What are the three main factors that determine ad Rank?

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Formally, a string is a finite, ordered sequence of characters such as letters, digits or spaces. The empty string is the special case where the sequence has length zero, so there are no symbols in the string. There is only one empty string, because two strings are only different if they have different lengths or a different sequence of symbols. In formal treatments,[1] the empty string is denoted with ε or sometimes Λ or λ.

The empty string should not be confused with the empty language ∅, which is a formal language (i.e. a set of strings) that contains no strings, not even the empty string.

The empty string has several properties:

In context-free grammars, a production rule that allows a symbol to produce the empty string is known as an ε-production, and the symbol is said to be "nullable".

Use in programming languages[edit]

In most programming languages, strings are a data type. Strings are typically stored at distinct memory addresses (locations). Thus, the same string (for example, the empty string) may be stored in two or more places in memory.

In this way, there could be multiple empty strings in memory, in contrast with the formal theory definition, for which there is only one possible empty string. However, a string comparison function would indicate that all of these empty strings are equal to each other.

Even a string of length zero can require memory to store it, depending on the format being used. In most programming languages, the empty string is distinct from a null reference (or null pointer) because a null reference points to no string at all, not even the empty string. The empty string is a legitimate string, upon which most string operations should work. Some languages treat some or all of the following in similar ways: empty strings, null references, the integer 0, the floating point number 0, the Boolean value false, the ASCII character NUL, or other such values.

The empty string is usually represented similarly to other strings. In implementations with string terminating character (null-terminated strings or plain text lines), the empty string is indicated by the immediate use of this terminating character.

Examples of empty strings[edit]

The empty string is a syntactically valid representation of zero in positional notation (in any base), which does not contain leading zeros. Since the empty string does not have a standard visual representation outside of formal language theory, the number zero is traditionally represented by a single decimal digit 0 instead.

Zero-filled memory area, interpreted as a null-terminated string, is an empty string.

Empty lines of text show the empty string. This can occur from two consecutive EOLs, as often occur in text files, and this is sometimes used in text processing to separate paragraphs, e.g. in MediaWiki.

Google Search is a fully-automated search engine that uses software known as web crawlers that explore the web regularly to find pages to add to our index. In fact, the vast majority of pages listed in our results aren't manually submitted for inclusion, but are found and added automatically when our web crawlers explore the web. This document explains the stages of how Search works in the context of your website. Having this base knowledge can help you fix crawling issues, get your pages indexed, and learn how to optimize how your site appears in Google Search.

A few notes before we get started

Before we get into the details of how Search works, it's important to note that Google doesn't accept payment to crawl a site more frequently, or rank it higher. If anyone tells you otherwise, they're wrong.

Google doesn't guarantee that it will crawl, index, or serve your page, even if your page follows the Google Search Essentials.

Introducing the three stages of Google Search

Google Search works in three stages, and not all pages make it through each stage:

1. Crawling: Google downloads text, images, and videos from pages it found on the internet with automated programs called crawlers.
2. Indexing: Google analyzes the text, images, and video files on the page, and stores the information in the Google index, which is a large database.
3. Serving search results: When a user searches on Google, Google returns information that's relevant to the user's query.

Crawling

The first stage is finding out what pages exist on the web. There isn't a central registry of all web pages, so Google must constantly look for new and updated pages and add them to its list of known pages. This process is called "URL discovery". Some pages are known because Google has already visited them. Other pages are discovered when Google follows a link from a known page to a new page: for example, a hub page, such as a category page, links to a new blog post. Still other pages are discovered when you submit a list of pages (a sitemap) for Google to crawl.

Once Google discovers a page's URL, it may visit (or "crawl") the page to find out what's on it. We use a huge set of computers to crawl billions of pages on the web. The program that does the fetching is called Googlebot (also known as a crawler, robot, bot, or spider). Googlebot uses an algorithmic process to determine which sites to crawl, how often, and how many pages to fetch from each site. Google's crawlers are also programmed such that they try not to crawl the site too fast to avoid overloading it. This mechanism is based on the responses of the site (for example, HTTP 500 errors mean "slow down") and settings in Search Console.

However, Googlebot doesn't crawl all the pages it discovered. Some pages may be disallowed for crawling by the site owner, other pages may not be accessible without logging in to the site.

During the crawl, Google renders the page and runs any JavaScript it finds using a recent version of Chrome, similar to how your browser renders pages you visit. Rendering is important because websites often rely on JavaScript to bring content to the page, and without rendering Google might not see that content.

Crawling depends on whether Google's crawlers can access the site. Some common issues with Googlebot accessing sites include:

• Problems with the server handling the site
• Network issues
• robots.txt directives preventing Googlebot's access to the page

Indexing

After a page is crawled, Google tries to understand what the page is about. This stage is called indexing and it includes processing and analyzing the textual content and key content tags and attributes, such as <title> elements and alt attributes, images, videos, and more.

During the indexing process, Google determines if a page is a duplicate of another page on the internet or canonical. The canonical is the page that may be shown in search results. To select the canonical, we first group together (also known as clustering) the pages that we found on the internet that have similar content, and then we select the one that's most representative of the group. The other pages in the group are alternate versions that may be served in different contexts, like if the user is searching from a mobile device or they're looking for a very specific page from that cluster.

Google also collects signals about the canonical page and its contents, which may be used in the next stage, where we serve the page in search results. Some signals include the language of the page, the country the content is local to, the usability of the page, and so on.

The collected information about the canonical page and its cluster may be stored in the Google index, a large database hosted on thousands of computers. Indexing isn't guaranteed; not every page that Google processes will be indexed.

Indexing also depends on the content of the page and its metadata. Some common indexing issues can include:

• The quality of the content on page is low
• Robots meta directives disallow indexing
• The design of the website might make indexing difficult

Serving search results

When a user enters a query, our machines search the index for matching pages and return the results we believe are the highest quality and most relevant to the user's query. Relevancy is determined by hundreds of factors, which could include information such as the user's location, language, and device (desktop or phone). For example, searching for "bicycle repair shops" would show different results to a user in Paris than it would to a user in Hong Kong.

Search Console might tell you that a page is indexed, but you don't see it in search results. This might be because:

• The content of the content on page is irrelevant to users' queries
• The quality of the content is low
• Robots meta directives prevent serving

While this guide explains how Search works, we are always working on improving our algorithms. You can keep track of these changes by following the Google Search Central blog.