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For example, one could define a dictionary having a string "toast" mapped to the integer 42 or vice versa. The keys in a dictionary must be of an immutable Python type, such as an integer or a string, because under the hood they are implemented via a hash function. This makes for much faster lookup times, but requires keys not change.
Python makes a distinction between lists and tuples. Lists are written as [1, 2, 3], are mutable, and cannot be used as the keys of dictionaries (dictionary keys must be immutable in Python). Tuples, written as (1, 2, 3), are immutable and thus can be used as keys of dictionaries, provided all of the tuple's elements are immutable.
A parsing expression is a kind of pattern that each string may either match or not match.In case of a match, there is a unique prefix of the string (which may be the whole string, the empty string, or something in between) which has been consumed by the parsing expression; this prefix is what one would usually think of as having matched the expression.
Note how the use of A[i][j] with multi-step indexing as in C, as opposed to a neutral notation like A(i,j) as in Fortran, almost inevitably implies row-major order for syntactic reasons, so to speak, because it can be rewritten as (A[i])[j], and the A[i] row part can even be assigned to an intermediate variable that is then indexed in a separate expression.
Other functions use parentheses around the input to avoid ambiguity. [6] [7] [a] The parentheses can be omitted if the input is a single numerical variable or constant, [2] as in the case of sin x = sin(x) and sin π = sin(π).
Parsing a number, for example, can require five function calls: one for each non-terminal in the grammar until reaching primary. An operator-precedence parser can do the same more efficiently. [ 1 ] The idea is that we can left associate the arithmetic operations as long as we find operators with the same precedence, but we have to save a ...
With some standard function when there is little chance of ambiguity, it is common to omit the parentheses around the argument altogether (e.g., ). Note that this is never done with a general function f {\displaystyle f} , in which case the parenthesis are always included
<EOL> represents the appropriate line-end specifier (in ASCII, carriage-return, line-feed or both depending on the operating system). <rule-name> and <text> are to be substituted with a declared rule's name/label or literal text, respectively. In the U.S. postal address example above, the entire block-quote is a <syntax>.