撤消换档而不会被截断 [英] Undoing shifts without truncating
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问题描述
我对此有些困惑.值在转换后不应该截断吗?
I'm a bit baffled by this. Shouldn't the values truncate after the shift?
有人知道为什么会这样吗?
Does anyone know why this happens?
long a, b, c, n;
//assign any value to a, set b and c to 0x000...0
n = 128; //any number works;
b = a << n;
c = b >> n;
a == (b >> n); // True
a == c; //True;
后记
我一直了解，如果您向任何方向移动缓冲区，落入"缓冲区大小之外的值都会被截断，并且除非您从原始缓冲区中获取它们，否则它们实际上会丢失. 对于我在汇编中使用过的每个平台，都是如此，我认为这将扩展到更高级别的语言.
I've always understood that if you shift a buffer in any direction, the values that "fall" outside the buffer size get truncated, and that they're essentially lost unless you get them from the original buffer. Thats been true for every platform I've ever worked with in assembly, and I thought that'd extend to the higher level languages.
但是，在c ++中，我可以将缓冲区(整数或长整数)移位的位数超出了它可以容纳的位数，并且如果我向相反的方向移位，则结果等于原始的缓冲区".
However, in c++ i can shift a buffer (int or long) by more bits than it can hold, and if I then shift in the opposite direction, the result is equal to the original "buffer".
推荐答案
看起来C编译器的工作原理有些奇怪，但它绝对合理.
如果移位数大于操作数长度(以位为单位)，则编译器将其除以操作数长度，并使用提示作为移位数
Looks like the C compiler works a little strange, but its absolutely reasonable
if the number of shifts be greater than the operand length (in bits) so compiler divide it to operand length and use reminder as number for shifts
// A << N
L = length of A
if L < N
then N = A % L;
因此，您可以为班次数设置这样的数字，以使其撤消禁用！！！
Therefore you can set such a number for number of shifts that makes it undoing disable !!!
N = 63可能是您的答案，对于A上的任何值(甚至A = 1)(A = 0除外)，它都不起作用(您的条件为FALSE)
PUT [61< N ＜ 64]，则A = 3导致无法撤消
[60< N ＜ 64] AND，A = 7
...
PUT [61 < N < 64] , A = 3 MAKES IMPOSSIBLE UNDO
[60 < N < 64] AND , A = 7
...
尝试以下代码以了解发生了什么
Try this code to understand what happen
#include <cstdlib>
#include <cstdio>
using namespace std;
void DisplayBinary(unsigned int n, int l)
{
for (int i = l - 1 ; i >= 0; i--)
{
printf("%x", (n & (1 << i)) >> i);
}
}
int main(int argc, char** argv)
{
// x = 340
unsigned int x = 0x154;
for (int numberOfShifts = 0; numberOfShifts < 100; numberOfShifts++) {
printf("numberOfShifts = %d\tresult = ", numberOfShifts);
DisplayBinary(x << numberOfShifts, 32);
printf("\n");
}
return 0;
}
查看输出:
numberOfShifts = 0 result = 00000000000000000000000101010100
numberOfShifts = 1 result = 00000000000000000000001010101000
numberOfShifts = 2 result = 00000000000000000000010101010000
numberOfShifts = 3 result = 00000000000000000000101010100000
numberOfShifts = 4 result = 00000000000000000001010101000000
numberOfShifts = 5 result = 00000000000000000010101010000000
numberOfShifts = 6 result = 00000000000000000101010100000000
numberOfShifts = 7 result = 00000000000000001010101000000000
numberOfShifts = 8 result = 00000000000000010101010000000000
numberOfShifts = 9 result = 00000000000000101010100000000000
numberOfShifts = 10 result = 00000000000001010101000000000000
numberOfShifts = 11 result = 00000000000010101010000000000000
numberOfShifts = 12 result = 00000000000101010100000000000000
numberOfShifts = 13 result = 00000000001010101000000000000000
numberOfShifts = 14 result = 00000000010101010000000000000000
numberOfShifts = 15 result = 00000000101010100000000000000000
numberOfShifts = 16 result = 00000001010101000000000000000000
numberOfShifts = 17 result = 00000010101010000000000000000000
numberOfShifts = 18 result = 00000101010100000000000000000000
numberOfShifts = 19 result = 00001010101000000000000000000000
numberOfShifts = 20 result = 00010101010000000000000000000000
numberOfShifts = 21 result = 00101010100000000000000000000000
numberOfShifts = 22 result = 01010101000000000000000000000000
numberOfShifts = 23 result = 10101010000000000000000000000000
numberOfShifts = 24 result = 01010100000000000000000000000000
numberOfShifts = 25 result = 10101000000000000000000000000000
numberOfShifts = 26 result = 01010000000000000000000000000000
numberOfShifts = 27 result = 10100000000000000000000000000000
numberOfShifts = 28 result = 01000000000000000000000000000000
numberOfShifts = 29 result = 10000000000000000000000000000000
numberOfShifts = 30 result = 00000000000000000000000000000000
numberOfShifts = 31 result = 00000000000000000000000000000000
numberOfShifts = 32 result = 00000000000000000000000101010100 <<<===
