'using namespace std'正在導致一個奇怪的行爲

Question

我在同一組輸入上運行了以下2個代碼片段，除了第二個中的「namespace std」的用法相同之外。我無法理解他們產出不同的原因。有人能指導我嗎？謝謝！

程序1：

#include<cmath> 
#include<cstdio> 
//using namespace std; 
int main() { 
    int i, p, q, r, s; 
    while (scanf("%d %d %d %d", &p, &q, &r, &s) == 4) { 
     long double ans = 0; 
     if (p - q < q) 
      q = p - q; 
     if (r - s < s) 
      s = r - s; 
     for (i = 1; i <= q; i++) 
      ans += log(p - q + i) - log(i); 
     for (i = 1; i <= s; i++) 
      ans -= log(r - s + i) - log(i); 
     printf("%.5lf\n", exp(ans)); 
    } 
    return 0; 
} 

輸出1：

0.12587 
505606.46055 
1.28223 
0.48996 
2.00000 
3.99960 

http://ideone.com/x7n5cm

計劃2：

#include<cmath> 
#include<cstdio> 
using namespace std; 
int main() { 
    int i, p, q, r, s; 
    while (scanf("%d %d %d %d", &p, &q, &r, &s) == 4) { 
     long double ans = 0; 
     if (p - q < q) 
      q = p - q; 
     if (r - s < s) 
      s = r - s; 
     for (i = 1; i <= q; i++) 
      ans += log(p - q + i) - log(i); 
     for (i = 1; i <= s; i++) 
      ans -= log(r - s + i) - log(i); 
     printf("%.5lf\n", exp(ans)); 
    } 
    return 0; 
} 

輸出2：

0.00000 
0.00000 
0.00000 
0.00000 
0.00000 
0.00000 

http://ideone.com/meDkRh

輸入：

10 5 14 9 
93 45 84 59 
145 95 143 92 
995 487 996 488 
2000 1000 1999 999 
9998 4999 9996 4998 

編輯： 1）代替的#include 「比特/ STDC++ H」 與標準的報頭。 2）用實際的代碼段替換圖像。

Answer 1

擺弄log()並懷疑operator +=()我終於解決了這個難題。

實際的錯誤是由printf("%.5lf\n", ...)造成的。

我修改示例代碼略微證明：現在

#include<cmath> 
#include<cstdio> 
#include<iomanip> 
#include<iostream> 
using namespace std; 
int main() { 
    int i, p, q, r, s; 
    while (scanf("%d %d %d %d", &p, &q, &r, &s) == 4) { 
     long double ans = 0; 
     if (p - q < q) 
      q = p - q; 
     if (r - s < s) 
      s = r - s; 
     for (i = 1; i <= q; i++) 
      ans += log(p - q + i) - log(i); 
     for (i = 1; i <= s; i++) 
      ans -= log(r - s + i) - log(i); 
     printf("printf: %.5lf", exp(ans)); 
     cout << "\tcout: " << fixed << setprecision(5) << exp(ans) << endl; 
    } 
    return 0; 
} 

的輸出是：

printf: 0.00000 cout: 0.12587 
printf: 0.00000 cout: 505606.46055 
printf: 0.00000 cout: 1.28223 
printf: 0.00000 cout: 0.48996 
printf: 0.00000 cout: 2.00000 
printf: 0.00000 cout: 3.99960 

我檢查此上ideone。 當我在Windows 10（64位）上使用VS2013進行編譯和運行時，獲得相同的輸出。

我與海灣合作委員會在Cygwin中選中此也可以在Windows 10（64位），並得到了以下的輸出：

$ g++ -std=c++11 -o test-longdouble test-longdouble.cc ; echo '10 5 14 9 
> 93 45 84 59 
> 145 95 143 92 
> 995 487 996 488 
> 2000 1000 1999 999 
> 9998 4999 9996 4998 
> ' | ./test-longdouble 
printf: -0.00000  cout: 0.12587 
printf: -4234002535919089587818586571373347278663379000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00000  cout: 505606.46055 
printf: -0.00000  cout: 1.28223 
printf: -65094314467486612925155033958017324735054040000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00000  cout: 0.48996 
printf: nan  cout: 2.00000 
printf: nan  cout: 3.99960 

$ g++ -std=c++14 -o test-longdouble test-longdouble.cc ; echo '10 5 14 9 
93 45 84 59 
145 95 143 92 
995 487 996 488 
2000 1000 1999 999 
9998 4999 9996 4998 
' | ./test-longdouble 
printf: -0.00000  cout: 0.12587 
printf: -4234002535919089587818586571373347278663379000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00000  cout: 505606.46055 
printf: -0.00000  cout: 1.28223 
printf: -65094314467486612925155033958017324735054040000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00000  cout: 0.48996 
printf: nan  cout: 2.00000 
printf: nan  cout: 3.99960 

$ g++ --version 
g++ (GCC) 5.4.0 

我沒有看到它的第一眼，但cout輸出是正確的，只有printf輸出是不同的。

考慮到printf格式化程序選擇錯誤，您應該不會有任何期望關於其效果。

然後我定了錯誤的格式在printf()：

#include<cmath> 
#include<cstdio> 
#include<iomanip> 
#include<iostream> 
using namespace std; 
int main() { 
    int i, p, q, r, s; 
    while (scanf("%d %d %d %d", &p, &q, &r, &s) == 4) { 
    long double ans = 0; 
    if (p - q < q) 
     q = p - q; 
    if (r - s < s) 
     s = r - s; 
    for (i = 1; i <= q; i++) 
     ans += log(p - q + i) - log(i); 
    for (i = 1; i <= s; i++) 
     ans -= log(r - s + i) - log(i); 
    printf("printf: %.5Lf", exp(ans)); 
    cout << "\tcout: " << fixed << setprecision(5) << exp(ans) << endl; 
    } 
    return 0; 
} 

編譯和Cygwin中使用gcc再次測試在Windows 10（64位）：

$ g++ -std=c++14 -o test-longdouble test-longdouble.cc ; echo '10 5 14 9 
93 45 84 59 
145 95 143 92 
995 487 996 488 
2000 1000 1999 999 
9998 4999 9996 4998 
' | ./test-longdouble 
printf: 0.12587 cout: 0.12587 
printf: 505606.46055 cout: 505606.46055 
printf: 1.28223 cout: 1.28223 
printf: 0.48996 cout: 0.48996 
printf: 2.00000 cout: 2.00000 
printf: 3.99960 cout: 3.99960 

因此，我只能重複我已經在我的評論中說過：你真的應該使用C++流輸出。 printf()可能會導致奇怪的行爲，因爲格式化程序中有絲毫的錯誤。