Re: [CentOS] [OT] GNU bc base conversion

[Warren Young]

On Aug 31, 2015, at 6:55 AM, Mike - st257  wrote:
> 
> I suppose my only options for this are to use shell functions or write a
> script using a language that handles things properly (sanely?).

No, there’s a fairly common hack around this problem: ibase=A and obase=A 
*always* means “base 10” regardless of the current base, due to a quirk in the 
way values for these settings are interpreted.  Thus you can always force your 
way back to sanity.

My objection is that this is even necessary in the first place.
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Re: [CentOS] [OT] GNU bc base conversion

[Mike - st257]

On Mon, Aug 31, 2015 at 11:15 AM, Warren Young  wrote:

> On Aug 31, 2015, at 6:55 AM, Mike - st257  wrote:
> >
> > I suppose my only options for this are to use shell functions or write a
> > script using a language that handles things properly (sanely?).
>
> No, there’s a fairly common hack around this problem: ibase=A and obase=A
> *always* means “base 10” regardless of the current base, due to a quirk in
> the way values for these settings are interpreted.  Thus you can always
> force your way back to sanity.
>

Not sure how this helps me with my most recent example of bin_to_hex where
the ibase within the define clause wasn't honored.

Working with bc interfactively or by piping produce the desired/correct
values.
Testing indicates the ibase is defaulted or overrode as base10 despite what
is specified in the define clause. :-(


>
> My objection is that this is even necessary in the first place.


Agreed.


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Re: [CentOS] [OT] GNU bc base conversion

[Gordon Messmer]

On 08/31/2015 05:55 AM, Mike - st257 wrote:

I'm not an expert in bc, so I might be wrong, but it looks like setting

>the ibase inside a function is simply too late.  ibase affects how bc
>interprets input.
>

Thanks Gordon.
Big bummer given that behavior. :-/


I suppose that depends on what you're trying to accomplish.  Most 
conversions you can do entirely within bash, if that's your goal.


function dec_to_hex () { printf '%x\n' $1; }
function hex_to_dec() { printf '%d\n' $(( 16#$1 )); }
function hex_to_bin() { echo 'obase=2;' $(( 16#$1 )) | bc; }

$ dec_to_hex 10
a
$ hex_to_dec a
10
$ hex_to_bin a
1010

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Re: [CentOS] [OT] GNU bc base conversion

[Warren Young]

On Aug 31, 2015, at 10:05 AM, Mike - st257  wrote:
> 
> On Mon, Aug 31, 2015 at 11:15 AM, Warren Young  wrote:
> 
>> ibase=A and obase=A
> 
> Not sure how this helps me with my most recent example of bin_to_hex where
> the ibase within the define clause wasn't honored.

That’s because your bin_to_hex function is erroneously assuming that its input 
is just a string of digits that has no base interpretation, so that it can set 
ibase *after* bc has already seen the value, and that this will change the 
meaning of the prior input.

That’s just plain wrong: the prior input has already been interpreted.  You 
can’t wind back the clock like that.  That sort of thinking only works in a 
string-based language like shell or Tcl, where a numeric string doesn’t take a 
meaning until you assign one.

The correct form is:

ibase=A   /* needed only if ibase might not be 10 at this point */
obase=A   /* ditto */
obase=16  /* means 16-base-10 here */
ibase=2   /* no possibility of confusion */
10101011

Result: AB, as expected.

The tricky bit is that you can’t swap the second pair of ibase and obase 
settings, since that would cause bc(1) to interpret obase=16 in base 2: bc(1) 
will clamp “16” to 11, which in base 2 is 3-base-10.
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Re: [CentOS] [OT] GNU bc base conversion

[Mike - st257]

On Mon, Aug 31, 2015 at 12:50 PM, Gordon Messmer 
wrote:

> On 08/31/2015 05:55 AM, Mike - st257 wrote:
>
>> I'm not an expert in bc, so I might be wrong, but it looks like setting

>>> >the ibase inside a function is simply too late.  ibase affects how bc
>>> >interprets input.
>>> >
>>>
>> Thanks Gordon.
>> Big bummer given that behavior. :-/
>>
>
> I suppose that depends on what you're trying to accomplish.  Most
> conversions you can do entirely within bash, if that's your goal.
>

Yes indeed it can be done in bash. I figure at this point I'm going that
route.
Thanks!


