Thread: fseek(), moving ptr to specific line after every loop.

Hi

I have a array of numbers in a txt file as shown below

139
344
140
345
139
344
140
345

I want to read 2numbers fore every loop.(139.344 for first loop. 140.345 for 2nd loop)
I am using fseek(file,(j-1)*6,SEEK_SET);
wher j-1 give 3,5,6..

but I am not able to read the correct numbers fore every-loop. It read first 139,344 correctly and then 0, it is not pointing he correct line.
can anybody help me with this.

Originally Posted by mbr123

Hi

I have a array of numbers in a txt file as shown below

139
344
140
345
139
344
140
345

I want to read 2numbers fore every loop.(139.344 for first loop. 140.345 for 2nd loop)

If you are using the numbers sequentially, read the file sequentially using fscanf() or fgets() don't mess with the file pointer. In text files fseek() is extremely unreliable due to the possibility of hidden characters (trailing tabs, trailing spaces, cr/lf, etc.) fseek() is useful only in records based files using structs (sometimes called random access files).

I use fopen to open the text file
and I want to read every 2number for a loop, and read next set of 2number for 2nd loop, How Can I do that?

Just as you say. Make a loop, and inside the loop read two numbers (using fscanf or fgets at your leisure).

Originally Posted by mbr123

I use fopen to open the text file
and I want to read every 2number for a loop, and read next set of 2number for 2nd loop, How Can I do that?

You can't.... not and have it work reliably. As I already pointed out fseek() is totally unreliable in text files it is intended for fixed size records, not variable length lines.

Why don't you read them into two arrays in one loop... something like this...

Code:

int EvenLines[MaxLines/2] = {0};
int OddLines[MaxLines/2] = {0};
int idx = 0;

FILE f = fopen("textfile","r")
while ((!eof(f)) && (idx < MaxLines/2)))    
  { fscanf("%d",EvenLines[idx]);
     fscanf(%d",OddLines[idx]);
     idx++; }

Now you have two separate arrays one with lines 0, 2, 4, etc the other with lines 1, 3, 5, etc.

Or you could just read them all into one array and access them by odd and even indexes.

@adak... you're faster... mines better

I will explain you my situation

I was trying to change the start position of the txt file, so that every time the file open, it start position is set to the number I require and the file is read from that point.
I want to have that accessibility,
In my code, I can read the number I require once the start point in the file is set,
So now I tired to set the start point using fseek as I mentioned before. but now I have a problem using it for 3 digit numbers.

sgfile = fopen(params->SliceGroupConfigFileName,"r");
fseek(sgfile,(j-1)*4,SEEK_SET);
ret = fscanf(sgfile,"%d",(params->top_left+i)); //1st number
ret = fscanf(sgfile,"%d",(params->bottom_right+i)); //2nd number

so I am trying to change the start position of the text file for ever loop.

Originally Posted by mbr123

Code:

sgfile = fopen(params->SliceGroupConfigFileName,"r");

Try to open in binary mode.

If I recall correctly like this

Code:

sgfile = fopen(params->SliceGroupConfigFileName,"rb");

Note: This will change how file operations work.

Tim S.