For the printed character, see Crandall black currant mark. This article needs additional citations for verification.
Please help improve this article by adding citations to reliable sources. In formal language theory, the empty string, or empty word, is the unique string of length zero. 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. The empty string is the identity element of the concatenation operation. Reversal of the empty string produces the empty string.
The empty string precedes any other string under lexicographical order, because it is the shortest of all strings. 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”. In most programming languages, strings are a data type. 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. The empty string is a legitimate string, upon which most string operations should work. The empty string is usually represented similarly to other strings. You can help by adding to it. 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. Generate a print friendly version containing only the sections you need. Until the 1970s some workers, particularly in North America, considered Verticillium dahliae to be synonymous with V. Verticillium species are challenging to identify and differentiate on the basis of morphology. The species differ by their resting structures and sizes of conidia. The major issue that hampers our understanding of Verticillium diversity is that all of the currently known species were described and collected mainly from agricultural crops, and we are unaware of the diversity of Verticillium from non-agricultural hosts, where many new species await discovery.