How to read the latest date from file in C

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#include <stdio.h>
#include <string.h>
#define MAXC 1024    
#define MAXDT 16

int record(char *string){
  char datestr[MAXDT]; 
  unsigned dateval;
  unsigned mo, day, yr;    
  unsigned tmpu = 0;      

  char *p = strrchr (string, ','), tmpstr[MAXDT] = "";

  do{
    if(!p)
      continue;    
    if(sscanf(p + 1, "%u-%u-%4u", &day, &mo, &yr)!= 3)
      continue;

    sprintf(tmpstr, "%04u%02u%02u", yr, mo, day);  
    if(sscanf(tmpstr, "%d", &tmpu) != 1) 
      continue;
          
    if(tmpu > dateval){   
      dateval = tmpu;     
      strcpy(datestr, p + 1);
      datestr[strcspn (datestr, "\n")] = 0;  
    }
  }while(0);
    
    printf("latest date in file: %s\n", datestr); 
    return 0;
}

int main () {
  char string[MAXC];
  FILE * fp1 = fopen("file.csv", "r");
  FILE * fp2 = fopen("new.csv", "r");

  while(fgets(string, MAXC, fp1)){
    record(string);
  } 
  printf("\n");
  while(fgets(string, MAXC, fp2)){
    record(string);
  }
}

What I have tried:

In this code, I am just trying to read the latest date in the file but it is printing the whole data from file, which I don't want. How to make this working?

Posted 21-Oct-20 8:44am

ibilalkayy

Updated 21-Oct-20 18:06pm

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Richard MacCutchan 22-Oct-20 4:35am

How many more times do you plan to post this same issue? You have been given a number of suggestion as to how you can parse data from lines of text, how you can capture specific items, etc.

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OriginalGriff · Answer 1 · 2020-10-21T08:54:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Rick York · Answer 2 · 2020-10-21T18:07:00

You know, if you use the parsing function I showed you earlier, you can define '-' to be another delimiter and it will separate the date components out for you. You could use ",-\n" as the delimiter string and one call would separate everything for you. I really don't understand your problem with it. It is just a simple loop calling strtok. That's all there is to it. I think it is a far better alternative than calling strrchr, sccanf, sprintf, sccanf, strcpy, and strcspn. That is six different string handling functions instead of just one.

Here is a hypothetical example. Let's say the text string you read from each line of a file looks like this :

Last Name,First Name,PIN,accountNumber,PhoneNumber,YYYY-MM-DD,postalCode,"a comment that might have spaces"

and assume no items have any dashes except for the date field. You can use a delimiter string of ",-\"\n" and you will get these tokens from the function :

0. 'Last Name'
1. 'First Name'
2. 'PIN'
3. 'accountNumber'
4. 'PhoneNumber'
5. 'YYYY'
6. 'MM'
7. 'DD'
8. 'postalCode'
9. 'a comment that might have spaces'

I added the single quotes so you can see exactly what it returns.

To get a value for the date you can use in numerical comparisons I would do this :

C++

int year = atoi( tokens[ 5 ] );
int month = atoi( tokens[ 6 ] );
int day = atoi( tokens[ 7 ] );
int dateValue = ( year * 10000 ) + ( month * 100 ) + day;

That puts the date into the standard format YYYYMMDD [^]. The date should have been written in that format in the first place.

This is not tricky at all. It is very, very simple.

How to read the latest date from file in C

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