Writing data to a specific address in EEPROM

Question

I'm trying to write two strings to an Arduino Uno's EEPROM. Here, the first code block works fine: it writes string(True/False) at 'address 0'. But the second block, which writes data at 'address 5', is not working as expected.Reading data at address 5 prints a blank line. What am I missing here? Is this the correct way to write two different strings to EEPROM?

First code block:

int state = LOW;
char first_eeprom_value;
if (state == LOW) {
 state = HIGH;
 char example_string[4] = "True";
 first_eeprom_value = EEPROM.read(0);
 Serial.println(first_eeprom_value);
 for (int i = 0 ; i < 5 ; i++) {
 EEPROM.update(i, example_string[i]);
 }
} else {
 state = LOW;
 char example_string[5] = "False";
 first_eeprom_value = EEPROM.read(0);
 Serial.println(first_eeprom_value);
 for (int i = 0 ; i < 5; i++) {
 EEPROM.update(i, example_string[i]);
 }
}
Time = millis();

Second code block:

char first_eeprom_value1;
if (state1 == LOW) {
 state1 = HIGH;
 char example_string1[4] = "True";
 first_eeprom_value1 = EEPROM.read(5); 
 Serial.println(first_eeprom_value1);
 for (int i = 5 ; i < 11 ; i++) {
 EEPROM.update(i, example_string1[i]);
 }
} else {
 state1 = LOW;
 char example_string1[5] = "False";
 first_eeprom_value1 = EEPROM.read(5); 
 Serial.println(first_eeprom_value1);
 for (int i = 5 ; i < 11 ; i++) {
 EEPROM.update(i, example_string1[i]);
 }
}
Time = millis();
//reading data at address 5
int addr = 5;
char value = EEPROM.read(addr);
Serial.println(value);// this prints blank line

• Nick Gammon: is not working as expected - what do you expect, and what is actually happening?
• smc: Nothing is getting written when using 2nd code block.
• Nick Gammon: How do you know? Can you please amend your question to include the output from the serial prints that you have in your code?
• smc: I have edited the question and 2nd code block.
• jsotola: remove the if ... then ... else statements ... minimize the code as much as possible ... do just a write and then a read ... do you still have the same problem?

Answer 1

You are cycling outside the string, thus creating undefined behavior.

Try replacing the second block with this:

char first_eeprom_value1;
if (state1 == LOW) {
 state1 = HIGH;
 char example_string1[4] = "True";
 first_eeprom_value1 = EEPROM.read(5); 
 Serial.println(first_eeprom_value1);
 for (int i = 0; i < sizeof(example_string1); i++) {
 EEPROM.update(i + 5, example_string1[i]);
 }
} else {
 state1 = LOW;
 char example_string1[5] = "False";
 first_eeprom_value1 = EEPROM.read(5); 
 Serial.println(first_eeprom_value1);
 for (int i = 0; i < sizeof(example_string1); i++) {
 EEPROM.update(i + 5, example_string1[i]);
 }
}

Please note that this is not optimal; I suggest you to use fixed-length strings, and fill the unused spaces with blanks or 0s, so you can read back all the bytes you wrote in a safe way