Question

C program question:

If i have a program that asks the user for a positive integer an i save that in the variable n. And then i have a while statement like so :
while (n) {
invert (P)
n = n & n-1
}

what is that while loop saying? Is it saying "while true"? isn't it always true?

Answer 1

Below is a list of `n = n & n-1` output.
There are cases where `n==0` and thus the loop will terminate.
I'm not sure what it is doing.
```
n: n & n-1
20: 16
19: 18
18: 16
17: 16
16: 0
15: 14
14: 12
13: 12
12: 8
11: 10
10: 8
9: 8
8: 0
7: 6
6: 4
5: 4
4: 0
3: 2
2: 0
1: 0
0: 0
```

Answer 2

Here is how the loop would iterate out for various `n`
It gives `n`, the number iterations, and the values of `n` for the iterations.
```
0: 1 (0)
1: 1 (0)
2: 1 (0)
3: 2 (2, 0)
4: 1 (0)
5: 2 (4, 0)
6: 2 (4, 0)
7: 3 (6, 4, 0)
8: 1 (0)
9: 2 (8, 0)
10: 2 (8, 0)
11: 3 (10, 8, 0)
12: 2 (8, 0)
13: 3 (12, 8, 0)
14: 3 (12, 8, 0)
15: 4 (14, 12, 8, 0)
16: 1 (0)
17: 2 (16, 0)
18: 2 (16, 0)
19: 3 (18, 16, 0)
20: 2 (16, 0)
21: 3 (20, 16, 0)
22: 3 (20, 16, 0)
23: 4 (22, 20, 16, 0)
24: 2 (16, 0)
25: 3 (24, 16, 0)
26: 3 (24, 16, 0)
27: 4 (26, 24, 16, 0)
28: 3 (24, 16, 0)
29: 4 (28, 24, 16, 0)
30: 4 (28, 24, 16, 0)
31: 5 (30, 28, 24, 16, 0)
32: 1 (0)
33: 2 (32, 0)
34: 2 (32, 0)
35: 3 (34, 32, 0)
36: 2 (32, 0)
37: 3 (36, 32, 0)
38: 3 (36, 32, 0)
39: 4 (38, 36, 32, 0)
40: 2 (32, 0)
41: 3 (40, 32, 0)
42: 3 (40, 32, 0)
43: 4 (42, 40, 32, 0)
44: 3 (40, 32, 0)
45: 4 (44, 40, 32, 0)
46: 4 (44, 40, 32, 0)
47: 5 (46, 44, 40, 32, 0)
48: 2 (32, 0)
49: 3 (48, 32, 0)
50: 3 (48, 32, 0)
51: 4 (50, 48, 32, 0)
52: 3 (48, 32, 0)
53: 4 (52, 48, 32, 0)
54: 4 (52, 48, 32, 0)
55: 5 (54, 52, 48, 32, 0)
56: 3 (48, 32, 0)
57: 4 (56, 48, 32, 0)
58: 4 (56, 48, 32, 0)
59: 5 (58, 56, 48, 32, 0)
60: 4 (56, 48, 32, 0)
61: 5 (60, 56, 48, 32, 0)
62: 5 (60, 56, 48, 32, 0)
63: 6 (62, 60, 56, 48, 32, 0)
64: 1 (0)
65: 2 (64, 0)
66: 2 (64, 0)
67: 3 (66, 64, 0)
68: 2 (64, 0)
69: 3 (68, 64, 0)
70: 3 (68, 64, 0)
71: 4 (70, 68, 64, 0)
72: 2 (64, 0)
73: 3 (72, 64, 0)
74: 3 (72, 64, 0)
75: 4 (74, 72, 64, 0)
76: 3 (72, 64, 0)
77: 4 (76, 72, 64, 0)
78: 4 (76, 72, 64, 0)
79: 5 (78, 76, 72, 64, 0)
80: 2 (64, 0)
81: 3 (80, 64, 0)
82: 3 (80, 64, 0)
83: 4 (82, 80, 64, 0)
84: 3 (80, 64, 0)
85: 4 (84, 80, 64, 0)
86: 4 (84, 80, 64, 0)
87: 5 (86, 84, 80, 64, 0)
88: 3 (80, 64, 0)
89: 4 (88, 80, 64, 0)
90: 4 (88, 80, 64, 0)
91: 5 (90, 88, 80, 64, 0)
92: 4 (88, 80, 64, 0)
93: 5 (92, 88, 80, 64, 0)
94: 5 (92, 88, 80, 64, 0)
95: 6 (94, 92, 88, 80, 64, 0)
96: 2 (64, 0)
97: 3 (96, 64, 0)
98: 3 (96, 64, 0)
99: 4 (98, 96, 64, 0)
100: 3 (96, 64, 0)
```

Answer 3

-------------------------------------------------------------
"what is that while loop saying? Is it saying "while true"? isn't it always true?"
-------------------------------------------------------------
That means
while (n !=0)
or
while( !(n==0))

n & (n-1) is equals to “0” if "n" have the form:
\[2^k\]
where k is positive integer.
if we start for instance with n=63; the loop ends when its value become equals to 32 because there is no value between 63 and 32 that can be written as \[2^k\].

N.B: the instruction
invert (P)
in your code is not taken under consideration in this comment.