为什么我的位移给出不正确的数字
Why are my bit shifts giving incorrect numbers
我正在尝试将一个数字存储在 4 个整数的数组中。数组位于类Num中。我的问题是，当我调用getValue时，该函数返回不正确的数字。我尝试在纸上浏览程序，在Microsoft的计算器中进行所有计算，程序应该给出正确的输出。我什至不知道哪个功能可能有问题，因为没有任何错误或警告，并且两者都在纸上工作。
21 in 二进制：10101
我想做什么：
输入到setValue功能：21
setValue将21的前四位(0101(放入num[3]。所以num[3]现在以二进制形式0101。然后它应该将接下来的四位21放入num[2]。接下来的四个位是0001的，所以0001进入num[2]其余位是0，所以我们忽略它们。现在num{0,0,1,5}.getValue首先去num[3].有5以二进制形式0101。因此，它将其放入返回值的前四位中。然后，它将0001放入接下来的四个位中。其余的数字是0的，所以应该忽略它们。然后直接打印出函数getValue的输出。实际输出在底部。
我的代码：
#include <iostream>
class Num {
char len = 4;
int num[4];
public:
void setValue(int);
int getValue();
};
void Num::setValue(int toSet)
{
char len1=len-1;
for (int counter = len1;counter>=0;counter--)
{
if(toSet&(0xF<<(len1-counter))!=0)
{
num[counter]=(toSet&(0xF<<(len1-counter)))>>len1-counter;
} else {
break;
}
}
}
int Num::getValue()
{
char len1 = len-1;
int returnValue = 0;
for(char counter = len1; counter>=0;counter--)
{
if (num[counter]!=0) {
returnValue+=(num[counter]<<(len1-counter));
} else {
break;
}
}
return returnValue;
}
int main()
{
int x=260;
Num number;
while (x>0)
{
number.setValue(x);
std::cout<<x<<"Test: "<<number.getValue()<<std::endl;
x--;
}
std::cin>>x;
return 0;
}
输出：
260Test: -1748023676
259Test: 5
258Test: 5
257Test: 1
256Test: 1
255Test: 225
254Test: 225
253Test: 221
252Test: 221
251Test: 213
250Test: 213
249Test: 209
248Test: 209
247Test: 193
246Test: 193
245Test: 189
244Test: 189
243Test: 181
242Test: 181
241Test: 177
240Test: 177
239Test: 177
238Test: 177
237Test: 173
236Test: 173
235Test: 165
234Test: 165
233Test: 161
232Test: 161
231Test: 145
230Test: 145
229Test: 141
228Test: 141
227Test: 133
226Test: 133
225Test: 1
224Test: 1
223Test: 161
222Test: 161
221Test: 157
220Test: 157
219Test: 149
218Test: 149
217Test: 145
216Test: 145
215Test: 129
214Test: 129
213Test: 125
212Test: 125
211Test: 117
210Test: 117
209Test: 113
208Test: 113
207Test: 113
206Test: 113
205Test: 109
204Test: 109
203Test: 101
202Test: 101
201Test: 97
200Test: 97
199Test: 81
198Test: 81
197Test: 77
196Test: 77
195Test: 5
194Test: 5
193Test: 1
192Test: 1
191Test: 161
190Test: 161
189Test: 157
188Test: 157
187Test: 149
186Test: 149
185Test: 145
184Test: 145
183Test: 129
182Test: 129
181Test: 125
180Test: 125
179Test: 117
178Test: 117
177Test: 113
176Test: 113
175Test: 113
174Test: 113
173Test: 109
172Test: 109
171Test: 101
170Test: 101
169Test: 97
168Test: 97
167Test: 81
166Test: 81
165Test: 77
164Test: 77
163Test: 69
162Test: 69
161Test: 1
160Test: 1
159Test: 97
158Test: 97
157Test: 93
156Test: 93
155Test: 85
154Test: 85
153Test: 81
152Test: 81
151Test: 65
150Test: 65
149Test: 61
148Test: 61
147Test: 53
146Test: 53
145Test: 49
144Test: 49
143Test: 49
142Test: 49
141Test: 45
140Test: 45
139Test: 37
138Test: 37
137Test: 33
136Test: 33
135Test: 17
134Test: 17
133Test: 13
132Test: 13
131Test: 5
130Test: 5
129Test: 1
128Test: 1
127Test: 225
126Test: 225
125Test: 221
124Test: 221
123Test: 213
122Test: 213
121Test: 209
120Test: 209
119Test: 193
118Test: 193
117Test: 189
116Test: 189
115Test: 181
114Test: 181
113Test: 177
112Test: 177
111Test: 177
110Test: 177
109Test: 173
108Test: 173
107Test: 165
106Test: 165
105Test: 161
104Test: 161
103Test: 145
102Test: 145
101Test: 141
100Test: 141
99Test: 133
98Test: 133
97Test: 1
96Test: 1
95Test: 161
94Test: 161
93Test: 157
92Test: 157
91Test: 149
90Test: 149
89Test: 145
88Test: 145
