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在C中扩展int的符号

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在C中扩展int的符号,c,C,所以我很难得到一个整型的字段，然后对它进行符号扩展。我有一个方法，可以得到int的字段 getField(int value, int hi, int lo); Value是我从中获取字段的int，hi和lo是字段的大小 所以我可以在getFieldSignExtended（int-value，int-hi，int-lo）中调用这个getField方法，但是如何进行符号扩展呢 e、 g.value=7，hi=1，lo=0 getField（7,1,0）返回3，因为二进制中的7是111，hi和l

所以我很难得到一个整型的字段，然后对它进行符号扩展。我有一个方法，可以得到int的字段

getField(int value, int hi, int lo);
Value是我从中获取字段的int，hi和lo是字段的大小

所以我可以在getFieldSignExtended（int-value，int-hi，int-lo）中调用这个getField方法，但是如何进行符号扩展呢

e、 g.
value=7，hi=1，lo=0

getField（7,1,0）
返回3，因为二进制中的7是111，hi和lo取0到1的字段

从getField返回3时，我得到的值等于0x0003

到目前为止，我所做的都是正面的，但负面的却非常糟糕。当我说“搞砸”时，我的意思是它根本不起作用。所以如果我尝试在-1上使用它，它会显示为一个大的int而不是负数

谢谢你的帮助！：]

编辑：很抱歉，我用一个自相矛盾的说法把自己和你们中的一些人弄糊涂了：p.已修复。

您可以采取多种方法。如果您刚刚开始，您可以通过算术计算字段中的位数。例如：

if (value %2 >= 1)
{
    // you know that the value has a `1` as the lest significant digit. 
    // if that's one of the digits you're looking for, you can do something like count++ here
}
else
{
    // least significant digit is a '0'
}
然后

if (value % 4 >=2)
{
    // you know that the second least significant digit is `1`
    // etc.
}
如果你这样做的话，你可能会想把它们做成某种循环

现在，更好的方法是使用按位anding，如下所示：

if (value & 8 != 0)
    // here you know that the fourth least significant digit (the one representing 8) is 1.
    // do a Google search on bitwise anding to get more information.
这里有很多字里行间的阅读。但是，如果
getField（7,1,0）
返回3，并且您需要
getFieldSignExtended（15,2,0）
返回
-3
和
getFieldSignExtended（3,2,0）
返回
+3
，那么这可能就是您所追求的

其概念是将原始值的hi:lo位中的n位字段视为2的补码。如果n位中的第一位是1，则需要将n位字段视为负数。如果3位字段的第一位是0，则希望将其视为正数

#include <assert.h>
#include <limits.h>
#include <stdio.h>

extern int getFieldSignExtended(int value, int hi, int lo);

enum { INT_BITS = CHAR_BIT * sizeof(int) };

int getFieldSignExtended(int value, int hi, int lo)
{
    assert(lo >= 0);
    assert(hi > lo);
    assert(hi < INT_BITS - 1);
    int bits = (value >> lo) & ((1 << (hi - lo + 1)) - 1);
    if (bits & (1 << (hi - lo)))
        return(bits | (~0U << (hi - lo)));
    else
        return(bits);
}
这是穷举测试前测试代码的输出，加上从穷举测试中选择的少量输出：

GFSX(15, 2, 0) = -1 = 0xFFFFFFFF
GFSX( 3, 2, 0) = +3 = 0x00000003

Systematic test
== PASS == GFSX( +15 = 0x000F, 1, 0) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 2, 0) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 2, 1) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 3, 1) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 4, 2) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX( +15 = 0x000F, 5, 0) =  +15 = 0x0000000F (wanted  +15 = 0x0000000F)
== PASS == GFSX( +15 = 0x000F, 5, 1) =   +7 = 0x00000007 (wanted   +7 = 0x00000007)
== PASS == GFSX( +15 = 0x000F, 5, 2) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX( +15 = 0x000F, 5, 3) =   +1 = 0x00000001 (wanted   +1 = 0x00000001)
== PASS == GFSX( +15 = 0x000F, 5, 4) =   +0 = 0x00000000 (wanted   +0 = 0x00000000)
== PASS == GFSX(  +3 = 0x0003, 2, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 2, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 3, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 4, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 5, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 6, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 7, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 7, 1) =   -7 = 0xFFFFFFF9 (wanted   -7 = 0xFFFFFFF9)
== PASS == GFSX(+243 = 0x00F3, 7, 2) =   -4 = 0xFFFFFFFC (wanted   -4 = 0xFFFFFFFC)
== PASS == GFSX(+243 = 0x00F3, 7, 3) =   -2 = 0xFFFFFFFE (wanted   -2 = 0xFFFFFFFE)
== PASS == GFSX(+243 = 0x00F3, 7, 4) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX(+243 = 0x00F3, 8, 0) = +243 = 0x000000F3 (wanted +243 = 0x000000F3)
== PASS ==

