From .txt file in sd card to string variable in Arduino

Question

I'm trying to read a text file in an Arduino SD card reader and copy its text into a string variable, but the function .read always returns -1. How can I solve this problem?

Here's the code:

#include <SPI.h>
#include <SD.h>

File mappa;
String text;

void setup() {
Serial.begin(9600);
while (!Serial) {
  ;
}
Serial.print("Initializing SD card...");
if (!SD.begin(4)) {
  Serial.println("initialization failed!");
  return;
}
Serial.println("initialization done.");

// open the file. note that only one file can be open at a time,
// so you have to close this one before opening another.
mappa = SD.open("map.txt");

// if the file opened okay, write to it:
if (mappa) {
  Serial.println("File aperto");
} else {
  // if the file didn't open, print an error:
  Serial.println("error opening map.txt");
}
Serial.println("map.txt:");

// read from the file until there's nothing else in it:
while (mappa.available()) {
  Serial.write(mappa.read());
 // text = parseInt(mappa.read());
}
Serial.println(text);
  // close the file:
  mappa.close();  
}
void loop() {
  // nothing happens after setup
}

我知道它.read()返回一个整数数组，但我不知道如何分别访问它们。

Answer 1

经过进一步研究，我了解了.read工作原理：它在推进光标时读取其光标指向的字符。

因此，为了读取整个文件，您必须删除该Serial.write部分并将字符转换为char：

String finalString = "";
while (mappa.available())
{
  finalString += (char)mappa.read();
}