Weird things in programming

Weird things often occur in programming. We can tell so-called reasons to some of them. For others, the only thing we can do is, how the hell could these be? We have to pay extra attention to avoid these in order not to fall into debugging nightmares.

10000000000000000 != 10000000000000000 ???

1 2	float a = 10000000000000000.0; float b = a - 10000000000000000.0;

When printing them, it turns out:

1 2	a = 10000000272564224.000000 b = 272564224.000000

When viewing them in Watch under Debug, it turns out:

Name	Value	Type
a	1.0000000e+016	float
b	2.7256422e+008	float

Reason: discussed in here.

215510*10000 != 2155100000 ???

1 2	UINT64 time1 = 21551010000; UINT64 time2 = (UINT64)(215510 10000);

When printing them or in Watch, it turns out:

1 2	time1 = 18446744071569684320 time2 = 18446744071569684320

We have to use one of the following codes in order to get correct answer:

1
2
3

UINT64 time3 = (UINT64)215510 * 10000;
UINT64 time4 = 215510 * (UINT64)10000;
UINT64 time5 = (UINT64)215510 * (UINT64)10000;

Reason: discussed in here.

3 < –1 ???

int start = 3;

vector<int> data;
data.push_back(0);
data.push_back(0);

for (int i=start; i<data.size()-start; i++)
    printf("In...\n");

When running the above code, it will run printf("In...\n"); infinitely. Although based on the condition (3<-1) of the for loop, it should never do this. Weird, huh?

To avoid this, you have to compute the long condition equation first, like:

... ...
int end = data.size()-start;
for (int i=start; i<end; i++)
    printf("In...\n");

Reason: discussed in here.