Exhaustive test
       GFSX(1604336871 = 0x5FA03CE7,  1,  0) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  2,  0) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  2,  1) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  3,  0) =         +7 = 0x00000007
       GFSX(1604336871 = 0x5FA03CE7,  3,  1) =         +3 = 0x00000003
       GFSX(1604336871 = 0x5FA03CE7,  3,  2) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7,  4,  0) =         +7 = 0x00000007
       GFSX(1604336871 = 0x5FA03CE7,  4,  1) =         +3 = 0x00000003
       GFSX(1604336871 = 0x5FA03CE7,  4,  2) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7,  4,  3) =         +0 = 0x00000000
       GFSX(1604336871 = 0x5FA03CE7,  5,  0) =        -25 = 0xFFFFFFE7
       GFSX(1604336871 = 0x5FA03CE7,  5,  1) =        -13 = 0xFFFFFFF3
       GFSX(1604336871 = 0x5FA03CE7,  5,  2) =         -7 = 0xFFFFFFF9
       GFSX(1604336871 = 0x5FA03CE7,  5,  3) =         -4 = 0xFFFFFFFC
       GFSX(1604336871 = 0x5FA03CE7,  5,  4) =         -2 = 0xFFFFFFFE
       GFSX(1604336871 = 0x5FA03CE7,  6,  0) =        -25 = 0xFFFFFFE7
       GFSX(1604336871 = 0x5FA03CE7,  6,  1) =        -13 = 0xFFFFFFF3
       GFSX(1604336871 = 0x5FA03CE7,  6,  2) =         -7 = 0xFFFFFFF9
       GFSX(1604336871 = 0x5FA03CE7,  6,  3) =         -4 = 0xFFFFFFFC
       GFSX(1604336871 = 0x5FA03CE7,  6,  4) =         -2 = 0xFFFFFFFE
       GFSX(1604336871 = 0x5FA03CE7,  6,  5) =         -1 = 0xFFFFFFFF
...
       GFSX(1604336871 = 0x5FA03CE7, 29, 28) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7, 30,  0) = -543146777 = 0xDFA03CE7
       GFSX(1604336871 = 0x5FA03CE7, 30,  1) = -271573389 = 0xEFD01E73
       GFSX(1604336871 = 0x5FA03CE7, 30,  2) = -135786695 = 0xF7E80F39
       GFSX(1604336871 = 0x5FA03CE7, 30,  3) =  -67893348 = 0xFBF4079C
       GFSX(1604336871 = 0x5FA03CE7, 30,  4) =  -33946674 = 0xFDFA03CE
       GFSX(1604336871 = 0x5FA03CE7, 30,  5) =  -16973337 = 0xFEFD01E7
       GFSX(1604336871 = 0x5FA03CE7, 30,  6) =   -8486669 = 0xFF7E80F3
       GFSX(1604336871 = 0x5FA03CE7, 30,  7) =   -4243335 = 0xFFBF4079
       GFSX(1604336871 = 0x5FA03CE7, 30,  8) =   -2121668 = 0xFFDFA03C
       GFSX(1604336871 = 0x5FA03CE7, 30,  9) =   -1060834 = 0xFFEFD01E
       GFSX(1604336871 = 0x5FA03CE7, 30, 10) =    -530417 = 0xFFF7E80F
       GFSX(1604336871 = 0x5FA03CE7, 30, 11) =    -265209 = 0xFFFBF407
       GFSX(1604336871 = 0x5FA03CE7, 30, 12) =    -132605 = 0xFFFDFA03
       GFSX(1604336871 = 0x5FA03CE7, 30, 13) =     -66303 = 0xFFFEFD01
       GFSX(1604336871 = 0x5FA03CE7, 30, 14) =     -33152 = 0xFFFF7E80
       GFSX(1604336871 = 0x5FA03CE7, 30, 15) =     -16576 = 0xFFFFBF40
       GFSX(1604336871 = 0x5FA03CE7, 30, 16) =      -8288 = 0xFFFFDFA0
       GFSX(1604336871 = 0x5FA03CE7, 30, 17) =      -4144 = 0xFFFFEFD0
       GFSX(1604336871 = 0x5FA03CE7, 30, 18) =      -2072 = 0xFFFFF7E8
       GFSX(1604336871 = 0x5FA03CE7, 30, 19) =      -1036 = 0xFFFFFBF4
       GFSX(1604336871 = 0x5FA03CE7, 30, 20) =       -518 = 0xFFFFFDFA
       GFSX(1604336871 = 0x5FA03CE7, 30, 21) =       -259 = 0xFFFFFEFD
       GFSX(1604336871 = 0x5FA03CE7, 30, 22) =       -130 = 0xFFFFFF7E
       GFSX(1604336871 = 0x5FA03CE7, 30, 23) =        -65 = 0xFFFFFFBF
       GFSX(1604336871 = 0x5FA03CE7, 30, 24) =        -33 = 0xFFFFFFDF
       GFSX(1604336871 = 0x5FA03CE7, 30, 25) =        -17 = 0xFFFFFFEF
       GFSX(1604336871 = 0x5FA03CE7, 30, 26) =         -9 = 0xFFFFFFF7
       GFSX(1604336871 = 0x5FA03CE7, 30, 27) =         -5 = 0xFFFFFFFB
       GFSX(1604336871 = 0x5FA03CE7, 30, 28) =         -3 = 0xFFFFFFFD
       GFSX(1604336871 = 0x5FA03CE7, 30, 29) =         -2 = 0xFFFFFFFE
“用零扩展的符号”在术语上是矛盾的“符号扩展”表示“将符号位（0或1）复制到扩展中”；如果类型为未签名且已扩展，则仅用零扩展它。您的问题未被指定，因此有可能会将其标记为“关闭”而不是“真正的问题”。另外，您尝试过什么方法？在您的示例中，符号扩展可能意味着返回-1，因为如果我们从
..0000111
中获取位
11
，我们必须将
11
解释为两位2的补码值，表示-1。在C语言中，在2的补码平台上，一个两位宽的有符号结构/联合位域（包含
11
）的计算结果为-1。我想你的意思是零填充字符串，根据上面的注释，这与符号扩展不同。要对字符串进行零填充，请执行类似于
printf（“%05d”，value）的操作抱歉，伙计们，刚刚编辑了：）。欢迎来到堆栈溢出。你的答案没有提供一个完整的解决方案，这意味着它的价值有限。我不完全清楚你打算如何把你所说的扩展到解决问题上。一般来说，一个好的答案要么是完整的，要么是容易完成的。如果你对这个问题有一个很好的替代解释（这个问题确实需要解释；它不是一个很好的问题），那么看到你的替代答案就太好了。

