manipulating user input in dc

Mike Rhodes

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Oct 29, 2012, 12:47:51 PM10/29/12

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Hi all,

I'm trying to understand how to manipulate user input in dc. I have a
script for calculating monthly payments on a mortgage:

---
#!/usr/bin/dc

9k # set precision value

[Enter interest rate: ]P?Sili12/si # get i
[Enter loan term in years: ]P?Snln12*sn # get n
[Enter present value: ]P?Sp # get p

li1+ln^li*li1+ln^1-/lp* # compute payment
p # print result
---

Is there no shorter way to read input, manipulate it, and store it in a
register than "?Sili12/si"? I tried "?12/Si" but that didn't work.

Any help would be greatly appreciated.

Regards,
Mike

Ed Morton

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Oct 29, 2012, 2:59:00 PM10/29/12

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Wouldn't it make more sense to do it in awk or similar? I don't read "dc" so I
don't know what your algorithm says but here's an awk script to calculate
something - apply your own algorithm:

$ cat tst.sh
#!/usr/bin/awk -f
BEGIN {
printf "[Enter interest rate: ] "
getline rate

printf "[Enter loan term in years: ] "
getline term

printf "[Enter present value: ] "
getline principal

interest = principal * (rate/100) * term

payment = ( (principal + interest) / term ) / 12

print payment
}

$ ./tst.sh
[Enter interest rate: ] 4.5
[Enter loan term in years: ] 30
[Enter present value: ] 200000
1305.56

Ed.

Alan Curry

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Oct 29, 2012, 4:01:48 PM10/29/12

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In article <508eb339$0$7645$c3e8da3$f626...@news.astraweb.com>,

Mike Rhodes <M8R-1...@mailinator.com> wrote:
>
>Is there no shorter way to read input, manipulate it, and store it in a
>register than "?Sili12/si"? I tried "?12/Si" but that didn't work.

Why do you use the S command if you don't want to do an L later? The oldest
versions of dc didn't have the S and L commands, only s and l, so maybe
that's the problem.

?12/Si should work with GNU dc at least, but ?12/si would work on all of
them.

--
Alan Curry

Mike Rhodes

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Oct 29, 2012, 4:05:14 PM10/29/12

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On 10/29/12 2:59 PM, Ed Morton wrote:
> On 10/29/2012 11:47 AM, Mike Rhodes wrote:
>> Hi all,
>>
>> I'm trying to understand how to manipulate user input in dc. I have a
>> script for calculating monthly payments on a mortgage:
>>

>> [...]

>>
>> Is there no shorter way to read input, manipulate it, and store it in a
>> register than "?Sili12/si"? I tried "?12/Si" but that didn't work.
>

> Wouldn't it make more sense to do it in awk or similar?
>

> Ed.

Yes, it would. Just an exercise on my part, in an effort to understand
dc a little better. AWK is definitely the more straightforward solution:

#!/usr/bin/awk -f

BEGIN {
printf "Enter interest rate: "

getline i

printf "Enter loan term in years: "

getline n

printf "Enter present value: "
getline p

i = i / 100 / 12
n = n * 12

pmt = p * (i * (1 + i) ^ n) / ((1 + i) ^ n - 1)
print
print "Monthly payment: $" pmt
}

--
Mike

Ben Bacarisse

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Oct 29, 2012, 4:13:04 PM10/29/12

to

Mike Rhodes <M8R-1...@mailinator.com> writes:

> I'm trying to understand how to manipulate user input in dc. I have a
> script for calculating monthly payments on a mortgage:
>
> ---
> #!/usr/bin/dc
>
> 9k # set precision value
>
> [Enter interest rate: ]P?Sili12/si # get i
> [Enter loan term in years: ]P?Snln12*sn # get n
> [Enter present value: ]P?Sp # get p
>
> li1+ln^li*li1+ln^1-/lp* # compute payment
> p # print result
> ---
>
> Is there no shorter way to read input, manipulate it, and store it in a
> register than "?Sili12/si"? I tried "?12/Si" but that didn't work.

I tried it and it did, so maybe it was something else? If you post the
code that didn't work, maybe someone can say why.

BTW, I tried ?12/si as well as ?12/Si -- I'd use s rather S unless you
need the stacking behaviour.

--
Ben.

Mike Rhodes

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Oct 29, 2012, 4:21:07 PM10/29/12

to

I get a "stack empty" error unless I use S. I can't make it work without
the uppercase S for some reason. I'm using OpenBSD dc, which supports S
and L.

Mike

Ben Bacarisse

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Oct 29, 2012, 4:33:58 PM10/29/12

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In case it helps, here:

$ dc -V
dc (GNU bc 1.06.95) 1.3.95
...

's' works in place of 'S' as does your shortened version:
$ printf "1\n2\n100\n" | dc dco
Enter interest rate: Enter loan term in years: Enter present value: 9.763223400
$ printf "1\n2\n100\n" | dc dcs
Enter interest rate: Enter loan term in years: Enter present value: 9.763223400
$ printf "1\n2\n100\n" | dc dc
Enter interest rate: Enter loan term in years: Enter present value: 9.763223400

'dco' is your version.

$ diff dco dcs
3,5c3,5

< [Enter interest rate: ]P?Sili12/si # get i
< [Enter loan term in years: ]P?Snln12*sn # get n
< [Enter present value: ]P?Sp # get p

---
> [Enter interest rate: ]P?sili12/si # get i
> [Enter loan term in years: ]P?snln12*sn # get n
> [Enter present value: ]P?sp # get p
$ diff dco dc
3,4c3,4

< [Enter interest rate: ]P?Sili12/si # get i
< [Enter loan term in years: ]P?Snln12*sn # get n

---
> [Enter interest rate: ]P?12/si # get i
> [Enter loan term in years: ]P?12*sn # get n

--
Ben.