Reading line by line in Arduino IDE

Question

I'm trying to read txt file (has numeric values) line by line. I used SPIFFS and I used this function

void readFile(fs::FS &fs, const char * path){
   Serial.printf("Reading file: %s\r\n", path);

   File file = fs.open(path);
   if(!file || file.isDirectory()){
       Serial.println("− failed to open file for reading");
       return;
   }
   
   int count = 0;
   Serial.println(" read from file:");

   while(file.available()){
    if (count < 100)  
      Serial.write(file.read());  
   } 
}

What is the alternative function for "file.read()" something like "readline" because I need to read the file from first line to 100 and from 101 to 200 and so on .

Answer 1

You need to use readStringUntil. Although it's not the most efficient way, i'll show you how it's done.

#include <vector>
#include <SPIFFS.h>

std::vector<double> readLine(String path, uint16_t from, uint16_t to) {
  std::vector<double> list;
  if(from > to) return list;
  File file = SPIFFS.open(path.c_str());
  if(!file) return list;
  uint16_t counter = 0;
  while(file.available() && counter <= to) {
    counter++;
    if(counter < from) {
      file.readStringUntil('\n');
    } else {
      String data = file.readStringUntil('\n');
      list.push_back(data.toDouble());
    }
  }
  return list;
}

void setup() {
  Serial.begin(115200);
  SPIFFS.begin(true);
  
  std::vector<double> numbers = readLine("file.txt", 0, 100);
  Serial.println("Data from 0 to 100:");
  uint16_t counter = 0;
  for (auto& n : numbers) {
    counter++;
    Serial.print(String(n) + "\t");
    if(counter % 10 == 0) {
      Serial.println();
    }
  }

  numbers = readLine("file.txt", 101, 200);
  Serial.println("\nData from 101 to 200:");
  counter = 0;
  for (auto& n : numbers) {
    counter++;
    Serial.print(String(n) + "\t");
    if(counter % 10 == 0) {
      Serial.println();
    }
  }
}

UPDATE

Supposed you have 1050 values and you want to parse it for each 100 values.

int i = 0;
while(i < 1050) {
  int start = i + 1;
  int end = (i + 100) > 1050 ? 1050 : i + 100;
  std::vector<double> numbers = readLine("file.txt", start, end);
  
  Serial.println("Data from " + String(start) + " to " + String(end) + ":");
  uint16_t counter = 0;
  for (auto& n : numbers) {
    counter++;
    Serial.print(String(n) + "\t");
    if(counter % 10 == 0) {
      Serial.println();
    }
  }

  i += 100;
}