In computer science , parsing reveals the grammatical structure of linear input text, as a first step in working out its meaning. Bottom-up parsing recognizes the text's lowest-level small details first, before its mid-level structures, and leaving the highest-level overall structure to last. The bottom-up name comes from the concept of a parse tree , in which the most detailed parts are at the bottom of the upside-down tree, and larger structures composed from them are in successively higher layers, until at the top or "root" of the tree a single unit describes the entire input stream. A bottom-up parse discovers and processes that tree starting from the bottom left end, and incrementally works its way upwards and rightwards. Bottom-up parsing patiently waits until it has scanned and parsed all parts of some construct before committing to what the combined construct is. The opposite of this is top-down parsing , in which the input's overall structure is decided or guessed at first, before dealing with mid-level parts, leaving completion of all lowest-level details to last.
Difference between Bottom-Up Model and Top-Down Model
The difference between top-down parsing and bottom-up parsing @ Things Of Interest
Tools like flex work a different way. As users we specify tokens using regular expressions. Then flex builds FA's for each regular expression and merges all the resulting FA's. This is then converted into code usually with a state-character table lookup. The advantage, as I hope you saw in the lab, is that changes in the language that add or remove or modify tokens usually require minor changes to a flex file. The major changes to the DFA that result are all handled automatically.
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Top-down and bottom-up are strategies of information processing and knowledge ordering,in system theories see systemics. This vocabulary is employed in neuroscience and psychology. The study of visual attention provides an example. If your attention is drawn to a flower in a field, it may be simply that the flower is more visually salient than the surrounding field. The information which caused you to attend to the flower came to you in a bottom-up fashion -- your attention was not contingent upon knowledge of the flower; the outside stimulus was sufficient on its own.
Top-down and bottom-up approaches are methods used to analyze and choose securities. However, the terms also appear in many other areas of business, finance, investing, and economics. While the two schemes are common terms, many investors get them confused or don't fully understand the differences between the approaches. Each approach can be quite simple—the top-down approach goes from the general to the specific, and the bottom-up approach begins at the specific and moves to the general.