Array value changes when running through For loop

[Chazza]

Hi
I am seeing an issue when populating an array with values.
Please see code below.

Under void setup()
I am populating an array with a For loop.
Then I print out the last value of the array

Under void loop()
If I read back the array with a For loop I get a strange value at the end of the array.
If I comment out the For loop it prints the correct last value of the array.

I would be grateful if anybody could point me to what I am missing here please? I have tried this on a Mega 2560 and an ESP32
Thank you in advance
Chazza

int myArray[255];

void setup() {
	Serial.begin(115200);

	for (int i = 0; i <= 255; i++) {
		myArray[i] = i;
	}


	Serial.print("myArray[255] under setup = ");
	Serial.println(myArray[255]);

} // end of setup



void loop() {

	for (int i = 0; i <= 255; i++) {
		Serial.print(i);
		Serial.print(". ");
		Serial.println(myArray[i]);
	}


	Serial.print("myArray[255] under loop = ");
	Serial.println(myArray[255]);

	while (1) {
	}

}// end of loop

[johnwasser]

@Chazza

for (int i = 0; i <= 255; i++) {

You are trying to fill 256 entries in a 255-entry array. Either change the array to "int myArray[256];" or change the loops so that the maximum index is 254, not 255.

[b707]

@Chazza
Please note that arrays in C are indexed from zero.
Therefore the array

@Chazza

int myArray[255];

doesn't has a element myArray[255], only elements with indexes 0-254

[Chazza]

Hi
Thank you all for the answers. I did suspect something along those lines but could not understand why Serial.print(myArray[255]) under setup yielded the right result but not when it was used under loop.

[gfvalvo]

@Chazza

but could not understand why Serial.print(myArray[255]) under setup yielded the right result but not when it was used under loop.

When you exceed an array's boundaries, the resultant behavior is undefined. Therefore, there is no point in trying to understand the behavior.

BTW, the conventional technique is to use '<' in the for loop, not '<='. So this is the preferred way to solve the problem.

	for (int i = 0; i <  255; i++) {
		myArray[i] = i;
	}

[gfvalvo]

Also note that if you set the compiler warnings to "All" in the Arduino IDE's 'preferences' menu, then you'll see the following warning which would have tipped you off to a problem.

C:\arduino-1.8.15\portable\packages\arduino\hardware\avr\1.8.6\cores\arduino\main.cpp: In function 'main':
C:\Users\TR001221\AppData\Local\Temp\arduino_modified_sketch_379691\sketch_jan09a.ino:23:19: warning: iteration 255 invokes undefined behavior [-Waggressive-loop-optimizations]
     Serial.println(myArray[i]);
                   ^
C:\Users\TR001221\AppData\Local\Temp\arduino_modified_sketch_379691\sketch_jan09a.ino:20:21: note: within this loop
   for (int i = 0; i <= 255; i++) {
                     ^
C:\Users\TR001221\AppData\Local\Temp\arduino_modified_sketch_379691\sketch_jan09a.ino:7:16: warning: iteration 255 invokes undefined behavior [-Waggressive-loop-optimizations]
     myArray[i] = i;
                ^
C:\Users\TR001221\AppData\Local\Temp\arduino_modified_sketch_379691\sketch_jan09a.ino:6:21: note: within this loop
   for (int i = 0; i <= 255; i++) {
                     ^

[Chazza]

Thank you