>
> function dec_to_hex () { printf '%x\n' $1; }
> function hex_to_dec() { printf '%d\n' $(( 16#$1 )); }
> function hex_to_bin() { echo 'obase=2;' $(( 16#$1 )) | bc; }
>
> $ dec_to_hex 10
> a
> $ hex_to_dec a
> 10
> $ hex_to_bin a
> 1010
>
>
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Re: [CentOS] [OT] GNU bc base conversion

[Mike - st257]

On Mon, Aug 31, 2015 at 1:06 PM, Warren Young  wrote:

> On Aug 31, 2015, at 10:05 AM, Mike - st257  wrote:
> >
> > On Mon, Aug 31, 2015 at 11:15 AM, Warren Young  wrote:
> >
> >> ibase=A and obase=A
> >
> > Not sure how this helps me with my most recent example of bin_to_hex
> where
> > the ibase within the define clause wasn't honored.
>
> That’s because your bin_to_hex function is erroneously assuming that its
> input is just a string of digits that has no base interpretation, so that
> it can set ibase *after* bc has already seen the value, and that this will
> change the meaning of the prior input.
>

Explains the behavior.


>
> That’s just plain wrong: the prior input has already been interpreted.
> You can’t wind back the clock like that.  That sort of thinking only works
> in a string-based language like shell or Tcl, where a numeric string
> doesn’t take a meaning until you assign one.
>
> The correct form is:
>
> ibase=A   /* needed only if ibase might not be 10 at this point */
> obase=A   /* ditto */
> obase=16  /* means 16-base-10 here */
> ibase=2   /* no possibility of confusion */
> 10101011
>
> Result: AB, as expected.
>

Thank you.
We're in agreement, your solution _does_ work when piped or interactively.
Any order does not work from a definition stanza since base10 is
(apparently) not capable of being overrode with definitions.

tmp]$ grep bin_to_hex ~/.bcrc
#define bin_to_hex(b) { obase=16; ibase=2; return b; }
define bin_to_hex(b) { ibase=A; obase=A; obase=16; ibase=2; return b; }

tmp]$ echo "bin_to_hex(10101011)" | bc
9A2113
tmp]$ echo "ibase=A; obase=A; obase=16; ibase=2; 10101011" | bc
AB
tmp]$ echo "obase=16; ibase=2; 10101011" | bc
AB


>
> The tricky bit is that you can’t swap the second pair of ibase and obase
> settings, since that would cause bc(1) to interpret obase=16 in base 2:
> bc(1) will clamp “16” to 11, which in base 2 is 3-base-10.


Agreed.
Thank you, Warren.


I don't believe bc is the tool for this job since define stanzas don't
replicate the behavior as seen when used via piping or interactively. :-/

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Re: [CentOS] [OT] GNU bc base conversion

[Warren Young]

On Aug 31, 2015, at 11:54 AM, Mike - st257  wrote:
> 
> define stanzas don't
> replicate the behavior as seen when used via piping or interactively. :-/

Again, it comes down to order of evaluation.  You can look at the following in 
three stages:

   define bin_to_hex(b) { obase=16; ibase=2; return b; }
   bin_to_hex(10101011)

1. Here is a number, 10101011, which you shall interpret in base 10, since 
that’s the default and no one has said otherwise yet.

2. Pass that value to bin_to_hex()

3. Change the obase and ibase, then return the parameter with those new 
settings.

You are expecting that because the define line appears first that it affects 
what follows, but it doesn’t affect anything until you call it for the first 
time.

That’s clear.

What I *don’t* understand is why this doesn’t work:

   define bin_to_hex(b) { obase=16; ibase=2; return b; }
   bin_to_hex(0)/* bogus call to force obase and ibase */
   bin_to_hex(10101011)

For some strange reason, I get 20100 out of this with both GNU and BSD bc, even 
though ibase=2 and obase=10 by the time of the second bin_to_hex() call.

(If you’re confused about obase=10, realize that 10-base-16 is 16-base-10. :) )

Bottom line, don’t use bc for this kind of thing.  It’s not the right tool for 
the job.
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Re: [CentOS] [OT] GNU bc base conversion

[Mike - st257]

On Fri, Aug 28, 2015 at 4:14 PM, Warren Young  wrote:

> On Aug 28, 2015, at 9:50 AM, Gordon Messmer 
> wrote:
> >
> > On 08/28/2015 07:15 AM, Mike - st257 wrote:
> >> Thoughts as to why my BC functions aren't properly converting between
> bases?
> >>
> >> Decimal to binary or hex works fine, but not binary or hex to decimal
> and
> >> so forth.
> >
> > I'm not an expert in bc, so I might be wrong, but it looks like setting
> the ibase inside a function is simply too late.  ibase affects how bc
> interprets input.
>

Thanks Gordon.
Big bummer given that behavior. :-/
I had (and did test) definitions for other conversions (though I didn't
post them), but this one also drives it home.