GFSX(15, 2, 0) = -1 = 0xFFFFFFFF
GFSX( 3, 2, 0) = +3 = 0x00000003

Systematic test
== PASS == GFSX( +15 = 0x000F, 1, 0) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 2, 0) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 2, 1) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 3, 1) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 4, 2) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX( +15 = 0x000F, 5, 0) =  +15 = 0x0000000F (wanted  +15 = 0x0000000F)
== PASS == GFSX( +15 = 0x000F, 5, 1) =   +7 = 0x00000007 (wanted   +7 = 0x00000007)
== PASS == GFSX( +15 = 0x000F, 5, 2) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX( +15 = 0x000F, 5, 3) =   +1 = 0x00000001 (wanted   +1 = 0x00000001)
== PASS == GFSX( +15 = 0x000F, 5, 4) =   +0 = 0x00000000 (wanted   +0 = 0x00000000)
== PASS == GFSX(  +3 = 0x0003, 2, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 2, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 3, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 4, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 5, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 6, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 7, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 7, 1) =   -7 = 0xFFFFFFF9 (wanted   -7 = 0xFFFFFFF9)
== PASS == GFSX(+243 = 0x00F3, 7, 2) =   -4 = 0xFFFFFFFC (wanted   -4 = 0xFFFFFFFC)
== PASS == GFSX(+243 = 0x00F3, 7, 3) =   -2 = 0xFFFFFFFE (wanted   -2 = 0xFFFFFFFE)
== PASS == GFSX(+243 = 0x00F3, 7, 4) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX(+243 = 0x00F3, 8, 0) = +243 = 0x000000F3 (wanted +243 = 0x000000F3)
== PASS ==

