value pair:** The representation of the mapping from a key to a value.

**item:** In a dictionary, another name for a key-value pair.

**key:** An object that appears in a dictionary as the first part of a key-value pair.

**value:** An object that appears in a dictionary as the second part of a key-value pair. This is more specific than our previous use of the word “value”.

**implementation:** A way of performing a computation.

**hashtable:** The algorithm used to implement Python dictionaries.

**hash function:** A function used by a hashtable to compute the location for a key.

**hashable:** A type that has a hash function. Immutable types like integers, floats and strings are hashable; mutable types like lists and dictionaries are not.

**lookup:** A dictionary operation that takes a key and finds the corresponding value.

**reverse lookup:** A dictionary operation that takes a value and finds one or more keys that map to it.

**raise statement:** A statement that (deliberately) raises an exception.

**singleton:** A list (or other sequence) with a single element.

**call graph:** A diagram that shows every frame created during the execution of a program, with an arrow from each caller to each callee.

**memo:** A computed value stored to avoid unnecessary future computation.

**global variable:** A variable defined outside a function. Global variables can be accessed from any function.

**global statement:** A statement that declares a variable name global.

**flag:** A boolean variable used to indicate whether a condition is true.

**declaration:** A statement like global that tells the interpreter something about a variable.

## Tuples

**tuple:** An immutable sequence of elements.

**tuple assignment:** An assignment with a sequence on the right side and a tuple of variables on the left. The right side is evaluated and then its elements are assigned to the variables on the left.

**gather:** An operation that collects multiple arguments into a tuple.

**scatter:** An operation that makes a sequence behave like multiple arguments.

**zip object:** The result of calling a built-in function zip; an object that iterates through a sequence of tuples.

**iterator:** An object that can iterate through a sequence, but which does not provide list operators and methods.

**data structure:** A collection of related values, often organized in lists, dictionaries, tuples, etc.

**shape error:** An error caused because a value has the wrong shape; that is, the wrong type or size.