使用C ++中的字符串堆排序代码获取分段错误

Question

int heapSize = 20; //variable 
int left(int i) {
return (2 * i) + 1;
}

int right(int i) {
return (2 * i) + 2;
}

void heapify(string arr[], int i) {
int l = left(i);
int great=0;
int r = right(i);
if ( ((strcmp(arr[l].c_str(),arr[i].c_str()))>0) && (l < heapSize)) {
great = l;
}
else {
great = i;
}

if ( ((strcmp(arr[r].c_str(),arr[great].c_str()))>0) && (r < heapSize)) {
great = r;
}
if (great != i) {
string temp = arr[i];
arr[i] = arr[great];
arr[great] = temp;
heapify(arr, great); //Getting segment here 
}

}

void BuildMaxHeap(string arr[]) {
for (int i = (heapSize - 1) / 2; i >= 0; i--) {
heapify(arr, i);
}
}

void HeapSort(string arr[]) {
BuildMaxHeap(arr); //
for (int i = heapSize; i > 0; i--) {
string temp = arr[0];
arr[0] = arr[heapSize - 1];
arr[heapSize - 1] = temp;
heapSize = heapSize - 1;
heapify(arr, 0);
}

}

Getting segmentation fault with this string heap sort code for heapSize larger than 15, for example 20 elements. Any idea about the reason?

I backtraced it in gdb and I get this

#0  0x0038124b in ?? () from /lib/tls/i686/cmov/libc.so.6
#1  0x08048f62 in heapify(std::string*, int) ()
#2  0x08049050 in heapify(std::string*, int) ()
#3  0x080490ae in BuildMaxHeap(std::string*) ()
#4  0x080490d3 in HeapSort(std::string*) ()
#5  0x080494f5 in main ()

So, what is wrong with the heapify function?

Answer 1

在if s中，在尝试比较字符串之前，应检查索引是否在数组的边界内：

if ( (l < heapSize) && (arr[l] > arr[great]) ) {

Answer 2

It is your array indexes. Check out the left & right functions and the for loop parameters that feed them. With an odd numbered 'heapSize' value left will eventually return an index past the end of the array. With an even numbered 'heapSize', right will do it.

eg when 'heapSize' == 15, then the max 'i' passed to 'heapify' will be 7 ((15-1) / 2) . Pass that into 'left' and you get 15. Too big for the array, so it croaks when trying to get the string for the 'strcmp'.

Maybe I've done something wrong, but I'm not getting your code to run regardless of heap size.

PS. build with '-g3' flag to get better debug symbols in gdb. g++ -g3 heap.cc