Reading file with different format in each line in C

Question

I just start learning C(optional school course). I stuck on a small problem for 2 days. So the basic idea is, I have bunch of data in a file that I want to extract. However, there are 2 formats that the data has, and the first letter on each line determines what action I need to take.

For example, the data in file looks like these:

S:John,engineer,male,30
S:Alice,teacher,female,40
C:Ford Focus,4-door,25000
C:Chevy Corvette,sports,56000
S:Anna,police,female,36

What I want to do is, after open the file, read each line. If the first letter is S, then use
fscanf(fp, "%*c:%[^,],%[^,],%[^,],%d%*c",name,job,sex,&age)
to store all variable so I can pass them to function people().

But if the first letter is C, then use
fscanf(fp, "%*c:%[^,],%[^,],%d%*c",car,type,&price)
to store so I can pass them to function vehicle().

Would really appreciated if anyone can give me some pointer on how to do this. Thanks.

chux · Accepted Answer · 2014-04-11 01:15:34Z

1

There are many approaches, but separating IO from parsing is a good first step.

With line oriented data, it is so much cleaner to simply

FILE *inf = ...;
char buf[100];
if (fgets(buf, sizeof buf, inf) == NULL) Handle_EOForIOError();

Then parse it.

char name[sizeof buf];
char job[sizeof buf]; 
char sex[sizeof buf]; 
unsigned age;
char car[sizeof buf]; 
char type[sizeof buf]; 
unsigned cost; 
int n;

if (sscanf(buf, "S:%[^,],%[^,],%[^,],%u %n", name, job, sex, &age, &n) == 4 && 
    buf[n] == '\0') 
  Good_SRecord();
else if (sscanf(buf, "C:%[^,],%[^,],%u %n", car, type, &cost &n) == 3 && 
     buf[n] == '\0')  
  Good_CRecord();
else Garbage();

The " %n" trick is really good at making sure all the data parsed as expected without extra junk.

answered Apr 11, 2014 at 1:15

chux

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Comments

fvu · Accepted Answer · 2014-04-11 00:32:33Z

0

Here is the rough idea: parse the string in 2 steps:

"%c:%s"

will get you the first character and the rest of the string. Then, based on what you read in the first character, you can continue parsing the remaining part as

"%[^,],%[^,],%[^,],%d%c"

or

"%c:%[^,],%[^,],%d%c"

edited Apr 11, 2014 at 0:32

answered Apr 11, 2014 at 0:20

fvu

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5 Comments

user2848576 Over a year ago

oh, I just saw the %*c part on my lecture notes and I assume it suppose to be there whenever I'm using %[^ ] to scan for string. Must be my teacher's habbit. Will try to use it with your method and report back!

fvu Over a year ago

* indicates that you are not going to assign the result of that format specifier, ie throw the element away. It is not useful if you use the technique I propose. On the other hand, it would allow you to scan the unmodified input string for both steps instead of first chopping it in 2 parts, which would allow you to have 1 less intermediate variable.

user2848576 Over a year ago

Well, I put the concept to a smaller test program and it works. Just gonna pop it into my main program now. Thanks so much!

fvu Over a year ago

Glad to hear that! Good luck with your course - and if there aren't any better answers that appear in say the next 24 hours, may I ask that you accept this answer? That way future visitors know that the proposed solution actually works.

user2848576 Over a year ago

Works great on the main program with some modification(I used "%c:%[^\n]" instead of "%c:%s"). Answer accepted! Thank you so much!

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