XRAD'S How to get last line of SD data file? - c++

Question

My SD data file does not have a set 'line' limit, nor does it have data of fixed lengths on each 'line.'
Is there a way to read the SD data file to the last 'line' AND then only Serial.print() just that last 'line'?
That last line of SD data is going to be used as the first 'home' coordinate in a GPS tracker if one was to continue a journey from last location.....
even if I add white space to make the first and second saved values XXXXXX (999999), I still am not sure how to find JUST the last line in an SD data file of an unknown length...(trying to keep the SD file as ONE file, not of fixed length)
//teensy 4.1 with BUILTIN_SDCARD
#include <SD.h>
File myFile;
String buffer;//buffer to redraw saved SD data
void lastLine()
{
//myFile = SD.open ("GPS", O_APPEND);
myFile = SD.open("GPS");
while (myFile.available()) {
if (myFile) {
buffer = myFile.readStringUntil('\n');
//either from the buffer or SD I would like to
//just read the last line of the file
Serial.println(buffer);
//Serial.println(myFile.size());
}
}
}
void setup()
{
Serial.begin(9600);
Serial.print("Initializing SD card...");
if (!SD.begin(BUILTIN_SDCARD)) {
Serial.println("initialization failed!");
return;
}
Serial.println("initialization done.");
// open the file.
myFile = SD.open("GPS", FILE_WRITE);
// if the file opened okay, write to it:
if (myFile) {
Serial.print("Writing to GPS...");
//GPS device / code continually writes data to SD
//but using this below for brevity/debugging
myFile.println("123, 144");//these stand for the x/y data values
myFile.println("1424, 254");//and the length of x/y may go as high
myFile.println("11235, 333");//as x= 999,999, y = 999,999 etc...
//etc..for hundreds/thousands of 'lines'
// close the file:
myFile.close();
Serial.println("done.");
}
lastLine();//just read the LAST line of SD data...
}
void loop()
{
// nothing happens after setup
}
EDIT: Thank you for ideas! OK ..so i tried your suggestions I think. I can get a counter going (first step) but it only increments to 20! I'm confused as to why/how this happens.....
here is my current code. Even though there are 50 'lines' in the SD file, I'm only reading 20 (get to sizeof())....IF I change the code to print ALL the lines to serial, it won't count lines....and IF there are LESS than 20 lines, serial monitor adds empty lines until '20' is reached..
and sizeof(myFile) is also 20..not sure why
//teensy 4.1 with BUILTIN_SDCARD
#include <SD.h>
File myFile;
int counter = 0;
String buffer;//buffer to redraw saved SD data
void lastLine()
{
//myFile = SD.open ("GPS", O_APPEND);
myFile = SD.open("GPS");
while (myFile.available()) {
for (int i = 0; i < sizeof(myFile); i++) {
String line = myFile.readStringUntil('\n');
buffer = myFile.readStringUntil('\n');
const char* listConv = line.c_str();
Serial.print ("line: ");
Serial.println (line);
counter ++;
}
if (counter == sizeof(myFile)) {
Serial.print ("Done. Counter: ");
Serial.println (counter);
Serial.print ("sizeof myFile: ");
Serial.println (sizeof(myFile));
myFile.close();
counter = 0;
}
}
}
//buffer = myFile.readStringUntil('\n');
//either from the buffer or SD I would like to
//just read the last line of the file
//Serial.println(buffer);
//Serial.println(myFile.size());
void setup()
{
Serial.begin(9600);
Serial.print("Initializing SD card...");
if (!SD.begin(BUILTIN_SDCARD)) {
Serial.println("initialization failed!");
return;
}
Serial.println("initialization done.");
// open the file.
myFile = SD.open("GPS", FILE_WRITE);
// if the file opened okay, write to it:
if (myFile) {
Serial.print("Writing to GPS...");
//GPS device / code continually writes data to SD
//but using this below for brevity/debugging
myFile.println("123, 144");//these stand for the x/y data values
myFile.println("1424, 254");//and the length of x/y may go as high
myFile.println("11235, 333");//as x= 999,999, y = 999,999 etc...
//etc..for hundreds/thousands of 'lines'
// close the file:
myFile.close();
Serial.println("done.");
}
lastLine();//just read the LAST line of SD data...
}
void loop()
{
// nothing happens after setup
}
ok so with some rewriting, I can now count to last file line using:
while (myFile.available()) {
for (int i = 0; i < sizeof(myFile.available()); i++) {
String line = myFile.readStringUntil('\n');
Serial.print ("line: ");
Serial.println (line);
counter ++;
Serial.print ("Counter: ");
Serial.println (counter);
if (counter == 400) {//testing different lines, works...
Serial.print ("Last Line: ");//prints the line at counter 'x'
Serial.println (line);
}
}
}
next step is to grab that last line after calculating the 'maximum' number of lines (minus a few that don't hold data I need at the end of the SD file).....

Answer 1

It was a bug in the library. conflict between SDFat, adafruit fork, and arduino SD. Had to delete all SD libs and reload....
working now...