Making a calculator using an Arduino

Other Hardware LEDs and Multiplexing
16

I think one reason you have a bug is that there is just too much going on in the one function. As Nis$hant inferred you should compartmentalise things.

Also I don't think you need all the array shuffling and IMO read_keypad() should return a human-readable value, ie 0-9 and + etc.

For example here's is a simpler way (I think)

long num1;
long num2;
long result;
long *curr_num = num1;
byte key;

loop () {
	key = read_keypad();	// this should return an int == to the numeric key entered or obvious values for '=' etc

	switch (key) {

		case '+':
		    if (curr_num == num1) {	// if first number just start accululating into second number
				curr_num = num2;
			} else {
				addNumbers();		// if the second number add them
			}
			break;
		
		case '=':
			addNumbers();
			break;
				
		default:
			accumulateNumber();
			display_number(curr_num);

	}
}

void addNumbers () {
	result = num1 + num2;
	display_number(result);
	curr_num = num1;
	num1 = 0;
	num2 = 0;
}

void accumulateNumber () {
	*curr_num *= 10;
	*curr_num += key;
}

void display_number(long * num){
	for (int x = 0; x < 8, x++) {
		shiftNum (num % 10);	// send digit out
		num /= 10;
	}
}

This is little more than psuedo code and there's no error checking etc but I think it's easy to follow and if there's a bug it should be easy to isolate.

Rob