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How to compare a string coming from serial monitor with some predefined text stored as a local variable?
if i say:

int led = 2;
String a = " abcds";
void setup()
Serial.begin(9600);


void loop(){
String b = Serial.read();
Serial.println(b);

if(b != a)
 digitalWrite(2,LOW);

else

digitalWrite(2,HIGH);

just as an example, this code will not compile because on the serial i receive bytes and i want to compare with a string.
So my question is...
how should be done?

Thanks in advance !

If you do a Google search on "Arduino String" you should find a class reference on the String class. https://www.arduino.cc/reference/en/language/variables/data-types/stringobject/

It has a function compareTo() that should do what you need.

C has strcmp() function that is used to compare two strings. It will return zero if two strings are equal non zero when not.

version using String (not recommended)

#define LED 2
const char* a = "abcd";

void setup() 
 Serial.begin(115200);
 pinMode(LED, OUTPUT);


void loop() 
 if (Serial.available()) 
 String s = Serial.readStringUntil('n');
 s.trim();
 if (s == a) 
 digitalWrite(LED, HIGH);
 else 
 digitalWrite(LED, LOW);
 
 

the version with C-string:

#define LED 2
const char* a = "abcd";
char buffer[32];

void setup() 
 Serial.begin(115200);
 pinMode(LED, OUTPUT);


void loop() 
 if (Serial.available()) 
 size_t l = Serial.readBytesUntil('n', buffer, sizeof(buffer - 1));
 if (buffer[l - 1] == 'r') 
 l--;
 
 buffer[l] = 0; // the terminating zero
 Serial.println(buffer);
 if (strcmp(buffer, a) == 0) 
 digitalWrite(LED, HIGH);
 else 
 digitalWrite(LED, LOW);
 
 

Here is a test sketch that uses a char array VS the String object. Please remember to set the serial monitor to send a newline only.

char inputBuffer[16];
char compareToThisString[] = "test string";

void setup()
 Serial.begin(9600);


void loop()

 if(Serial.available() > 0)

 Serial.readBytesUntil('n', inputBuffer, 16);

 if(strcmp(compareToThisString, inputBuffer) == 0)
 Serial.println("Matches");
 
 else
 Serial.println("No Match");
 
 
 memset(inputBuffer, 0, sizeof(inputBuffer));

As Egar Bonet mentions in his comments, there is a (up to) one second delay before Serial.readBytesUntil() terminates. That does not apply to the sketch I've written because the function terminates as soon as it receives the n character. Serial.readBytesUntil() is blocking code, but that is a different matter which may or may not be an issue for you, depending on what you're building and how much data you are sending. To reduce the timeout period, there is a Serial.setTimeout() function which could be set to whatever you want, but it only comes into play if you don't send the n character.