Transcranial Doppler (TCD), first described in 1982 [1], is a noninvasive ultrasound (US) study that involves the use of a low-frequency (â¤2 MHz) transducer probe to insonate the basal cerebral arteries through relatively thin bone windows.
Details. as.character and is.character are generic: you can write methods to handle specific classes of objects, see InternalMethods. Further, for as.character the default method calls as.vector, so dispatch is first on methods for as.character and then for methods for as.vector.
A character set is a fixed collection of symbols. For example, the English alphabet âAâ to âZâ and âaâ to âzâ can be a character set, with a total of 52 symbols. One of the simplest standardized character set is âASCIIâ, started from 1960s, and is almost the only one used in USA up to 1990s. (ASCII = American Standard Code for Information Interchange).