Writing last n lines from one file to another - c++

Question

This is the program I have written which copies all the lines from diskData.dat file to 24HrDiskData.dat file. As of now I am copying all the lines from one file to another I wish to copy last n lines from diskData.dat to 24HrDiskData.dat
#include <iostream>
#include <stdio.h>
#include <fstream>
#include <stdlib.h>
using namespace std;
int main(int argc, char *argv[]) {
FILE *HrData;
char tempData[1024];
int flag = 0;
ofstream fout;
fout.open("24HrDiskData.dat", ios::app); // open file for appending
assert (!fout.fail());
if ((HrData = fopen("/home/xvishuk/diskData.dat", "r")) == NULL) {
printf("\nFile cannot be opened");
}
while (fgets(tempData, 1024, HrData) != NULL) {
fout << tempData; // write the data to the file
}
return 0;
}
Sample pattern of Delimited data
|Sat Mar 26 18:47:57 2016|1|1|182|60.66|3|30|4782|31|68|4|3467750|110|43.1562|64|
|Sat Mar 26 19:01:49 2016|1|1|140|46.26|3.03|30|4782|30|68|4|3467764|96|43.1562|64|
|Sat Mar 26 19:15:40 2016|1|1|184|61.07|3.01|30|4782|30|68|4|3467777|112|43.1562|64|
|Sat Mar 26 19:29:30 2016|1|1|180|59.91|3|30|4782|32|68|4|3467791|98|43.1562|64|
|Sat Mar 26 19:43:20 2016|1|1|194|64.61|3|30|4782|32|68|4|3467805|114|43.1562|64|
|Sat Mar 26 19:57:17 2016|1|1|170|56.62|3|30|4782|30|68|4|3467818|102|43.1562|64|
|Sat Mar 26 20:11:14 2016|1|1|140|46.32|3.02|30|4782|30|68|4|3467832|118|43.1562|64|
|Sat Mar 26 20:25:12 2016|1|1|176|58.35|3.02|30|4782|30|68|4|3467846|104|43.1562|64|
|Sat Mar 26 20:39:10 2016|1|1|202|66.9|3.02|30|4782|30|68|4|3467859|120|43.1562|64|
|Sat Mar 26 20:53:11 2016|1|1|198|65.85|3.01|30|4782|31|68|4|3467873|106|43.1562|64|
|Sat Mar 26 21:07:12 2016|1|1|184|60.97|3.02|30|4782|32|68|4|3467887|92|43.1562|64|
|Sat Mar 26 21:21:11 2016|1|1|152|50.28|3.02|30|4782|31|68|4|3467901|108|43.1562|64|
|Sat Mar 26 21:35:16 2016|1|1|168|55.77|3.01|30|4782|30|68|4|3467915|94|43.1562|64|
|Sat Mar 26 21:49:20 2016|1|1|172|57.03|3.02|30|4782|31|68|4|3467928|112|43.1562|64|
|Sat Mar 26 22:03:26 2016|1|1|152|50.56|3.01|30|4782|33|68|4|3467942|98|43.1562|64|
|Sat Mar 26 22:17:32 2016|1|1|174|57.86|3.01|30|4782|31|68|4|3467956|114|43.1562|64|
|Sat Mar 26 22:31:38 2016|1|1|156|51.86|3.01|30|4782|30|68|4|3467970|100|43.1562|64|
|Sat Mar 26 22:45:44 2016|1|1|202|66.57|3.03|30|4782|30|68|4|3467984|116|43.1562|64|
|Sat Mar 26 22:59:55 2016|1|1|188|62.4|3.01|30|4782|31|68|4|3467998|102|43.1562|64|
|Sat Mar 26 23:14:06 2016|1|1|164|53.95|3.04|30|4782|32|68|4|3468012|118|43.1562|64|
|Sat Mar 26 23:28:17 2016|1|1|168|55.78|3.01|30|4782|31|68|4|3468026|104|43.1562|64|
|Sat Mar 26 23:42:28 2016|1|1|176|58.33|3.02|30|4782|30|68|4|3468040|120|43.1562|64|
|Sat Mar 26 23:56:39 2016|1|1|170|55.76|3.05|30|4782|30|68|4|3468053|106|43.1562|64|
|Sun Mar 27 00:10:54 2016|1|1|172|56.97|3.02|30|4782|31|68|4|3468067|92|43.1562|64|
|Sun Mar 27 00:25:13 2016|1|1|184|61.23|3.01|30|4782|33|68|4|3468081|108|43.1562|64|
|Sun Mar 27 00:39:26 2016|1|1|188|62.12|3.03|30|4782|31|68|4|3468096|94|43.1562|64|
|Sun Mar 27 00:53:42 2016|1|1|170|56.42|3.01|30|4782|30|68|4|3468110|110|43.1562|64|
|Sun Mar 27 01:07:58 2016|1|1|174|57.8|3.01|30|4782|31|68|4|3468124|96|43.1562|64|
Expected Output
The last 10 records.
|Sat Mar 26 22:45:44 2016|1|1|202|66.57|3.03|30|4782|30|68|4|3467984|116|43.1562|64|
|Sat Mar 26 22:59:55 2016|1|1|188|62.4|3.01|30|4782|31|68|4|3467998|102|43.1562|64|
|Sat Mar 26 23:14:06 2016|1|1|164|53.95|3.04|30|4782|32|68|4|3468012|118|43.1562|64|
|Sat Mar 26 23:28:17 2016|1|1|168|55.78|3.01|30|4782|31|68|4|3468026|104|43.1562|64|
|Sat Mar 26 23:42:28 2016|1|1|176|58.33|3.02|30|4782|30|68|4|3468040|120|43.1562|64|
|Sat Mar 26 23:56:39 2016|1|1|170|55.76|3.05|30|4782|30|68|4|3468053|106|43.1562|64|
|Sun Mar 27 00:10:54 2016|1|1|172|56.97|3.02|30|4782|31|68|4|3468067|92|43.1562|64|
|Sun Mar 27 00:25:13 2016|1|1|184|61.23|3.01|30|4782|33|68|4|3468081|108|43.1562|64|
|Sun Mar 27 00:39:26 2016|1|1|188|62.12|3.03|30|4782|31|68|4|3468096|94|43.1562|64|
|Sun Mar 27 00:53:42 2016|1|1|170|56.42|3.01|30|4782|30|68|4|3468110|110|43.1562|64|

