For cycle bugged?

Question

I made a for cycle to calculate the population of an alien species growth. This is the cycle:

int mind = 96;
int aliens = 1;
for (int i=0; i <= mind; i++)
{
    aliens = aliens * 2; 
}
cout << aliens;

Oddly, the cout is returning 0, and it makes no sense, it should return a very high value. Is the cycle badly coded?

Answer 1

The issue is simple. you have a int (most likely 32-bit signed integer). The operation you're doing (x2 each cycle) can be expressed as a shift arithmetic left. Beware the powers of 2! Doing 1 << 31 on a 32-bit signed integer will effectively go back to 0 (after an overflow).

Let's see how your loop goes.

0 2
1 4
2 8
3 16
4 32
5 64
6 128
7 256
8 512
9 1024
10 2048
11 4096
12 8192
13 16384
14 32768
15 65536
16 131072
17 262144
18 524288
19 1048576
20 2097152
21 4194304
22 8388608
23 16777216
24 33554432
25 67108864
26 134217728
27 268435456
28 536870912
29 1073741824
30 -2147483648 // A.K.A. overflow
31 0

At this point I don't think I need to tell you 0 x 2 = 0 The point being: use a double or a integer variable that's at least mind + 1 bits long