how do i read odd numbered lines from a file

[Richard Bos]

Quite right. I was thinking about reading only the odd numbered lines (IOW:
skipping lines) which can't really be done without some sort of assumptions
or extensions.

It can't be done with a database, either, because in that case it will
be the database that has to read the entire file to extract the odd
lines. Ok, so your C code won't, but it's cheating.

Richard

 

[Dann Corbit]

It can't be done with a database, either, because in that case it will
be the database that has to read the entire file to extract the odd
lines. Ok, so your C code won't, but it's cheating.

If a row is exactly one page, or if you cluster on (line number) %2, it will
not read the even numbered lines at all.

A database will only do a table scan if that is the cheapest way to examine
the data.

 

[Richard Bos]

If a row is exactly one page, or if you cluster on (line number) %2, it will
not read the even numbered lines at all.

If the database can do that, so can the C program. Sure, it won't be ISO
C without any dependencies, but then, neither is code that relies on a
database.
Of course, if you want to do this repeatedly to the same file, a
database is always the way to go.

Richard

 

[Dann Corbit]

If the database can do that, so can the C program. Sure, it won't be ISO
C without any dependencies, but then, neither is code that relies on a
database.
Of course, if you want to do this repeatedly to the same file, a
database is always the way to go.

One could argue that the database was probably written in C. Therefore, C
can do the job. But lots of effort is required.

 

[CBFalconer]

Quite right. I was thinking about reading only the odd numbered
lines (IOW: skipping lines) which can't really be done without
some sort of assumptions or extensions.

You can't advance to the next line in a C text file without reading
the line and scanning for a '\n'. So where do databases come into
it? A solution might be (using stdin/stdout):

#include <stdio.h>
int main(void) {
unsigned int lines;
int ch;

lines = 0;
while (EOF != (ch = getchar())) {
if (lines & 1) putchar(ch);
if ('\n' == ch) lines++;
}
return 0;
}

If you really want even numbered lines you can just alter the
initialization of lines. It depends if you consider the first line
the zeroth or the first

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