87Test: 129
86Test: 129
85Test: 125
84Test: 125
83Test: 117
82Test: 117
81Test: 113
80Test: 113
79Test: 113
78Test: 113
77Test: 109
76Test: 109
75Test: 101
74Test: 101
73Test: 97
72Test: 97
71Test: 81
70Test: 81
69Test: 77
68Test: 77
67Test: 5
66Test: 5
65Test: 1
64Test: 1
63Test: 161
62Test: 161
61Test: 157
60Test: 157
59Test: 149
58Test: 149
57Test: 145
56Test: 145
55Test: 129
54Test: 129
53Test: 125
52Test: 125
51Test: 117
50Test: 117
49Test: 113
48Test: 113
47Test: 113
46Test: 113
45Test: 109
44Test: 109
43Test: 101
42Test: 101
41Test: 97
40Test: 97
39Test: 81
38Test: 81
37Test: 77
36Test: 77
35Test: 69
34Test: 69
33Test: 1
32Test: 1
31Test: 97
30Test: 97
29Test: 93
28Test: 93
27Test: 85
26Test: 85
25Test: 81
24Test: 81
23Test: 65
22Test: 65
21Test: 61
20Test: 61
19Test: 53
18Test: 53
17Test: 49
16Test: 49
15Test: 49
14Test: 49
13Test: 45
12Test: 45
11Test: 37
10Test: 37
9Test: 33
8Test: 33
7Test: 17
6Test: 17
5Test: 13
4Test: 13
3Test: 5
2Test: 5
1Test: 1
我用命令g++ 6.3.0编译了这个g++ a.cpp -o a.exe
使用-Wall编译时，有许多警告：
orig.cpp: In member function ‘void Num::setValue(int)’:
orig.cpp:15:39: warning: suggest parentheses around comparison in operand of ‘&’ [-Wparentheses]
if(toSet&(0xF<<(len1-counter))!=0)
~~~~~~~~~~~~~~~~~~~~~^~~
orig.cpp:17:61: warning: suggest parentheses around ‘-’ inside ‘>>’ [-Wparentheses]
num[counter]=(toSet&(0xF<<(len1-counter)))>>len1-counter;
~~~~^~~~~~~~
orig.cpp: In member function ‘int Num::getValue()’:
orig.cpp:30:24: warning: array subscript has type ‘char’ [-Wchar-subscripts]
if (num[counter]!=0) {
^
orig.cpp:31:38: warning: array subscript has type ‘char’ [-Wchar-subscripts]
returnValue+=(num[counter]<<(len1-counter));
^
如果您在更改num之前打印它们的值，您会发现有些值可能不为零(即它们未初始化(，这会导致未定义的行为，并可能中断getValue和setValue中的for循环。
所以改变：
int num[4];
到：
int num[4] = { 0 };
这是一个修复了警告的清理版本：
#include <iostream>
class Num {
int len = 4;
int num[4] = { 0 };
public:
void setValue(int);
int getValue();
void showval();
};
void Num::setValue(int toSet)
{
int len1=len-1;
for (int counter = len1;counter>=0;counter--)
{
if ((toSet & (0xF << (len1-counter))) != 0)
{
num[counter] = (toSet & (0xF << (len1-counter))) >> (len1-counter);
} else {
break;
}
}
}
int Num::getValue()
{
int len1 = len-1;
int returnValue = 0;
for(int counter = len1; counter>=0;counter--)
{
if (num[counter]!=0) {
returnValue+=(num[counter]<<(len1-counter));
} else {
break;
}
}
return returnValue;
}
void Num::showval()
{
for (int i = 0; i < len; ++i)
std::cout << i << ": show: " << num[i] << "n";
#if 0
for (int i = 0; i < len; ++i)
num[i] = 0;
#endif
}
int main()
{
int x=260;
Num number;
number.showval();
while (x>0)
{
number.setValue(x);
std::cout << x << " Test: " << number.getValue() << std::endl;
x--;
}
std::cin>>x;
return 0;
}
要将数字分解为半字节，移位计数应为 4 的倍数。否则，将提取不对齐的 4 位切片。
00010101 (21)
^^^^ first nibble
^^^^ second nibble
第二个半字节被移位 4 位，因此它需要向右移动 4，而不是 1。
您可以将班次数乘以 4，但有一种更简单的方法：只将班次乘以 4。例如：
for (int i = len - 1; i >= 0; i--) {
num[i] = toSet & 0xF;
toSet >>= 4;
}
然后每次迭代都会提取toSet中最低的半字节，并移toSet，使下一个半字节变成最低的半字节。 我没有放break，也不应该有。它绝对不应该是你所拥有的那种break，每当一个数字中间有一个零时，它也会停止循环(例如，0x101中间的 0 会导致循环停止(。当数字的其余部分为零时，循环也不应停止，因为这会在num的其他条目中留下垃圾。
更常见的是将最低的半字节存储在第 0 个元素中，依此类推(这样您就不必处理所有带有向下计数循环和从长度中减去东西的"反向逻辑"(，但这取决于您。
提取值可以对称地完成，在移动结果的同时建立结果，而不是立即将每个部分移动到其最终位置。或者只是将(len1-counter)乘以 4。在提取值时，当num[i]为零时，您也无法停止，因为这并不能证明其余数字也是零。
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