Using for loop to set pinMode

Question

I notice problems with my output pins when I use this method of setting their pinMode:

int allOutputPins[] = {3, 4, 9, 10, 5, A3, 11, 12, 7, 8, A1, A2};
for(int a = 0; a < sizeof(allOutputPins); a++){
 pinMode(allOutputPins[a], OUTPUT); 
 }//end for

Am I doing something wrong?

Answer 1

Re "Am I doing something wrong?", yes :)

The sizeof(allOutputPins) expression returns the size of allOutputPins[] in terms of bytes, so is 24 because the array contains 12 two-byte integers. The loop's last 12 pinMode() calls will be garbage.

Among other ways of fixing the problem, you could change the expression to sizeof(allOutputPins)/sizeof(int) [which has the same value, 12, as (sizeof allOutputPins)/sizeof(int); eg see the syntax entries for the sizeof operator at cppreference.com] or you could change the base type of the array to byte or uint8_t.

• Edgar Bonet: I would recommend using sizeof allOutputPins / sizeof allOutputPins[0] instead. This idiom is more robust because it works independently of the type of the array. I would use it even if I change allOutputPins base type to uint8_t.
• JRobert: What you needed and were expecting was the count of the items in the array. I need this so regularly, I keep a countof macro, #define countof(a) (sizeof(a)/sizeof(a[0])) in my local.h file (personal file of common idioms).
• Michael Rader: Thanks @JRobert that's a good idea. Is that all I need to define to use it?
• Nick Gammon: You also need to use the countof define like this: for(int a = 0; a < countof(allOutputPins); a++){
• JRobert: You would need to #include <local.h> in your sketch (the Arduino IDE won't know to do it for you); thereafter you can use it anywhere in your sketch, such as how @NickGammon just described.