Question

I have a vector containing integer values, which I wish to sum. However, it’s summing the total wrong – if the vector holds 10 ints between -10 and 10, my total should not be 18446744073709551602.

I declare the vector, and populate it with values:

std::vector<int> X;
for ( int i = 0; i < readings.size() ; ++i ) { 
    X.push_back(readings[i].x);

I then sum and display:

unsigned long temptotal = 0;
for ( int i = 0; i < readings.size() ; ++i ) {
    std::cout << "Value in X: " << X[i] << std::endl;
    temptotal += X[i];
}
std::cout << "Temp total: " << temptotal << std::endl;

which produces the following output: Temp total: 18446744073709551603.

When I step through the summation loop in GDB, (at temptotal += X[i];), I can print X[i]:

(gdb) p X[i]
$1 = (reference) @0x100100c80: -1

Printing temptotal before and after the addition is performed:

(gdb) p temptotal
$2 = 0
(gdb) p temptotal
$3 = 18446744073709551615

I’m fairly sure that I’m adding the memory locations rather than the values in the memory locations – I could be wrong on that – but I’m not sure why I’m not adding the values at X[i]. When I populate X with the values from readings[i].x, they’re doubles, which I then type-cast to ints. Is this where my error is? Checking in GDB, my type-cast of the readings[i].x value should be an int, or, at least, a number of some kind:

(gdb) p (int)readings[i].x
$1 = -1

I’m stumped; if somebody could shed some light on why I’m not adding what I think I’m adding, I’d be grateful.

EDIT
It’s the casting from int to unsigned long. I went over it and over it, pressed “submit question” and spotted my error. Although, I still don’t understand why it’s doing what it’s doing – any light shed on that would be wonderful – why, if I’m adding an int to an unsigned long, does it not take the value from the int?

Answer 1

This is because your temptotal is unsigned. All numbers added to it are interpreted as unsigned, hence small negative numbers become large positive numbers in two’s complement representation.