Answer 1

To only write the last n lines, you could allocate an array of n strings and store the lines into it as you read them, only keeping the last n when the array fills up, using modular index to avoid unnecessary copying.
When you reach the end of file, output the lines from the array.
Also avoid mixing C and C++. Either use <stdio.h> or <iostream>, but using both at the same time is error prone and inelegant.
Here is a simple implementation in C:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 100
int main(void) {
char line[1024];
char *stash[N] = { NULL };
int i, j;
FILE *HrData, *fout;
if ((fout = fopen("24HrDiskData.dat", "a")) == NULL) {
fprintf(stderr, "cannot open 24HrDiskData.dat for appending: %s\n",
strerror(errno));
return 1;
}
if ((HrData = fopen("/home/xvishuk/diskData.dat", "r")) == NULL) {
fprintf(stderr, "cannot open input file: %s\n",
strerror(errno));
return 1;
}
for (i = 0; fgets(line, 1024, HrData) != NULL;) {
free(stash[i]);
stash[i] = strdup(line);
i = (i + 1) % N;
}
fclose(HrData);
for (j = i;;) {
if (stash[i])
fputs(stash[i], fout);
i = (i + 1) % N;
if (i == j)
break;
}
fclose(fout);
return 0;
}

Answer 2

Two passes.
First pass, count the lines and store their file positions.
Second pass, calculate the line number you want to start copying from. Look up its file position in the container. Seek to the file position. Start copying.
Edit 1: Example code
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
int main(void)
{
std::string text_line;
std::vector<std::streampos> file_positions;
std::ifstream input_file("my_file.txt");
file_positions.push_back(input_file.tellg());
while (std::getline(input_file, text_line))
{
file_positions.push_back(input_file.tellg());
}
// The total lines is file_positions.size().
// Return the last 13 lines
const unsigned int total_lines = file_positions.size();
std::streampos seek_position = 0;
unsigned int index = 0U;
if (total_lines > 13U)
{
index = total_lines - 13U;
}
input_file.clear();
input_file.seekg(file_positions[index]);
// Copy text lines to output file...
return 0;
}
Edit 2 - More efficient method
You could maintain a container of N text lines as you read through the file.
#include <deque>
//...
int main(void)
{
std::deque<std::string> text_lines(N);
while (std::getline(input_file, text_line))
{
if (text_lines.size() == N)
{
text_lines.pop_front();
}
text_lines.push_back(text_line);
}
// Now copy the text lines from the `std::deque` to the output file.
//...
return 0;
}