numberOfShifts = 33 result = 00000000000000000000001010101000
numberOfShifts = 34 result = 00000000000000000000010101010000
numberOfShifts = 35 result = 00000000000000000000101010100000
numberOfShifts = 36 result = 00000000000000000001010101000000
numberOfShifts = 37 result = 00000000000000000010101010000000
numberOfShifts = 38 result = 00000000000000000101010100000000
numberOfShifts = 39 result = 00000000000000001010101000000000
numberOfShifts = 40 result = 00000000000000010101010000000000
numberOfShifts = 41 result = 00000000000000101010100000000000
numberOfShifts = 42 result = 00000000000001010101000000000000
numberOfShifts = 43 result = 00000000000010101010000000000000
numberOfShifts = 44 result = 00000000000101010100000000000000
numberOfShifts = 45 result = 00000000001010101000000000000000
numberOfShifts = 46 result = 00000000010101010000000000000000
numberOfShifts = 47 result = 00000000101010100000000000000000
numberOfShifts = 48 result = 00000001010101000000000000000000
numberOfShifts = 49 result = 00000010101010000000000000000000
numberOfShifts = 50 result = 00000101010100000000000000000000
numberOfShifts = 51 result = 00001010101000000000000000000000
numberOfShifts = 52 result = 00010101010000000000000000000000
numberOfShifts = 53 result = 00101010100000000000000000000000
numberOfShifts = 54 result = 01010101000000000000000000000000
numberOfShifts = 55 result = 10101010000000000000000000000000
numberOfShifts = 56 result = 01010100000000000000000000000000
numberOfShifts = 57 result = 10101000000000000000000000000000
numberOfShifts = 58 result = 01010000000000000000000000000000
numberOfShifts = 59 result = 10100000000000000000000000000000
numberOfShifts = 60 result = 01000000000000000000000000000000
numberOfShifts = 61 result = 10000000000000000000000000000000
numberOfShifts = 62 result = 00000000000000000000000000000000
numberOfShifts = 63 result = 00000000000000000000000000000000
numberOfShifts = 64 result = 00000000000000000000000101010100 <<<===
numberOfShifts = 65 result = 00000000000000000000001010101000
numberOfShifts = 66 result = 00000000000000000000010101010000
numberOfShifts = 67 result = 00000000000000000000101010100000
numberOfShifts = 68 result = 00000000000000000001010101000000
numberOfShifts = 69 result = 00000000000000000010101010000000
numberOfShifts = 70 result = 00000000000000000101010100000000
numberOfShifts = 71 result = 00000000000000001010101000000000
numberOfShifts = 72 result = 00000000000000010101010000000000
numberOfShifts = 73 result = 00000000000000101010100000000000
numberOfShifts = 74 result = 00000000000001010101000000000000
numberOfShifts = 75 result = 00000000000010101010000000000000
numberOfShifts = 76 result = 00000000000101010100000000000000
numberOfShifts = 77 result = 00000000001010101000000000000000
numberOfShifts = 78 result = 00000000010101010000000000000000
numberOfShifts = 79 result = 00000000101010100000000000000000
numberOfShifts = 80 result = 00000001010101000000000000000000
numberOfShifts = 81 result = 00000010101010000000000000000000
numberOfShifts = 82 result = 00000101010100000000000000000000
numberOfShifts = 83 result = 00001010101000000000000000000000
numberOfShifts = 84 result = 00010101010000000000000000000000
numberOfShifts = 85 result = 00101010100000000000000000000000
numberOfShifts = 86 result = 01010101000000000000000000000000
numberOfShifts = 87 result = 10101010000000000000000000000000
numberOfShifts = 88 result = 01010100000000000000000000000000
numberOfShifts = 89 result = 10101000000000000000000000000000
numberOfShifts = 90 result = 01010000000000000000000000000000
numberOfShifts = 91 result = 10100000000000000000000000000000
numberOfShifts = 92 result = 01000000000000000000000000000000
numberOfShifts = 93 result = 10000000000000000000000000000000
numberOfShifts = 94 result = 00000000000000000000000000000000
numberOfShifts = 95 result = 00000000000000000000000000000000
numberOfShifts = 96 result = 00000000000000000000000101010100 <<<===
numberOfShifts = 97 result = 00000000000000000000001010101000
numberOfShifts = 98 result = 00000000000000000000010101010000
numberOfShifts = 99 result = 00000000000000000000101010100000
这篇关于撤消换档而不会被截断的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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