Accessing 2 dimensional array in PROGMEM inside loop

Question

I am unable to access elements of a 2D PROGMEM array from inside a loop. Please see the following example:

const byte a1[] PROGMEM = {'a', 'b', 'c', 'd'};
const byte a2[] PROGMEM = {'e', 'f', 'g', 'h', 'i'};
const byte a3[] PROGMEM = {'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q'};
const byte a4[] PROGMEM = {'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
const byte * const arr[] PROGMEM = {a1, a2, a3, a4};
void setup() {
 Serial.begin(9600);
 Serial.println(pgm_read_byte(&(arr[0][0])));
 Serial.println(pgm_read_byte(&(arr[1][0])));
 Serial.println(pgm_read_byte(&(arr[2][0])));
 Serial.println(pgm_read_byte(&(arr[3][0])));
 for (int i = 0; i < 4; i++) {
 Serial.print(i);
 Serial.print(":");
 Serial.println(pgm_read_byte(&(arr[i][0])));
 }
 int j = 0;
 Serial.println(pgm_read_byte(&(arr[j][0])));
 j = 1;
 Serial.println(pgm_read_byte(&(arr[j][0])));
 j = 2;
 Serial.println(pgm_read_byte(&(arr[j][0])));
 j = 3;
 Serial.println(pgm_read_byte(&(arr[j][0])));
}
void loop() {
}

The output is:

97
101
106
114
0:12
1:174
2:12
3:148
97
101
106
114

I verified and re-verified the syntax so many times. could not find anyone facing similar issue. I am at my wits end why it would not print correct values inside the loop. Any ideas?

EDIT: expected output is:

97
101
106
114
0:97
1:101
2:106
3:114
97
101
106
114

• Juraj: the constant uses like arr[1][0] are replaced at compile time with the items value. they are not read from the array in runtime. even with j with constant value the compiler evaluates it at compile time and replaces with the value
• Arundale Ramanathan: I think you are right.. is there any way I can achieve accessing the elements from inside the loop?
• Juraj: arduino.cc/reference/en/language/variables/utilities/progmem
• Arundale Ramanathan: I had already gone through it.. after you put it here, i took some ideas and made it work.. giving below as answer.. thanks..
• Jot: You can make it easier. For example when each array is a zero terminated string, or when all the arrays have the same fixed size, or one long string with markers. A two-dimensional array with a different number of elements without knowing how many elements are valid, that seems so very weird.

Answer 1

const byte a1[] PROGMEM = {'a', 'b', 'c', 'd'};
const byte a2[] PROGMEM = {'e', 'f', 'g', 'h', 'i'};
const byte a3[] PROGMEM = {'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q'};
const byte a4[] PROGMEM = {'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
const byte * const arr[] PROGMEM = {a1, a2, a3, a4};
void setup() {
 Serial.begin(115200);
 for (int i = 0; i < 4; i++) {
 Serial.print(i);
 Serial.print(':');
 for (int j = 0; j < 4; j++) {
 Serial.print((char) pgm_read_byte_near(pgm_read_word(arr + i) + j));
 Serial.print(',');
 }
 Serial.println();
 }
}
void loop() {
}

output

0:a,b,c,d,
1:e,f,g,h,
2:j,k,l,m,
3:r,s,t,u,

at runtime the size of the arrays is unknown, so I print the first 4 items

Answer 2

After going through link given by Juraj above again, I could make it work as follows:

const byte a1[] PROGMEM = {'a', 'b', 'c', 'd'};
const byte a2[] PROGMEM = {'e', 'f', 'g', 'h', 'i'};
const byte a3[] PROGMEM = {'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q'};
const byte a4[] PROGMEM = {'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
const byte * const arr[] PROGMEM = {a1, a2, a3, a4};
void setup() {
 Serial.begin(74880);
 Serial.println(pgm_read_byte(&(arr[0][0])));
 Serial.println(pgm_read_byte(&(arr[1][0])));
 Serial.println(pgm_read_byte(&(arr[2][0])));
 Serial.println(pgm_read_byte(&(arr[3][0])));
 for (int i = 0; i < 4; i++) {
 Serial.print(i);
 Serial.print(":");
 Serial.println(pgm_read_byte_near(pgm_read_word(&(arr[i])) + 0));
 }
 int j = 0;
 Serial.println(pgm_read_byte(&(arr[j][0])));
 j = 1;
 Serial.println(pgm_read_byte(&(arr[j][0])));
 j = 2;
 Serial.println(pgm_read_byte(&(arr[j][0])));
 j = 3;
 Serial.println(pgm_read_byte(&(arr[j][0])));
}
void loop() {
}

Now it shows the expected output and access from inside loop using variable index is possible. Thanks.