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3.1 About Expressions and Special Variables  
3.2 Basic Expressions  
3.3 Relational Expressions  
3.4 Boolean Expressions  
3.5 Precedence  
3.6 Special Expressions 
The numbers are manipulated by expressions and statements. Since the language was designed to be interactive, statements and expressions are executed as soon as possible. There is no main program. Instead, code is executed as it is encountered. (Functions, discussed in detail later, are defined when encountered.)
A simple expression is just a constant. bc
converts constants
into internal decimal numbers using the current input base, specified by
the variable ibase. (There is an exception in functions.) The
legal values of ibase are 2 through 16. Assigning a value outside
this range to ibase will result in a value of 2 or 16. Input
numbers may contain the characters 09 and AF. (Note: They must be
capitals. Lower case letters are variable names.) Single digit numbers
always have the value of the digit regardless of the value of
ibase. (i.e. A = 10.) For multidigit numbers, bc
changes all input digits greater or equal to ibase to the value of
ibase1. This makes the number FFF
always be the largest
3 digit number of the input base.
Full expressions are similar to many other high level languages. Since there is only one kind of number, there are no rules for mixing types. Instead, there are rules on the scale of expressions. Every expression has a scale. This is derived from the scale of original numbers, the operation performed and in many cases, the value of the variable scale. Legal values of the variable scale are 0 to the maximum number representable by a C integer.
In the following descriptions of legal expressions, "expr" refers to a complete expression and "var" refers to a simple or an array variable. A simple variable is just a
name
and an array variable is specified as
name[expr]
Unless specifically mentioned the scale of the result is the maximum scale of the expressions involved.
 expr
The result is the negation of the expression.
++ var
The variable is incremented by one and the new value is the result of the expression.
 var
The variable is decremented by one and the new value is the result of the expression.
var ++
The result of the expression is the value of the variable and then the variable is incremented by one.
var 
The result of the expression is the value of the variable and then the variable is decremented by one.
expr + expr
The result of the expression is the sum of the two expressions.
expr  expr
The result of the expression is the difference of the two expressions.
expr * expr
The result of the expression is the product of the two expressions.
expr / expr
The result of the expression is the quotient of the two expressions.
The scale of the result is the value of the variable scale
expr % expr
The result of the expression is the "remainder" and it is computed in the following way. To compute a%b, first a/b is computed to scale digits. That result is used to compute a(a/b)*b to the scale of the maximum of scale+scale(b) and scale(a). If scale is set to zero and both expressions are integers this expression is the integer remainder function.
expr ^ expr
The result of the expression is the value of the first raised to the second. The second expression must be an integer. (If the second expression is not an integer, a warning is generated and the expression is truncated to get an integer value.) The scale of the result is scale if the exponent is negative. If the exponent is positive the scale of the result is the minimum of the scale of the first expression times the value of the exponent and the maximum of scale and the scale of the first expression. (e.g. scale(a^b) = min(scale(a)*b, max(scale, scale(a))).) It should be noted that expr^0 will always return the value of 1.
( expr )
This alters the standard precedence to force the evaluation of the expression.
var = expr
The variable is assigned the value of the expression.
var <op>= expr
This is equivalent to "var = var <op> expr" with the exception that the "var" part is evaluated only once. This can make a difference if "var" is an array.
Relational expressions are a special kind of expression that always
evaluate to 0 or 1, 0 if the relation is false and 1 if the relation is
true. These may appear in any legal expression. (POSIX bc
requires that relational expressions are used only in if
,
while
, and for
statements and that only one relational
test may be done in them.) The relational operators are
expr1 < expr2
The result is 1 if expr1 is strictly less than expr2.
expr1 <= expr2
The result is 1 if expr1 is less than or equal to expr2.
expr1 > expr2
The result is 1 if expr1 is strictly greater than expr2.
expr1 >= expr2
The result is 1 if expr1 is greater than or equal to expr2.
expr1 == expr2
The result is 1 if expr1 is equal to expr2.
expr1 != expr2
The result is 1 if expr1 is not equal to expr2.
Boolean operations are also legal. (POSIX bc
does NOT have
boolean operations). The result of all boolean operations are 0 and 1
(for false and true) as in relational expressions. The boolean
operators are:
!expr
The result is 1 if expr is 0.
expr && expr
The result is 1 if both expressions are nonzero.
expr  expr
The result is 1 if either expression is nonzero.
The expression precedence is as follows: (lowest to highest)
 operator, left associative && operator, left associative ! operator, nonassociative Relational operators, left associative Assignment operator, right associative + and  operators, left associative *, / and % operators, left associative ^ operator, right associative unary  operator, nonassociative ++ and  operators, nonassociative 
This precedence was chosen so that POSIX compliant bc
programs
will run correctly. This will cause the use of the relational and
logical operators to have some unusual behavior when used with
assignment expressions. Consider the expression:
a = 3 < 5 
Most C programmers would assume this would assign the result of "3 <
5" (the value 1) to the variable "a". What this does in bc
is
assign the value 3 to the variable "a" and then compare 3 to 5. It is
best to use parentheses when using relational and logical operators
with the assignment operators.
There are a few more special expressions that are provided in
bc
. These have to do with userdefined functions and standard
functions. They all appear as
"name(
parameters)
". See section Functions, for
userdefined functions. The standard functions are:
length ( expression )
The value of the length function is the number of significant digits in the expression.
read ( )
The read
function (an extension) will read a number from the
standard input, regardless of where the function occurs. Beware, this
can cause problems with the mixing of data and program in the standard
input. The best use for this function is in a previously written
program that needs input from the user, but never allows program code to
be input from the user. The value of the read
function is the
number read from the standard input using the current value of the
variable ibase for the conversion base.
scale ( expression )
The value of the scale
function is the number of digits after the
decimal point in the expression.
sqrt ( expression )
The value of the sqrt
function is the square root of the
expression. If the expression is negative, a run time error is
generated.
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