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+ (plus)

Keyword Abbreviation Token (hex) Version(s) Classification
+ none AA 1.0 to 7.0 Operator (3)

augend addend
 ~ or ~
+ augend
Parameters Type Legal Value(s) Default Value Note(s)
augend Numeric or String
addend Numeric or String any
Type must be the "same" as augend (numeric or string)
Returns Type Value(s) Note(s)
sum Numeric
Form 1, numeric types
concatenation String 0 ~ 255 chars
Form 1, string types
augend Numeric or String  any  Form 2 
Calculate an arithmetic expression or concatenate two strings.

The plus operator has two primary roles:
  • calculates the sum of two numbers (I'm not going to explain how addition works),
  • or it appends the addend string to the end of the augend string
In these roles, the arguments must both be numeric or both be strings.  (If they are numeric, they don't have to be the exact same type... you can add an integer to a floating-point for example).  Trying to combine string and numeric types generates TYPE MISMATCH ERROR.  When combining two strings, the concatenation (result string) must have no more than 255 characters, otherwise STRING TOO LONG ERROR occurs.
Another role for the plus operator, shown by the second form of Syntax above is a "no operation": BASIC just skips the token.  This is not like REM because the plus "no operation" only works when BASIC is looking for an expression, and it only skips one token, not the remainder of the line.  (Trivia, when you enter a program line in direct mode, BASIC is looking for a literal number, not an expression; so trying to enter the line number 10 as +10 PRINT "HELLO" will generate an error instead of storing the text as a program line.)  More trivia: I think mathematicians would call this role the identity operator.
If the absolute value of the result is greater than 1.70141183e+38 then an OVERLOW ERROR occurs.  If the absolute value of the result is less than 2.93873588e-39 then it silently underflows to zero (I'm pretty sure this can only happen when the result really should be zero).
The plus operator has the third-highest fixed priority (the lowest arithmetic operator, but greater than relational and boolean operators).  The user of course may over-ride the built-in operator priorities by using parentheses "(" and ")".
Like all operators, the result may be printed, stored in an appropriate variable, or combined with other functions and operators in a larger expression.  For storing a numeric result (the sum of two numbers), if the variable is an integer then the minimum result that can be stored is -32768 and the maximum is +32767; otherwise an ILLEGAL QUANTITY ERROR is generated.
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