~]$ grep bin_to_hex ~/.bcrc
define bin_to_hex(b) { obase=16; ibase=2; return b; }

~]$ echo "bin_to_hex(10101011)" | bc
9A2113
# so we're sending in a string with implied base10 ... sure enough matches
what BC errantly decided to do
~]$ echo "obase=16; 10101011" | bc
9A2113

~]$ echo "obase=16; ibase=2; 10101011" | bc
AB


>
> Yes, and it’s a serious design mistake in bc, IMHO.  No other programmable
> system I’ve ever used changes how numbers in program text are interpreted
> based on prior commands to the system.
>
> I wrote a long answer explaining this on the Unix & Linux Stack Exchange
> here:
>
>   http://unix.stackexchange.com/a/199620
>
>
Thanks Warren.
The order of obase and ibase definitely are bizarre and tripped me up the
first time around. I expected I should set ibase first and no no no that
was not the case! ;-)


To all:
I suppose my only options for this are to use shell functions or write a
script using a language that handles things properly (sanely?).


Thanks for the input everyone!

Best,
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Re: [CentOS] [OT] GNU bc base conversion

[Gordon Messmer]

On 08/28/2015 07:15 AM, Mike - st257 wrote:

Thoughts as to why my BC functions aren't properly converting between bases?

Decimal to binary or hex works fine, but not binary or hex to decimal and
so forth.


I'm not an expert in bc, so I might be wrong, but it looks like setting 
the ibase inside a function is simply too late.  ibase affects how bc 
interprets input.  So echo bin_to_dec(1001) | bc is going to 
interpret the value of 1001 while reading it from input, not after 
passing it to a function where ibase is reset.


Supporting that theory:
$ bc
...
define bin_to_dec(b) { obase=10; ibase=2; return b; }
bin_to_dec(1001)
1001

Decimal to binary and hex work correctly because decimal input is the 
default.  Since ibase is already 10, those values are interpreted the 
way you want, but not because you're setting ibase in your function.


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[CentOS] [OT] GNU bc base conversion

[Mike - st257]

Hello CentOS List Members,

Thoughts as to why my BC functions aren't properly converting between bases?

Decimal to binary or hex works fine, but not binary or hex to decimal and
so forth.
No doubt the syntax is in some way wrong, but when I test from the CLI and
the right values are returned, I have to wonder.

Other than a few other define statements, the only other option I have set
is scale=5 in my bcrc.

In reference to the order of (o|i)base parameters, I have specified obase
before ibase [0].
[0] http://docstore.mik.ua/orelly/unix/upt/ch49_03.htm

See below for my examples.
Thanks!



]$ echo bin_to_dec(1001) | bc
1001
# should be decimal 9

]$ echo obase=10; ibase=2; 1001 | bc
9

]$ grep bin_to_dec ~/.bcrc
define bin_to_dec(b) { obase=10; ibase=2; return b; }



]$ echo hex_to_dec(AB) | bc
99
# should be decimal 171

]$ echo obase=10; ibase=16; AB | bc
171
]$ echo obase=A; ibase=16; AB | bc
171

]$ grep hex_to_dec ~/.bcrc
define hex_to_dec(h) { obase=A; ibase=16; return h; }

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Re: [CentOS] [OT] GNU bc base conversion

[Warren Young]

On Aug 28, 2015, at 9:50 AM, Gordon Messmer gordon.mess...@gmail.com wrote:
 
 On 08/28/2015 07:15 AM, Mike - st257 wrote:
 Thoughts as to why my BC functions aren't properly converting between bases?
 
 Decimal to binary or hex works fine, but not binary or hex to decimal and
 so forth.
 
 I'm not an expert in bc, so I might be wrong, but it looks like setting the 
 ibase inside a function is simply too late.  ibase affects how bc interprets 
 input.

Yes, and it’s a serious design mistake in bc, IMHO.  No other programmable 
system I’ve ever used changes how numbers in program text are interpreted based 
on prior commands to the system.

I wrote a long answer explaining this on the Unix  Linux Stack Exchange here:

  http://unix.stackexchange.com/a/199620
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