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Diffstat (limited to 'Doc')
| -rw-r--r-- | Doc/ref/ref5.tex | 19 | ||||
| -rw-r--r-- | Doc/tut/tut.tex | 21 |
2 files changed, 35 insertions, 5 deletions
diff --git a/Doc/ref/ref5.tex b/Doc/ref/ref5.tex index db98fe4..6b699bc 100644 --- a/Doc/ref/ref5.tex +++ b/Doc/ref/ref5.tex @@ -152,13 +152,22 @@ A list display is a possibly empty series of expressions enclosed in square brackets: \begin{verbatim} -list_display: "[" [expression_list] "]" +list_display: "[" [expression_list [list_iter]] "]" +list_iter: list_for | list_if +list_for: "for" expression_list "in" testlist [list_iter] +list_if: "if" test [list_iter] \end{verbatim} -A list display yields a new list object. If it has no expression -list, the list object has no items. Otherwise, the elements of the -expression list are evaluated from left to right and inserted in the -list object in that order. +A list display yields a new list object. Its contents are specified +by providing either a list of expressions or a list comprehension. +When a comma-separated list of expressions is supplied, its elements are +evaluated from left to right and placed into the list object in that +order. When a list comprehension is supplied, it consists of a +single expression followed by one or more "for" or "if" clauses. In this +case, the elements of the new list are those that would be produced +by considering each of the "for" or "if" clauses a block, nesting from +left to right, and evaluating the expression to produce a list element +each time the innermost block is reached. \obindex{list} \indexii{empty}{list} diff --git a/Doc/tut/tut.tex b/Doc/tut/tut.tex index b53d6ea..f811932 100644 --- a/Doc/tut/tut.tex +++ b/Doc/tut/tut.tex @@ -1753,6 +1753,27 @@ item, then to the result and the next item, and so on. For example, 0 \end{verbatim} +\subsection{List Comprehensions} + +List comprehensions provide a concise way to create lists without resorting +to use of the \func{map()} or \func{filter()} functions. The resulting +construct tends often to be clearer than use of those functions. + +\begin{verbatim} +>>> spcs = [" Apple", " Banana ", "Coco nut "] +>>> print [s.strip() for s in spcs] +['Apple', 'Banana', 'Coco nut'] +>>> vec = [2, 4, 6] +>>> print [3*x for x in vec] +[6, 12, 18] +>>> vec1 = [2, 4, 6] +>>> vec2 = [4, 3, -9] +>>> print [x*y for x in vec1 for y in vec2] +[8, 6, -18, 16, 12, -36, 24, 18, -54] +>>> print [x+y for x in vec1 for y in vec2] +[6, 5, -7, 8, 7, -5, 10, 9, -3] +\end{verbatim} + \section{The \keyword{del} statement \label{del}} There is a way to remove an item from a list given its index instead |