Why does subtraction of smaller character with greater character give a big positive integer instead of a negative integer?

Question

When I subtract a larger character with a smaller one, I'm getting a huge positive number instead of a much smaller, negative number, while assigning it to a long long int variable.

If I subtract ' N ' with ' A 'and if I assign it a long long int variable, it turns out to be 18446744073709551605 instead of -13.

When I use abs() function while computing, I get the value correctly, but obviously it will be positive instead of negative. So how to get the value correctly?

char s[] = "ABCT";
char t[] = "PBDI";
long long int curr_diff;
for(int i=0; i<4; i++){
curr_diff = (t[i] - '0') - (s[i] - '0');
printf("Current value : %llu\n", curr_diff);
}

Result from my code

Current value : 15
Current value : 0
Current value : 1
Current value : 18446744073709551605

Expected result

Current value : 15
Current value : 0
Current value : 1
Current value : -11

Possible duplicate of [When does underflow occur?](https://stackoverflow.com/questions/42277132/when-does-underflow-occur) — Adam Hess, Jan 09 '19 at 16:44

score 5 · Accepted Answer · answered Jan 09 '19 at 16:30

5

printf("Current value : %llu\n", curr_diff);

You are printing long long unsigned value. Use %lld to get the right output

answered Jan 09 '19 at 16:30

Wander3r

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10

Eliminating the risk of using the wrong format string is a good reason for using `std::cout` over `printf` as it's type safe. – François Andrieux Jan 09 '19 at 16:33
1

To re-word for clarity: OP is trying to print `long long int` using the specifier that is meant for `unsigned long long int`. The behaviour of the program is undefined. – eerorika Jan 09 '19 at 16:45

Why does subtraction of smaller character with greater character give a big positive integer instead of a negative integer?

1 Answers1