String parsing to extract int in C++ for Arduino - c++

Question

I'm trying to write a sketch that allows a user to access data in EEPROM using the serial monitor. In the serial monitor the user should be able to type one of two commands: “read” and “write. "Read" should take one argument, an EEPROM address. "Write" should take two arguments, an EEPROM address and a value. For example, if the user types “read 7” then the contents of EEPROM address 7 should be printed to the serial monitor. If the user types “write 7 12” then the value 12 should be written into address 7 of the EEPROM. Any help is much appreciated. I'm not an expert in Arudino, still learning ;). In the code below I defined inByte to be the serail.read(). Now how do I extract numbers from the string "inByte" to assign to "val" and "addr"
void loop() {
String inByte;
if (Serial.available() > 0) {
// get incoming byte:
inByte = Serial.read();
}
if (inByte.startsWith("Write")) {
EEPROM.write(addr, val);
}
if (inByte.startsWith("Read")) {
val= EEPROM.read(addr);
}
delay(500);
}

Answer 1

Serial.read() only reads a single character. You should loop until no more input while filling your buffer or use a blocking function like Serial.readStringUntil() or Serial.readBytes() to fill a buffer for you.
https://www.arduino.cc/en/Serial/ReadStringUntil
https://www.arduino.cc/en/Serial/ReadBytes
Or you can use Serial.parseInt() twice to grab the two values directly into a pair of integers. This function will skip the non numerical text and grab the values. This method is also blocking.
https://www.arduino.cc/en/Reference/StreamParseInt
A patch I wrote to improve this function is available in the latest hourly build, but the old versions still work fine for simple numbers with the previous IDE's
The blocking methods can be tweaked using Serial.setTimeout() to change how long they wait for input (1000ms default)
https://www.arduino.cc/en/Serial/SetTimeout

Answer 2

[missed the other answer, there's half my answer gone]
I was going to say use Serial.readStringUntil('\n') in order to read a line at a time.
To address the part:
how do I extract numbers from the string "inByte" to assign to "val" and "addr"
This is less trivial than it might seem and a lot of things can go wrong. For simplicity, let's assume the input string is always in the format /^(Read|Write) (\d+)( \d+)?$/.
A simple way to parse it would be to find the spaces, isolate the number strings and call .toInt().
...
int val, addr;
int addrStart = 0;
while(inByte[addrStart] != ' ' && addrStart < inByte.length())
addrStart++;
addrStart++; //skip the space
int addrEnd = addrStart + 1;
while(inByte[addrEnd] != ' ' && addrEnd < inByte.length())
addrEnd++;
String addrStr = inByte.substring(addrStart, addrEnd); //excludes addrEnd
addr = addrStr.toInt();
if (inByte.startsWith("Write")) {
int valEnd = addrEnd+1;
while(inByte[varEnd] != ' ' && varEnd < inByte.length())
valEnd++;
String valStr = inByte.substring(addrEnd+1, valEnd);
val = valStr.toInt();
EEPROM.write(addr, val);
}
else if (inByte.startsWith("Read")) {
val = EEPROM.read(addr);
}
This can fail in all sorts of horrible ways if the input string has a double space or the numbers are malformed, or has any other subtle error.
If you're concerned with correctness, I suggest you look into a regex library, or even an standard format such as JSON - see ArduinoJson.