File seek

[Arvind Varma Kalidindi]

Hi,
I was asked this question in an interview recently. "How do you
move to the 6th byte in a file?" ... My thinking would be to find the
data types in the file, set a base pointer and advance it by 6. I
mean, ptr+6. Another way to ask the same question is how do you move
to the 3rd record in a file (considering that the file is made up of
records).
I told the interviewer that we could do a seek to move to the
specific byte. At that time, the interviewer nodded his head to show
his approval of my answer, but I came to know that he thought there is
a more effective way of doing this. Can anyone throw some pointers on
what that effective way could be?

Regards,
Arvind.

[Mauricio Lange]

Hi,
To move to the 6th byte you dont need to find the data types in the file. A
byte is a byte. Maybe he refered to that.

Mauricio

"Arvind Varma Kalidindi" <ax*****@lycos. com> wrote in message
news:26******** *************** ***@posting.goo gle.com...

Hi,
I was asked this question in an interview recently. "How do you
move to the 6th byte in a file?" ... My thinking would be to find the
data types in the file, set a base pointer and advance it by 6. I
mean, ptr+6. Another way to ask the same question is how do you move
to the 3rd record in a file (considering that the file is made up of
records).
I told the interviewer that we could do a seek to move to the
specific byte. At that time, the interviewer nodded his head to show
his approval of my answer, but I came to know that he thought there is
a more effective way of doing this. Can anyone throw some pointers on
what that effective way could be?

Regards,
Arvind.

[Arthur J. O'Dwyer]

On Tue, 10 Aug 2004, Arvind Varma Kalidindi wrote in comp.lang.c:

I was asked this question in an interview recently. "How do you
move to the 6th byte in a file?" ... My thinking would be to find the
data types in the file, set a base pointer and advance it by 6.
This is not coherent English. Files don't have "data types," and
you can't set C or C++ pointers to point /into/ files. You have to
read out the data first.
I mean, ptr+6. Another way to ask the same question is how do you move
to the 3rd record in a file (considering that the file is made up of
records).
And assuming that the first two records in the file have a total of
6 bytes between them, yes. :)
I told the interviewer that we could do a seek to move to the
specific byte. At that time, the interviewer nodded his head to show
his approval of my answer, but I came to know that he thought there is
a more effective way of doing this. Can anyone throw some pointers on
what that effective way could be?

I doubt he thought there was a more /effective/ way. You could
have been more /detailed/ in your answer, though; he might have
expected an interview candidate in C to be able to write down

fseek(fp, 6, SEEK_SET);

from memory (although I personally have to look up the order of
those parameters every time), and perhaps to explain why

if (fseek(fp, 6, SEEK_SET)) {
puts("I/O error during fseek");
}

would be more appropriate. He might have expected you to
recall that 'fseek' does not have well-defined behavior on text
streams except in very specific circumstances---consider the
definition of "the 6th byte" with respect to the text file

apple
banana
EOF

Is "the 6th byte" an ASCII newline, a carriage return, the letter 'b',
or what? A complete answer must at least explain the problem; and
the interviewer might then follow up with, "What is the best
interpretation in your opinion, and how would you implement it?"
leading to

for (i=0; i < 6; ++i) getc(fp);

The interviewer might even have wondered if you'd realize that
'fseek(fp, 6, SEEK_SET)' does /not/ retrieve the sixth byte in
a binary file at all, but rather the /seventh/ byte, because
'fseek' counts from zero. (It took me a while, too.)

And of course there are a whole slew of other answers in C++.
Don't crosspost between groups for different languages like this;
it just confuses people, and half the answers you get will be
unusable.

-Arthur

[SM Ryan]

ax*****@lycos.c om (Arvind Varma Kalidindi) wrote:
# Hi,
# I was asked this question in an interview recently. "How do you
# move to the 6th byte in a file?" ... My thinking would be to find the
# data types in the file, set a base pointer and advance it by 6. I
# mean, ptr+6. Another way to ask the same question is how do you move
# to the 3rd record in a file (considering that the file is made up of
# records).

Something here is garbled or missing. There are many different ways of
encoding records in a file, and of indexing (or not indexing) a file's
record. Without information about file and record formats and indexing,
there's no real answer.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
A bunch of savages in this town.

[RCollins]

SM Ryan wrote:

ax*****@lycos.c om (Arvind Varma Kalidindi) wrote:
# Hi,
# I was asked this question in an interview recently. "How do you
# move to the 6th byte in a file?" ... My thinking would be to find the
# data types in the file, set a base pointer and advance it by 6. I
# mean, ptr+6. Another way to ask the same question is how do you move
# to the 3rd record in a file (considering that the file is made up of
# records).

