bc.html

unix 下的C开发手册,还用详细的例程。

The result will be the quotient of the two expressions.
The
scale of the result will be the value of <b>scale</b>.

<dt><i>expression</i> % <i>expression</i><dd>
For expressions
<i>a</i>
and
<i>b</i>,
a % b
will be evaluated equivalent to the steps:
<ol>

<li>
Compute
a/b
to current scale.

<li>
Use the result to compute:
<pre>
<code>
a - (a / b) * b
</code>
</pre>
to scale:
<pre>
<code>
max(scale + scale(b), scale(a))
</code>
</pre>

</ol>
The scale of the result will be:
<pre>
<code>
max(scale + scale(b), scale(a))
</code>
</pre>

When
<b>scale</b>
is zero, the "%" operator is the mathematical remainder operator.

</dl>
<p>
The additive operators ("+", "-") bind left to right.
<dl compact>

<dt><i>expression</i> + <i>expression</i><dd>
The result will be the sum of the two expressions.
The scale of
the result will be the maximum of the scales of the expressions.

<dt><i>expression</i> - <i>expression</i><dd>
The result will be the difference of the two expressions.
The
scale of the result will be the maximum of the scales of the
expressions.

</dl>
<p>
The assignment operators ("=", "+=", "-=", "*=", "/=", "%=", "^=")
bind right to left.
<dl compact>

<dt><i>named-expression</i> = <i>expression</i><dd>
This expression results in assigning the value of the
expression on the right to the named expression on the left.
The scale of both the named expression and the result will be the scale of
<i>expression</i>.

</dl>
<p>
The compound assignment forms:
<pre>
<code>
<i>named-expression &lt;operator&gt;</i>= <i>expression
</i></code>
</pre>
are equivalent to:
<pre>
<code>
<i>named-expression</i> = <i>named-expression &lt;operator&gt; expression</i>
</code>
</pre>
except that the
<i>named-expression</i>
will be evaluated only once.
<p>
Unlike all other operators, the relational operators
("&lt;", "&gt;", "&lt;=", "&gt;=", "==", "!=")
will be only valid as the object of an <b>if</b>, <b>while</b>
or inside a
<b>for</b>
statement.
<dl compact>

<dt><i>expression1</i> &lt; <i>expression2</i><dd>
The relation will be true if the value of
<i>expression1</i>
is
strictly less than the value of <i>expression2</i>.

<dt><i>expression1</i> &gt; <i>expression2</i><dd>
The relation will be true if the value of
<i>expression1</i>
is
strictly greater than the value of <i>expression2</i>.

<dt><i>expression1</i> &lt;= <i>expression2</i><dd>
The relation will be true if the value of
<i>expression1</i>
is less
than or equal to the value of <i>expression2</i>.

<dt><i>expression1</i> &gt;= <i>expression2</i><dd>
The relation will be true if the value of
<i>expression1</i>
is
greater than or equal to the value of <i>expression2</i>.

<dt><i>expression1</i> == <i>expression2</i><dd>
The relation will be true if the values of
<i>expression1</i>
and
<i>expression2</i>
are equal.

<dt><i>expression1</i> != <i>expression2</i><dd>
The relation will be true if the values of
<i>expression1</i>
and
<i>expression2</i>
are unequal.

