visual basic barcode program Small BASIC Expressions in Java

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Small BASIC Expressions
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As they apply to the small BASIC interpreter developed in this chapter, expressions are comprised of the following items: Integers The operators + / * ^ = ( ) < > >= <= <> Variables In Small BASIC, the ^ indicates exponentiation. The = is used for both assignments and for equality. However, relative to BASIC expressions, it is only an operator when used in a relational expression. (In standard BASIC, assignment is a statement and not an operation.) Not equal is denoted as < >. These items can be combined in expressions according to the rules of algebra. Here are some examples:
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3: Implementing Language Interpreters in Java
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7 8 (100 5) * 14/6 a+b c 10 ^ 5 A<B The precedence of the operators is shown here:
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Highest ( ) unary + ^ * / + Lowest < > <= >= <> =
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Operators of equal precedence evaluate from left to right. Small BASIC makes the following assumptions: All variables are single letters; this means that 26 variables, the letters A through Z, are available for use. The variables are not case sensitive; 'a' and 'A' will be treated as the same variable. All numbers are doubles. No string variables are supported, although quoted string constants can be used for writing messages to the screen. These assumptions are built in to the parser.
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Small BASIC Tokens
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At the core of the Small BASIC parser is the getToken( ) method. This method is an expanded version of the one shown in 2. The changes allow it to tokenize not just numeric expressions, but also other elements of the Small BASIC language, such as keywords and strings. In Small BASIC, each keyword token has two formats: external and internal. The external format is the text form that you use when writing a program. For example, "PRINT" is the external form of the PRINT keyword. Although it is possible for an interpreter to be designed in such a way that each token is used in its external string form, this is seldom (if ever) done because it is inefficient. Instead, Small BASIC operates on the internal format of a token, which is simply an integer value. For example, the PRINT command is represented by 1; the INPUT command by 2; and so on. The advantage of the internal representation is that much faster code can be written using integers rather than strings. It is the job of getToken( ) to convert the token from its external format into its internal format.
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The Small BASIC getToken( ) method is shown here. It progresses through the program one character at a time.
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// Obtain the next token. private void getToken() throws InterpreterException { char ch; tokType = NONE; token = ""; kwToken = UNKNCOM; // Check for end of program. if(progIdx == prog.length) { token = EOP; return; } // Skip over white space. while(progIdx < prog.length && isSpaceOrTab(prog[progIdx])) progIdx++; // Trailing whitespace ends program. if(progIdx == prog.length) { token = EOP; tokType = DELIMITER; return; } if(prog[progIdx] == '\r') { // handle crlf progIdx += 2; kwToken = EOL; token = "\r\n"; return; } // Check for relational operator. ch = prog[progIdx]; if(ch == '<' || ch == '>') { if(progIdx+1 == prog.length) handleErr(SYNTAX); switch(ch) { case '<': if(prog[progIdx+1] == '>') { progIdx += 2;;
3: Implementing Language Interpreters in Java
token = String.valueOf(NE); } else if(prog[progIdx+1] == '=') { progIdx += 2; token = String.valueOf(LE); } else { progIdx++; token = "<"; } break; case '>': if(prog[progIdx+1] == '=') { progIdx += 2;; token = String.valueOf(GE); } else { progIdx++; token = ">"; } break; } tokType = DELIMITER; return; } if(isDelim(prog[progIdx])) { // Is an operator. token += prog[progIdx]; progIdx++; tokType = DELIMITER; } else if(Character.isLetter(prog[progIdx])) { // Is variable or keyword. while(!isDelim(prog[progIdx])) { token += prog[progIdx]; progIdx++; if(progIdx >= prog.length) break; } kwToken = lookUp(token); if(kwToken==UNKNCOM) tokType = VARIABLE; else tokType = COMMAND; } else if(Character.isDigit(prog[progIdx])) {
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