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我想，以評估的近似計算的質量來解決一個組合問題蠻力。爲此，我寫了一個小R函數，它用於變量r輸出一個令人難以置信的醜陋rcpp函數，其中包含r嵌套循環加上一些中斷條件。但是，當r開始變爲20+的範圍時，使用rcpp :: cppFunction編譯函數需要非常長的時間來編譯。 ..
任何解釋爲什麼rcpp編譯爲這個大量的嵌套循環打破？當我使用g ++作爲一個普通的C++程序編譯函數時，它在不到一秒的時間內即可編譯並且運行完美（使用cout而不是Rcout）。
我可能錯過了一些明顯的東西，因爲當我刪除除了最內層的中斷條件之外的所有內容時，它會與rcpp進行精美的編譯。但是，當我也刪除這最後的休息條件，它不會完成編譯... 有什麼建議嗎？
P.S.這是一個r = 20的示例程序，我仍然在等待編譯完成。警告：它很醜，但會自動生成。
cppFunction('
int make_tList_rcpp() {
int r = 20;
std::cout << std::endl;
for (int t20=0; t20 <= floor(r/20); t20++) {
for (int t19=0; t19 <= floor(r/19); t19++) {
for (int t18=0; t18 <= floor(r/18); t18++) {
for (int t17=0; t17 <= floor(r/17); t17++) {
for (int t16=0; t16 <= floor(r/16); t16++) {
for (int t15=0; t15 <= floor(r/15); t15++) {
for (int t14=0; t14 <= floor(r/14); t14++) {
for (int t13=0; t13 <= floor(r/13); t13++) {
for (int t12=0; t12 <= floor(r/12); t12++) {
for (int t11=0; t11 <= floor(r/11); t11++) {
for (int t10=0; t10 <= floor(r/10); t10++) {
for (int t9=0; t9 <= floor(r/9); t9++) {
for (int t8=0; t8 <= floor(r/8); t8++) {
for (int t7=0; t7 <= floor(r/7); t7++) {
for (int t6=0; t6 <= floor(r/6); t6++) {
for (int t5=0; t5 <= floor(r/5); t5++) {
for (int t4=0; t4 <= floor(r/4); t4++) {
for (int t3=0; t3 <= floor(r/3); t3++) {
for (int t2=0; t2 <= floor(r/2); t2++) {
for (int t1=0; t1 <= floor(r/1); t1++) {
if ((1*t1+2*t2+3*t3+4*t4+5*t5+6*t6+7*t7+8*t8+9*t9+10*t10+11*t11+12*t12+13*t13+14*t14+15*t15+16*t16+17*t17+18*t18+19*t19+20*t20) == r) {
Rcout << t1 << "," << t2 << "," << t3 << "," << t4 << "," << t5 << "," << t6 << "," << t7 << "," << t8 << "," << t9 << "," << t10 << "," << t11 << "," << t12 << "," << t13 << "," << t14 << "," << t15 << "," << t16 << "," << t17 << "," << t18 << "," << t19 << "," << t20 << std::endl;
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7+6*t6+5*t5+4*t4+3*t3+2*t2+1*t1) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7+6*t6+5*t5+4*t4+3*t3+2*t2) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7+6*t6+5*t5+4*t4+3*t3) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7+6*t6+5*t5+4*t4) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7+6*t6+5*t5) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7+6*t6) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8+7*t7) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9+8*t8) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10+9*t9) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11+10*t10) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12+11*t11) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13+12*t12) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14+13*t13) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15+14*t14) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16+15*t15) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17+16*t16) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18+17*t17) > r) {
break;
}
}
if ((20*t20+19*t19+18*t18) > r) {
break;
}
}
if ((20*t20+19*t19) > r) {
break;
}
}
if ((20*t20) > r) {
break;
}
}
return(0);
}')
正如@spacedman建議的那樣，當使用sourceCpp代替時，這裏有更多的調試信息。寫在給sourceCpp建議進一步的評論，它出現在Linux所以大概在Mac相關的問題工作...：
> sourceCpp(file="foobar.cpp",verbose=TRUE, rebuild=TRUE)
Generated extern "C" functions
--------------------------------------------------------
#include <Rcpp.h>
// make_tList_rcpp
void make_tList_rcpp();
RcppExport SEXP sourceCpp_1_make_tList_rcpp() {
BEGIN_RCPP
Rcpp::RNGScope rcpp_rngScope_gen;
make_tList_rcpp();
return R_NilValue;
END_RCPP
}
Generated R functions
-------------------------------------------------------
`.sourceCpp_1_DLLInfo` <- dyn.load('/private/var/folders/bj/k_b2brs5443bmm8699v5fvxw0000gn/T/RtmpDSa3m8/sourceCpp-x86_64-apple-darwin13.4.0-0.12.9/sourcecpp_a7c1e15e92a/sourceCpp_8.so')
make_tList_rcpp <- Rcpp:::sourceCppFunction(function() {}, TRUE, `.sourceCpp_1_DLLInfo`, 'sourceCpp_1_make_tList_rcpp')
rm(`.sourceCpp_1_DLLInfo`)
Building shared library
--------------------------------------------------------
DIR: /private/var/folders/bj/k_b2brs5443bmm8699v5fvxw0000gn/T/RtmpDSa3m8/sourceCpp-x86_64-apple-darwin13.4.0-0.12.9/sourcecpp_a7c1e15e92a
/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB -o 'sourceCpp_8.so' --preclean 'foobar.cpp'
clang++ -I/Library/Frameworks/R.framework/Resources/include -DNDEBUG -I/usr/local/include -I/usr/local/include/freetype2 -I/opt/X11/include -I"/Users/hoehle/Library/R/3.3/library/Rcpp/include" -I"/Users/hoehle/Sandbox/Blog/_source" -fPIC -Wall -mtune=core2 -g -O2 -c foobar.cpp -o foobar.o
（而這正是它掛...）
附：這裏是sessionInfo（）
R version 3.3.2 (2016-10-31)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: macOS Sierra 10.12.2
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] Rcpp_0.12.9
loaded via a namespace (and not attached):
[1] compiler_3.3.2 tools_3.3.2
可能是最好的嵌套循環永遠 –
自己不能重現這個。上面的'cppfunction'代碼在我的筆記本電腦中以〜1s編譯。 Linux上的Rcpp 0.12.8，R 3.3.1 - 確實堅持一個'verbose = TRUE，rebuild = TRUE'告訴我們更多關於哪部分進程正在堅持？ – Spacedman
循環找到所有需要用不等概率解決生日問題的集合 - 請參閱http://www.stat.wisc.edu/sites/default/files/tr591.pdf – mhatsu