我们都知道，如果想对int型变量清极大值或极小值，我们一般选择memset(a,0x3f,sizeof a);或者memset(a,0xef,sizeof a);。然而，如果对double型清0x3f，我们经常会得到一个连1都不到的小数。那么对double清极值是否智能手动或者fill(a+1,a+n,0x7fffffff);呢？
首先我们运行以下程序
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
int main()
{
double x;
freopen("1.txt","w",stdout);
for(int i=1;i<=0xff;i++)
memset(&x,i,sizeof x),printf("%x %.2lf\n",i,x);
}
得到这个表：
1 0.00
2 0.00
3 0.00
4 0.00
5 0.00
6 0.00
7 0.00
8 0.00
9 0.00
a 0.00
b 0.00
c 0.00
d 0.00
e 0.00
f 0.00
10 0.00
11 0.00
12 0.00
13 0.00
14 0.00
15 0.00
16 0.00
17 0.00
18 0.00
19 0.00
1a 0.00
1b 0.00
1c 0.00
1d 0.00
1e 0.00
1f 0.00
20 0.00
21 0.00
22 0.00
23 0.00
24 0.00
25 0.00
26 0.00
27 0.00
28 0.00
29 0.00
2a 0.00
2b 0.00
2c 0.00
2d 0.00
2e 0.00
2f 0.00
30 0.00
31 0.00
32 0.00
33 0.00
34 0.00
35 0.00
36 0.00
37 0.00
38 0.00
39 0.00
3a 0.00
3b 0.00
3c 0.00
3d 0.00
3e 0.00
3f 0.00
40 32.50
41 2261634.51
42 156842099844.52
43 10843961455707782.00
44 747708026454360460000.00
45 51429126632076458000000000.00
46 3529536965341344500000000000000.00
47 241737052174609690000000000000000000.00
48 16525713066466281000000000000000000000000.00
49 1127805529726474100000000000000000000000000000.00
4a 76846329228755865000000000000000000000000000000000.00
4b 5228514198248326500000000000000000000000000000000000000.00
4c 355259342137649310000000000000000000000000000000000000000000.00
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4e 1634089943872471700000000000000000000000000000000000000000000000000000.00
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53 2519470002541514500000000000000000000000000000000000000000000000000000000000000000000000000000.00
54 173693439909239170000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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61 122176384420437770000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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70 408354876418796630000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
71 28396262443943004000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
72 1968084071671644300000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
73 135999696916777530000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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76 44207816532211711000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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78 206842847014058100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
79 14111782168453345000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
7a 961276249046606020000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
7b 65386757613253666000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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80 -0.00
81 -0.00
82 -0.00
83 -0.00
84 -0.00
85 -0.00
86 -0.00
87 -0.00
88 -0.00
89 -0.00
8a -0.00
8b -0.00
8c -0.00
8d -0.00
8e -0.00
8f -0.00
90 -0.00
91 -0.00
92 -0.00
93 -0.00
94 -0.00
95 -0.00
96 -0.00
97 -0.00
98 -0.00
99 -0.00
9a -0.00
9b -0.00
9c -0.00
9d -0.00
9e -0.00
9f -0.00
a0 -0.00
a1 -0.00
a2 -0.00
a3 -0.00
a4 -0.00
a5 -0.00
a6 -0.00
a7 -0.00
a8 -0.00
a9 -0.00
aa -0.00
ab -0.00
ac -0.00
ad -0.00
ae -0.00
af -0.00
b0 -0.00
b1 -0.00
b2 -0.00
b3 -0.00
b4 -0.00
b5 -0.00
b6 -0.00
b7 -0.00
b8 -0.00
b9 -0.00
ba -0.00
bb -0.00
bc -0.00
bd -0.00
be -0.00
bf -0.12
c0 -8577.51
c1 -595821443.51
c2 -41255400998277.52
c3 -2848394305499268600.00
c4 -196154140339432450000000.00
c5 -13476555094338099000000000000.00
c6 -923923411592226570000000000000000.00
c7 -63219126011291991000000000000000000000.00
c8 -4318036447754703500000000000000000000000000.00
c9 -294449594579902470000000000000000000000000000000.00
ca -20048271934734512000000000000000000000000000000000000.00
cb -1363115719988382000000000000000000000000000000000000000000.00
cc -92559631349317831000000000000000000000000000000000000000000000.00
cd -6277438562204192500000000000000000000000000000000000000000000000000.00
ce -425255837650091480000000000000000000000000000000000000000000000000000000.00
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d0 -1993854408381186000000000000000000000000000000000000000000000000000000000000000000.00
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d2 -9586051653338756600000000000000000000000000000000000000000000000000000000000000000000000000.00
d3 -661737160148046550000000000000000000000000000000000000000000000000000000000000000000000000000000.00
d4 -45563434706068069000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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d9 -68355779549259894000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
da -4653735800328269100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
db -316388621163970020000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
dc -21482046286569577000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
dd -1456815990147462900000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
de -98683099208633679000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
df -6677614145500822200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e0 -463468064771798250000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e1 -32180465730379564000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e2 -2227377823277027000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e3 -153732818170537500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e4 -10583553036438193000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e5 -726930037227653690000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e6 -49824159672519490000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e7 -3408411520223639500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e8 -232754178434599600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
e9 -15868539665109664000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
ea -1080249646601300100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
eb -73435622782400814000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
ec -4985717045666697800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
ed -338084306397821390000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
ee -22899894549927040000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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f0 -107730874267432140000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
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f2 -517539169214551540000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f3 -35714397827745240000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f4 -2458343745902715200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f5 -168827860796461440000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f6 -11570099379335498000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f7 -791405925073819980000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f8 -54037958965559871000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
f9 -3683800791481180300000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
fa -250751870841351620000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
fb -17044745290527055000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
fc -1157117770045540900000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
fd -78459116763945423000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
fe -5314010372517807600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
ff -1.#R
看了这么大的表，不难发现，对于double型变量，当我们想要清极大值的时候，我们可以选择0x7f，当然如果觉得这个数字过于夸张，我们可以选择0x42或者0x43。同样，想清最小值的时候，可以选择0xfe或0xc2。