change values in a file

[John Smith]

I have a file which contains two columns, name of students and
average. I would like to read in the name and average,
re-calculate the new average based on the latest test score and
write the new average to the file. I know how to do the first two
parts (reading the name and average and calculating the new
average). How do I do the last part, i.e. write the new average to
the file?

here is a sample file
---------------------------------
#comment: this is a sample file
student1 average1
student2 average2
student3 average3

[Arafangion]

John Smith wrote:
<snip> I would like to read in the name and average,

re-calculate the new average based on the latest test score and
write the new average to the file.

Don't you need to know all the original values in order to compute the
average correctly?

[Jack Klein]

On Mon, 16 May 2005 01:38:45 GMT, John Smith <js****@company.com>
wrote in comp.lang.c:

I have a file which contains two columns, name of students and
average. I would like to read in the name and average,
re-calculate the new average based on the latest test score and
write the new average to the file. I know how to do the first two
parts (reading the name and average and calculating the new
average). How do I do the last part, i.e. write the new average to
the file?

here is a sample file
---------------------------------
#comment: this is a sample file
student1 average1
student2 average2
student3 average3

This sounds like the general problem of replacing text in a text file,
which is addressed briefly in the FAQ for comp.lang.c, link in my
signature.

It is possible to do this directly if and only if the new text is
exactly the same length as the original text. File systems are not
like word processors or editors. They do not provide a method of
shoving the rest of the text down if you write more in the middle, or
pulling the rest of the file up if you take something out.

The standard way to do something like this is to create a new file for
writing with a temporary name, one you make up or one generated by the
standard tmpnam() function. Then open your original file for reading.

Read from the original file line-by-line, perhaps using fgets(), make
whatever modifications you need to the data, and write it to the new
file most likely with fputs() or fprintf().

When you have finished, fclose() both files. Either remove() or
rename() the original file (perhaps to some pattern used for back up
files), then rename() the new file to the name of the original.

This method handles modifications where deletions and additions are
different sizes or in different parts of the file.

All of the functions I mentioned are prototyped in <stdio.h>.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Kenny McCormack]

In article <42********@dnews.tpgi.com.au>,
Arafangion <Ar********@invalid.email.address.com> wrote:

John Smith wrote:
<snip> I would like to read in the name and average,

re-calculate the new average based on the latest test score and
write the new average to the file.

Don't you need to know all the original values in order to compute the
average correctly?

No. You just need to know the current average and the (current) number of
observations. You do the math.

[Arafangion]

Kenny McCormack wrote:

In article <42********@dnews.tpgi.com.au>,
Arafangion <Ar********@invalid.email.address.com> wrote:

John Smith wrote:
<snip> I would like to read in the name and average,

re-calculate the new average based on the latest test score and
write the new average to the file.

Don't you need to know all the original values in order to compute the
average correctly?

No. You just need to know the current average and the (current) number of
observations. You do the math.

I didn't see any current number of observations in the original description.

[Kenny McCormack]

In article <42******@dnews.tpgi.com.au>,
Arafangion <Ar********@invalid.email.address.com> wrote:

Kenny McCormack wrote:

In article <42********@dnews.tpgi.com.au>,
Arafangion <Ar********@invalid.email.address.com> wrote:

John Smith wrote:
<snip> I would like to read in the name and average,

re-calculate the new average based on the latest test score and
write the new average to the file.

Don't you need to know all the original values in order to compute the
average correctly?

No. You just need to know the current average and the (current) number of
observations. You do the math.

I didn't see any current number of observations in the original description.

Wasn't really stated one way or the other.

But, maybe that's already known - say, if they (the students) get one test
a week, and we know (from looking at a calendar) how many weeks have gone
by.