</dl>
<p>
There are only two storage classes in
<i>bc</i>,
global and automatic
(local).
Only identifiers that are to be local to a function need
be declared with the
<b>auto</b>
command.
The arguments to a function
will be local to the function.
All other identifiers are assumed to
be global and available to all functions.
All identifiers, global
and local, have initial values of zero.
Identifiers declared as
auto will be allocated on entry to the function and released on returning
from the function.
They therefore do not retain values between
function calls.
Auto arrays will be specified by the array name followed
by empty square brackets.
On entry to a function,
the old values of the names that appear as parameters and as automatic
variables are pushed onto a stack.
Until the function returns,
reference to these names refers only to the new values.
<p>
References to any of these names from other functions that
are called from this function also refer to the new value
until one of those functions uses the same name for a local variable.
<p>
When a statement is an expression, unless the main operator is
an assignment, execution of the statement will write the value
of the expression followed by a
newline
character.
<p>
When a statement is a string, execution of the statement
will write the value of the string.
<p>
Statements separated by semicolons or
newline characters
will be executed sequentially.
In an interactive invocation of
<i>bc</i>,
each time a
newline
character is read that satisfies the grammatical production:
<pre>
<code>
input_item : semicolon_list NEWLINE
</code>
</pre>
the sequential list of statements making up the
<b>semicolon_list</b>
will be executed immediately and any output produced by that
execution will be written without any delay due to buffering.
<p>
In an
<b>if</b>
statement
(<b>if</b> (<i>relation</i>) <i>statement</i>), the
<i>statement</i>
will be executed if the relation is true.
<p>
The
<b>while</b>
statement
(<b>while</b> (<i>relation</i>) <i>statement</i>)
implements a loop in which the
<i>relation</i>
is tested; each time the
<i>relation</i>
is true, the
<i>statement</i>
will be executed and the
<i>relation</i>
retested.
When the
<i>relation</i>
is false, execution will resume after
<i>statement</i>.
<p>
A
<b>for</b>
statement
(<b>for</b> (<i>expression</i>; <i>relation</i>; <i>expression</i>) <i>statement</i>)
is the same as:
<pre>
<code>
<i>first-expression</i>
while (<i>relation</i>) {
    <i>statement</i>
    <i>last-expression</i>
}
</code>
</pre>
All three expressions must be present.
<p>
The
<b>break</b>
statement causes termination
of a
<b>for</b>
or
<b>while</b>
statement.
<p>
The
<b>auto</b>
statement
(<b>auto</b> <i>identifier</i><b>[</b>,<i>identifier</i><b>]</b>&nbsp;...)
will cause the values of the identifiers to be pushed down.
The identifiers can be
ordinary identifiers or array identifiers.
Array identifiers are
specified by following the array name by empty square brackets.
The
<b>auto</b>
statement must be the first statement in a function definition.
<p>
A
<b>define</b>
statement:
<pre>
<code>
define <i>LETTER</i> ( <i>opt_parameter_list</i> ) {
    <i>opt_auto_define_list</i>
    <i>statement_list</i>
}
</code>
</pre>
defines a function named
<i>LETTER</i>.
If a function named
<i>LETTER</i>
was previously defined, the
<b>define</b>
statement will replace the previous definition.
The expression:
<pre>
<code>
<i>LETTER</i> ( <i>opt_argument_list</i> )
</code>
</pre>
will invoke the function named
<i>LETTER</i>.
The behaviour is undefined
if the number of arguments in the invocation does not match the
number of parameters in the definition.
Functions will be defined before they are invoked.
A function will be considered to be
defined within its own body, so recursive calls are valid.
The values of numeric constants within a function will be
interpreted in the base specified by the value of the
<b>ibase</b>
register when the function is invoked.
<p>
The
<b>return</b>
statements
(<b>return</b> and <b>return</b>(<i>expression</i>)) will cause
termination of a function, popping of its auto variables and
specifies the result of the function.
The first form is equivalent to
return(0).
The value and scale of an invocation of the function will be the
value and scale of the expression in parentheses.
<p>
The
<b>quit</b>
statement (<b>quit</b>) will stop execution of a
<i>bc</i>
program at the
point where the statement occurs in the input, even if it occurs
in a function definition, or in an
<b>if</b>,
<b>for</b>
or
<b>while</b>
statement.
<p>
The following functions will be defined when the
<b>-l</b>
option is specified:
<dl compact>

<dt><b>s</b> ( <i>expression</i> )<dd>
Sine of argument in radians.

<dt><b>c</b> ( <i>expression</i> )<dd>
Cosine of argument in radians.

<dt><b>a</b> ( <i>expression</i> )<dd>
Arctangent of argument.

<dt><b>l</b> ( <i>expression</i> )<dd>
Natural logarithm of argument.

<dt><b>e</b> ( <i>expression</i> )<dd>
Exponential function of argument.

