关于c ++：在分配内存时合并排序逻辑失败

Merge sort logic failure in allocating memory.

我正在研究合并排序的实现，它将按歌曲的长度对列表进行排序。我已经写了以下内容，并且无法发现我的逻辑中的缺陷。这些函数不返回排序列表，而是从原始列表的中间返回一个项目列表。

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vector<song> merge( vector<song> firstList, vector<song> secondList){
vector<song> outPut;
int i = 0; int n = 0;
while( outPut.size() < (firstList.size() + secondList.size()-1 )){
if( firstList[i].length < secondList[n].length){
outPut.push_back( firstList[i]);
i++;
}else{
outPut.push_back( secondList[n]);
n++;
}
}
return outPut;
}
vector<song> mergeSortLength( vector<song> playlist){
int scope = playlist.size()/2;
vector<song> firstHalf( &playlist[0], &playlist[scope]);
vector<song> secondHalf( &playlist[scope], &playlist[playlist.size()]);
if ( firstHalf.size() != 1){
firstHalf = mergeSortLength(firstHalf);
}
if ( secondHalf.size() != 1){
secondHalf = mergeSortLength( secondHalf);
}
return merge( firstHalf, secondHalf);
}

如果我将while循环条件从

1	( outPut.size() < (firstList.size() + secondList.size() -1)){

到

1	( outPut.size() < (firstList.size() + secondList.size())){

列表排序成功进行到一半，直到编译器吐出：

playList(27898,0x7fff78b7b000) malloc: * mach_vm_map(size=7526769998340063232) failed (error code=3)
* error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
libc++abi.dylib: terminating with uncaught exception of type std::bad_alloc: std::bad_alloc
Abort trap: 6

我真的很感激任何人的帮助。我的眼睛因为盯着这个而变得模糊。

相关讨论

在merge()中，代码不检查是否已到达向量的末尾，特别是如果i>=firstlist.size()或n>=secondlist.size()，在这种情况下，将复制其他向量的其余部分。while语句的结尾不应该有-1。

vs抱怨使用&；playlist[playlist.size()]创建临时矢量，因为下标超出范围。一种可选的方法是使用begin()+index，它是一个迭代器：

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vector<song> firstHalf( playlist.begin()+0, playlist.begin()+scope);
vector<song> secondHalf( playlist.begin()+scope,
playlist.begin()+playlist.size());

已经过去一天了，所以发布一个固定版本以防有人感兴趣。所有的向量创建和回推都会增加大量的开销，最好只进行一次分配，然后使用迭代器或索引来处理子向量。

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vector<song> merge( vector<song> firstList, vector<song> secondList){
vector<song> outPut;
size_t i = 0; size_t n = 0;
while( outPut.size() < (firstList.size() + secondList.size()) ){
if( firstList[i].length < secondList[n].length){
outPut.push_back( firstList[i]);
if(++i >= firstList.size()){
do
outPut.push_back( secondList[n]);
while(++n < secondList.size());
break;
}
}else{
outPut.push_back( secondList[n]);
if(++n >= secondList.size()){
do
outPut.push_back( firstList[i]);
while(++i < firstList.size());
break;
}
}
}
return outPut;
}

vector<song> mergeSortLength( vector<song> playlist){
size_t scope = playlist.size()/2;
vector<song> firstHalf( playlist.begin()+0, playlist.begin()+scope);
vector<song> secondHalf( playlist.begin()+scope, playlist.begin()+playlist.size());
if ( firstHalf.size() != 1){
firstHalf = mergeSortLength(firstHalf);
}
if ( secondHalf.size() != 1){
secondHalf = mergeSortLength( secondHalf);
}
return merge( firstHalf, secondHalf);
}