Exhaustive test
       GFSX(1604336871 = 0x5FA03CE7,  1,  0) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  2,  0) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  2,  1) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  3,  0) =         +7 = 0x00000007
       GFSX(1604336871 = 0x5FA03CE7,  3,  1) =         +3 = 0x00000003
       GFSX(1604336871 = 0x5FA03CE7,  3,  2) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7,  4,  0) =         +7 = 0x00000007
       GFSX(1604336871 = 0x5FA03CE7,  4,  1) =         +3 = 0x00000003
       GFSX(1604336871 = 0x5FA03CE7,  4,  2) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7,  4,  3) =         +0 = 0x00000000
       GFSX(1604336871 = 0x5FA03CE7,  5,  0) =        -25 = 0xFFFFFFE7
       GFSX(1604336871 = 0x5FA03CE7,  5,  1) =        -13 = 0xFFFFFFF3
       GFSX(1604336871 = 0x5FA03CE7,  5,  2) =         -7 = 0xFFFFFFF9
       GFSX(1604336871 = 0x5FA03CE7,  5,  3) =         -4 = 0xFFFFFFFC
       GFSX(1604336871 = 0x5FA03CE7,  5,  4) =         -2 = 0xFFFFFFFE
       GFSX(1604336871 = 0x5FA03CE7,  6,  0) =        -25 = 0xFFFFFFE7
       GFSX(1604336871 = 0x5FA03CE7,  6,  1) =        -13 = 0xFFFFFFF3
       GFSX(1604336871 = 0x5FA03CE7,  6,  2) =         -7 = 0xFFFFFFF9
       GFSX(1604336871 = 0x5FA03CE7,  6,  3) =         -4 = 0xFFFFFFFC
       GFSX(1604336871 = 0x5FA03CE7,  6,  4) =         -2 = 0xFFFFFFFE
       GFSX(1604336871 = 0x5FA03CE7,  6,  5) =         -1 = 0xFFFFFFFF
...
       GFSX(1604336871 = 0x5FA03CE7, 29, 28) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7, 30,  0) = -543146777 = 0xDFA03CE7
       GFSX(1604336871 = 0x5FA03CE7, 30,  1) = -271573389 = 0xEFD01E73
       GFSX(1604336871 = 0x5FA03CE7, 30,  2) = -135786695 = 0xF7E80F39
       GFSX(1604336871 = 0x5FA03CE7, 30,  3) =  -67893348 = 0xFBF4079C
       GFSX(1604336871 = 0x5FA03CE7, 30,  4) =  -33946674 = 0xFDFA03CE
       GFSX(1604336871 = 0x5FA03CE7, 30,  5) =  -16973337 = 0xFEFD01E7
       GFSX(1604336871 = 0x5FA03CE7, 30,  6) =   -8486669 = 0xFF7E80F3
       GFSX(1604336871 = 0x5FA03CE7, 30,  7) =   -4243335 = 0xFFBF4079
       GFSX(1604336871 = 0x5FA03CE7, 30,  8) =   -2121668 = 0xFFDFA03C
       GFSX(1604336871 = 0x5FA03CE7, 30,  9) =   -1060834 = 0xFFEFD01E
       GFSX(1604336871 = 0x5FA03CE7, 30, 10) =    -530417 = 0xFFF7E80F
       GFSX(1604336871 = 0x5FA03CE7, 30, 11) =    -265209 = 0xFFFBF407
       GFSX(1604336871 = 0x5FA03CE7, 30, 12) =    -132605 = 0xFFFDFA03
       GFSX(1604336871 = 0x5FA03CE7, 30, 13) =     -66303 = 0xFFFEFD01
       GFSX(1604336871 = 0x5FA03CE7, 30, 14) =     -33152 = 0xFFFF7E80
       GFSX(1604336871 = 0x5FA03CE7, 30, 15) =     -16576 = 0xFFFFBF40
       GFSX(1604336871 = 0x5FA03CE7, 30, 16) =      -8288 = 0xFFFFDFA0
       GFSX(1604336871 = 0x5FA03CE7, 30, 17) =      -4144 = 0xFFFFEFD0
       GFSX(1604336871 = 0x5FA03CE7, 30, 18) =      -2072 = 0xFFFFF7E8
       GFSX(1604336871 = 0x5FA03CE7, 30, 19) =      -1036 = 0xFFFFFBF4
       GFSX(1604336871 = 0x5FA03CE7, 30, 20) =       -518 = 0xFFFFFDFA
       GFSX(1604336871 = 0x5FA03CE7, 30, 21) =       -259 = 0xFFFFFEFD
       GFSX(1604336871 = 0x5FA03CE7, 30, 22) =       -130 = 0xFFFFFF7E
       GFSX(1604336871 = 0x5FA03CE7, 30, 23) =        -65 = 0xFFFFFFBF
       GFSX(1604336871 = 0x5FA03CE7, 30, 24) =        -33 = 0xFFFFFFDF
       GFSX(1604336871 = 0x5FA03CE7, 30, 25) =        -17 = 0xFFFFFFEF
       GFSX(1604336871 = 0x5FA03CE7, 30, 26) =         -9 = 0xFFFFFFF7
       GFSX(1604336871 = 0x5FA03CE7, 30, 27) =         -5 = 0xFFFFFFFB
       GFSX(1604336871 = 0x5FA03CE7, 30, 28) =         -3 = 0xFFFFFFFD
       GFSX(1604336871 = 0x5FA03CE7, 30, 29) =         -2 = 0xFFFFFFFE