Top-down parsing was introduced in the 1970s, and it remains a widespread strategy that has also become the basis for newer and more sophisticated algorithms. The top-down strategy was preferred during an extended period of time due to its comparative simplicity, usability, and an opportunity to customize. As the term suggests, top-down parsing involves the identification of the root of the parse tree. When the root is identified, the movement down the subtrees occurs, and eventually, the leaves of the tree are found. The parser examines the parse tree with the help of formal grammar rewriting rules.
Overall, this technique aims at detecting the leftmost derivations that form an input string. The analysis commences with the focus on the start symbol of the parse tree, and the leaf nodes of the parse tree are the final target of the process. The major decision is associated with choosing what production rule has to be utilized to create a string. Top-down parsing encompasses the consumption of tokens from left to the right. This approach is mainly employed by such programming languages as Fortran and COBOL. Some of its primary uses include module documentation and debugging.
In order to illustrate the peculiarities of the top-down model, it is possible to take a closer look at a simple addition example (68 + 124). Moving to the right, the lexer identifies the numerical 68, a space, the plus sign, and another numerical. First, the token of the NUM type is identified and followed by the token of the PLUS type. After that, the lexer finds another NUM type token (124). Finally, the lexer will define the sequence as a sum expression.
The advantages of the algorithm are quite numerous, which makes the model still applicable. The division of a problem into small parts makes it possible to understand who exactly should be done. Each step is characterized by the refinement of new elements that become more understandable and manageable. An important advantage of the method is potential multiple uses of some parts of the solution. For example, when the parser encounters the same sequence, the results obtained earlier are utilized. The model is also easy to customize, which is a positive feature for a parser as well.
As far as disadvantages are concerned, during top-down parsing, every segment is programmed separately, so redundancy is rather common. The linear nature of the process is also linked to a significant disadvantage. The following sequence can be impossible to address with a top-down parser: 3 * 5 * 8 + 4 * 2. Like in the example mentioned above, the parser identifies NUM type tokens and symbols. When the parser recognizes it as an expression, a solution to this sequence is a switch from the top-down parsing to the special-purpose algorithm.
In conclusion, it is possible to note that this type of computer language parsing algorithm can be used in diverse situations when linear sequences are available. The movement from the top to the leaves of the tree is regarded as simple, which makes it a usable approach. The fact that the parser moves from left to right contributes to the simplicity of its use, but it is also associated with a disadvantage of the model. The top-down parsing method can hardly tolerate any level of ambiguity, which is still a serious problem.