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9.5.1 Normal strings and E-strings

Normal strings are common to most programming languages. They are simply an array of characters, with the end of the string marked by a null character (ASCII zero). We've already met normal strings (see 2.4 Strings). The ones we used were constant strings contained in ' characters, and they denote pointers to the memory where the string data is stored. Therefore, you can assign a string constant to a pointer (to CHAR), and you've got a ready-filled array with the elements you want (an initialised array.).

  s:='This is a string constant'
  /* Now s[] is T and s[2] is i */

Remember that LONG is actually PTR TO CHAR so this code is precisely the same as:

  DEF s
  s:='This is a string constant'

The following diagram illustrates the above assignment to s. The first two characters s[0] and s[1]) are `T' and `h', and the last character (before the terminating null) is `t'. Memory marked as `Unknown' is not part of the string constant.

   |  's'   |
   |Address +----*
   +--------+     \
        +-------+ +--\----+ +-------+   +-------+ +-------+ +-------+
        |Unknown| | s[0]  | | s[1]  |   | s[24] | | s[25] | |Unknown|
Memory: |+-----+| |+-----+| |+-----+|...|+-----+| |+-----+| |+-----+|
        || XXX || || "T" || || "h" ||   || "t" || ||  0  || || XXX ||
        +=======+ +=======+ +=======+   +=======+ +=======+ +=======+

E-strings are very similar to normal strings and, in fact, an E-string can be used wherever a normal string can. However, the reverse is not true, so if something requires an E-string you cannot use a normal string instead. The difference between a normal string and an E-string was hinted at in the introduction to this section: E-strings can be safely altered without exceeding their bounds. A normal string is just an array so you need to be careful not to exceed its bounds. However, an E-string knows what its bounds are, and so any of the string manipulation functions can alter them safely.

An E-string (STRING type) variable is declared as in the following example, with the maximum size of the E-string given just like an array declaration.

  DEF s[30]:STRING

As with an array declaration, the variable s is actually a pointer to the string data. To initialise an E-string you need to use the function StrCopy as we shall see.

There are some worked examples in Part Three (see 19 String Handling and I/O) which show how to use normal strings and E-strings.

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