Efficient coding to decode the sd txt file

Hi Community,

My txt file in sd card looks like this :




and I want to go through this file line by line, then convert each value after & into Hex value and save as array of byte. Converting the values into Hex is easy. However, i cant figure out a nice and novel way to go thorugh the values. readStringUntil("&") was my first option, but then the second value contains the first value and the third value contains the fist and second …

Does anybody have a tip?

#include <SPI.h>
#include <SD.h>
int i ;
File myFile;
char ch;
String arr;

void setup() {
  // Open serial communications and wait for port to open:
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only

  Serial.print("Initializing SD card...");

  if (!SD.begin(4)) {
    Serial.println("initialization failed!");
    while (1);
  Serial.println("initialization done.");

  myFile = SD.open("test2.txt");

  arr = "";

void loop() {
  // re-open the file for reading:
  if (myFile.available()  ) {
    // read from the file until there's nothing else in it:
     ch = myFile.read();

     if(ch != '\n')
          arr = arr + ch;

     if(ch == '\n')

      arr ="";

Above is my current code.

if(ch == '\n')

Do you want to do something when you get a null? Or when you see an '&'?

Or maybe both but the two things are different?

Does anybody have a tip?

Avoid Strings, they cause memory problems and program crashes.

Use C-strings instead, and the standard functions associated with them.

It is trivial to read a line like you've posted, separating out the ASCII numbers from the resulting C-string using the function strtok().

The ASCII numbers (which will be short C-strings) can be converted to binary using the function atoi().

So now, I changed my txt in SD a bit to make the reading more convenient.

Before :




The 385 indicates the ID of the data and 100100000255 indicates the dataset of INT8U (consisting of 8 INT8U).

Another problem occured :


with val = 385 and len = 8 and data_arr = 100100000255 (data_arr is a char array ; char data_arr[100])

void print_received_buf(unsigned long rxId,  byte *len, byte *rxBuf)
  //received data
      sprintf(msgString, "Standard ID: 0x%.3lX       DLC: %1d  Data:", rxId, len);
      for(int i=0; i<8;i++)
        //Serial.print("  ");

I expected that the output is like

Standard ID: 0x381 DLC: 8 Data:100100000255

However the actual output is

Standard ID: 0x381 DLC: 8 Data:4848484848484850

What am I missing here?

You can't mash integers together like that. How is the receiving code supposed to know that "100100000255" is "10 0 10 0 0 0 0 255" and not "100 10 0 0 0 0 2 55"?

If you keep spaces or '&' between integers you could use myFile.parseInt();. It skips anything that doesn't look like a number (like ' ' or '&'), gathers up stuff that looks like a number (digits) and returns the value when it hits something that doesn't look like a number (like ' ' or '&'). Use "if (myFile.available())" to see if you have reached the end of the file. Note: If your file doesn't end with a digit you will get an extra 0 at the end when the .parseInt() times out.

What am I missing here?

strtok(), mentioned in reply #2.