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#470913 - 02/12/18 21:31 Read a line from a .txt file?
Hredot Offline
Member

Registered: 09/11/17
Posts: 229
I guess one could use a generic C code to read the first line from a txt file, as for instance described here.
Does Zorro by any chance have a more convenient function to accomplish this task?
Or, how would one read the LAST line in a .txt file?


Edited by Hredot (02/12/18 21:48)

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#470914 - 02/13/18 00:05 Re: Read a line from a .txt file? [Re: Hredot]
AndrewAMD Online
Senior Member

Registered: 02/21/17
Posts: 384
Loc: Chicago
Zorro has some handy file functions, which you can easily wrap with a helper function.
http://zorro-project.com/manual/en/file_.htm

But how you would wrap them depends on what you are trying to do.

For example, you can convert a file into a char* string and change the first newline character '\n' to a null character '\0' to get the first line.

For last line, simply find the last '\n' and point to the character after it.

I might be able to demo this first thing in the morning tomorrow unless someone beats me to it.

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#470915 - 02/13/18 07:05 Re: Read a line from a .txt file? [Re: AndrewAMD]
Hredot Offline
Member

Registered: 09/11/17
Posts: 229
@AndrewAMD
Awesome link, thanks! Looking forward to your demo!

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#470932 - 02/13/18 13:52 Re: Read a line from a .txt file? [Re: Hredot]
AndrewAMD Online
Senior Member

Registered: 02/21/17
Posts: 384
Loc: Chicago
Well, I didn't write helpers, but I left you a ton of clues on how to write helpers for your own use case.

See attached.

I suppose that if I had more time, I can write it into an identical style function, like "file_read_firstline" and "file_read_lastline".

Code:
int main()
{
	// create a file
	const string f = "./mumble.txt";
	file_write(f,"Many mumbling mice",0);
	file_append(f,"\nAre making merry music in the moonlight!",0);
	file_append(f,"\nMighty nice!",0);
	
	// check file length
	long l = file_length(f), l2 = 0;
	printf("\nlength: %d",l);
	
	// allocate memory for reading the file into a string
	char* str = malloc(l+1);
	memset(str,0,l+1);
	
	// fill the string with data and check how much data was copied
	l2 = file_read(f,str,l+1);
	printf("\nlength2: %d",l2);
	
	// let's see where the newline characters are
	int i = 0;
	for(i = 0; i<l; i++)
	{
		if(str[i]=='\n') printf("\ni: %d, found newline!",i);
	}
	// let's print the string
	printf("\n%s",str);
	
	// let's find the last newline character (if not found, zero)
	for(i = l-1; i>0; i--)
	{
		if(str[i]=='\n') {break;}
	}
	// point to the last part of the string by offsetting the pointer
	char* lastline = str + i;
	printf("%s",lastline);
	
	// okay, now let's alter the original string to convert it into the first line only.
	// Remember, a string is all data before a null character.
	for(i=0; i<l; i++)
	{
		if(str[i]=='\n') {str[i]=0;break;}
	}
	// let's print the altered string (first line)
	printf("\n%s",str);
	
	
	// always free memory after using malloc
	free(str);
	return 0;
}



Attachments
mumble.c (17 downloads)


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#470935 - 02/13/18 14:24 Re: Read a line from a .txt file? [Re: AndrewAMD]
Hredot Offline
Member

Registered: 09/11/17
Posts: 229
@AndrewAMD:
Thanks for the code!

I wonder, does malloc remember how much memory was allocated originally, or does free(str); only free up memory up to the null character, which could potentially lead to a memory leak?

Isn't loading the entire file just to read one line a bit excessive? Perhaps some ideas presented here could help ease the workload.

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#470937 - 02/13/18 14:41 Re: Read a line from a .txt file? [Re: Hredot]
AndrewAMD Online
Senior Member

Registered: 02/21/17
Posts: 384
Loc: Chicago
Originally Posted By: Hredot
does free(str); only free up memory up to the null character, which could potentially lead to a memory leak?
No. You can use malloc and free for any kind of data, not just char arrays.
Quote:
Isn't loading the entire file just to read one line a bit excessive? Perhaps some ideas presented here could help ease the workload.
I was trying to help you solve your problem in five minutes or less. You're writing in C... why do you care about these optimizations? Are you reading gigabytes of data?

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#470941 - 02/13/18 15:25 Re: Read a line from a .txt file? [Re: AndrewAMD]
Hredot Offline
Member

Registered: 09/11/17
Posts: 229
I see, OK!

Oh, I don't know... Perhaps I don't have a good perception for what could cause slowdown in C. Coming from a more higher level languages, I get worried about that too much I guess. laugh

In any case, thank you for your help! This is very useful code!


Edited by Hredot (02/13/18 15:25)

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#470942 - 02/13/18 15:31 Re: Read a line from a .txt file? [Re: Hredot]
AndrewAMD Online
Senior Member

Registered: 02/21/17
Posts: 384
Loc: Chicago
Ah, I see. Yes, this is not python or R. wink

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