<dt><b>j</b> ( <i>expression</i> , <i>expression</i> )<dd>
Bessel function of integer order.

</dl>
<p>
The scale of an invocation of each of these functions will be the
value of the
<b>scale</b>
register when the function is invoked.
The behaviour is undefined if any of these functions is
invoked with an argument outside the domain of the mathematical function.
</blockquote><h4><a name = "tag_000_000_166">&nbsp;</a>EXIT STATUS</h4><blockquote>
The following exit values are returned:
<dl compact>

<dt>0<dd>All input files were processed successfully.

<dt><i>unspecified</i><dd>An error occurred.

</dl>
</blockquote><h4><a name = "tag_000_000_167">&nbsp;</a>CONSEQUENCES OF ERRORS</h4><blockquote>
If any
<i>file</i>
operand is specified and the named file cannot be accessed,
<i>bc</i>
will write a diagnostic message to standard
error and terminate without any further action.
<p>
In an interactive invocation of
<i>bc</i>,
the utility should
print an error message and recover following any error in the input.
In a non-interactive invocation of
<i>bc</i>,
invalid input causes undefined behaviour.
</blockquote><h4><a name = "tag_000_000_168">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
Automatic variables in
<i>bc</i>
do not work in exactly the same way as in either C or PL/1.
<p>
For historical reasons, the exit status from
<i>bc</i>
cannot be relied upon to indicate that an error has occurred.
Returning zero after an error is possible.
Therefore,
<i>bc</i>
should be used primarily by interactive users (who can
react to error messages) or by application programs
that can somehow validate the answers returned
as not including error messages.
<p>
The
<i>bc</i>
utility always uses the period
(.)
character to represent a
radix point, regardless of any decimal-point character specified
as part of the current locale.
In languages like C or
<i><a href="awk.html">awk</a></i>,
the period character is used in program source, so it can be
portable and unambiguous, while the locale-specific character is
used in input and output.
Because there is no distinction
between source and input in
<i>bc</i>,
this arrangement would not be possible.
Using the locale-specific character in
<i>bc</i>'s
input would introduce ambiguities into the language; consider the
following example in a locale with a comma as the decimal-point character:
<pre>
<code>
define f(a,b) {
    ...
}
...

f(1,2,3)
</code>
</pre>
<p>
Because of such ambiguities, the period character is used in input.
Having input follow different conventions from output
would be confusing in either pipeline usage or interactive
usage, so the period is also used in output.
</blockquote><h4><a name = "tag_000_000_169">&nbsp;</a>EXAMPLES</h4><blockquote>
In the shell, the following assigns an approximation of the first ten digits
of  to the variable
<i>x</i>:
<pre>
<code>
x=$(printf "%s\n" 'scale = 10; 104348/33215' | bc)
</code>
</pre>
<p>
The following
<i>bc</i>
program prints the same approximation of ,
with a label, to standard output:
<pre>
<code>
scale = 10
"pi equals "
104348 / 33215
</code>
</pre>
<p>
The following defines a function to compute an approximate value of
the exponential function
(note that such a function is predefined if the
<b>-l</b>
option is specified):
<pre>
<code>
scale = 20
define e(x){
    auto a, b, c, i, s
    a = 1
    b = 1
    s = 1
    for (i = 1; 1 == 1; i++){
        a = a*x
        b = b*i
        c = a/b
        if (c == 0) {
             return(s)
        }
        s = s+c
    }
}
</code>
</pre>
<p>
The following prints approximate values of the exponential function of
the first ten integers:
<pre>
<code>
for (i = 1; i &lt;= 10; ++i) {
    e(i)
}
</code>
</pre>
</blockquote><h4><a name = "tag_000_000_170">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
The IEEE PASC 1003.2 Interpretations Committee has forwarded concerns about parts of this interface
definition to the IEEE PASC Shell and Utilities Working Group which is identifying the corrections.
A future revision of this specification will align with
IEEE Std. 1003.2b when finalised.
</blockquote><h4><a name = "tag_000_000_171">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="awk.html">awk</a></i>.
</blockquote><hr size=2 noshade>
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