Something here is garbled or missing. There are many different ways of
encoding records in a file, and of indexing (or not indexing) a file's
record. Without information about file and record formats and indexing,
there's no real answer.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
A bunch of savages in this town.

No, at the C source-code level, all files are just an un-differentiated
list of bytes. If the OS stores files as files of fixed record types
(such as VAX/VMS), then it is up to the implementation to "hide" these
messy details from the programmer.

--
Ron Collins
Air Defense/RTSC/BCS
"I have a plan so cunning, you could put a tail on it and call it a weasel"

[CBFalconer]

SM Ryan wrote:

ax*****@lycos.c om (Arvind Varma Kalidindi) wrote:

# I was asked this question in an interview recently. "How do you
# find the move to the 6th byte in a file?" ... My thinking would
# be to data types in the file, set a base pointer and advance it
# by 6. I mean, ptr+6. Another way to ask the same question is how
# do you move to the 3rd record in a file (considering that the
# file is made up of records).

Something here is garbled or missing. There are many different
ways of encoding records in a file, and of indexing (or not
indexing) a file's record. Without information about file and
record formats and indexing, there's no real answer.

Please don't use a non-standard quote marker (your #). It fouls
up reformatting software.

Answering the first sentence in the OPs article: Just read the
first 6 bytes of the newly opened stream with fread(). Discard
them. You are positioned.

--
"Churchill and Bush can both be considered wartime leaders, just
as Secretariat and Mr Ed were both horses." - James Rhodes.
"A man who is right every time is not likely to do very much."
- Francis Crick, co-discover of DNA

[Keith Thompson]

RCollins <rc***@nospam.t heriver.com> writes:
[...]

No, at the C source-code level, all files are just an un-differentiated
list of bytes. If the OS stores files as files of fixed record types
(such as VAX/VMS), then it is up to the implementation to "hide" these
messy details from the programmer.

For purposes of C stdio, yes. In addition, the OS can (and does)
provide system-specific interfaces that expose all the fixd record
stuff to the programmer.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[SM Ryan]

Keith Thompson <ks***@mib.or g> wrote:
# RCollins <rc***@nospam.t heriver.com> writes:
# [...]
# > No, at the C source-code level, all files are just an un-differentiated
# > list of bytes. If the OS stores files as files of fixed record types
# > (such as VAX/VMS), then it is up to the implementation to "hide" these
# > messy details from the programmer.
#
# For purposes of C stdio, yes. In addition, the OS can (and does)
# provide system-specific interfaces that expose all the fixd record
# stuff to the programmer.

I didn't realise fgetpos and ftell had been redefined so they no longer
can return a cookie that is system dependent, but now always return a
byte offset. I bet that really cheesed off the VMS people.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
Raining down sulphur is like an endurance trial, man. Genocide is the
most exhausting activity one can engage in. Next to soccer.

[RCollins]

Keith Thompson wrote:

RCollins <rc***@nospam.t heriver.com> writes:
[...]

No, at the C source-code level, all files are just an un-differentiated
list of bytes. If the OS stores files as files of fixed record types
(such as VAX/VMS), then it is up to the implementation to "hide" these
messy details from the programmer.

For purposes of C stdio, yes. In addition, the OS can (and does)
provide system-specific interfaces that expose all the fixd record

^^^^^^^^^^^^^^^ ^^^^^^^^^^^ stuff to the programmer.

Non-standard stuff. Do we care about that in c.l.c. ?

--
Ron Collins
Air Defense/RTSC/BCS
"I have a plan so cunning, you could put a tail on it and call it a weasel"

[RCollins]

SM Ryan wrote:

Keith Thompson <ks***@mib.or g> wrote:
# RCollins <rc***@nospam.t heriver.com> writes:
# [...]
# > No, at the C source-code level, all files are just an un-differentiated
# > list of bytes. If the OS stores files as files of fixed record types
# > (such as VAX/VMS), then it is up to the implementation to "hide" these
# > messy details from the programmer.
#
# For purposes of C stdio, yes. In addition, the OS can (and does)
# provide system-specific interfaces that expose all the fixd record
# stuff to the programmer.

I didn't realise fgetpos and ftell had been redefined so they no longer
can return a cookie that is system dependent, but now always return a
byte offset. I bet that really cheesed off the VMS people.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
Raining down sulphur is like an endurance trial, man. Genocide is the
most exhausting activity one can engage in. Next to soccer.

Actually, it really made life easier (at least, in my group). VMS
provides about a ga-zillion different record formats, and we were
writing specific code for each format (when using FORTRAN). With the
more simplistic <stdio> stuff, all we had to do was parse our data
from a single input format.

--
Ron Collins
Air Defense/RTSC/BCS
"I have a plan so cunning, you could put a tail on it and call it a weasel"