C++ array of linkedlist segmentation fault

Question

I am trying to fix a method that takes a function that takes a root of a linked list and the number of vertices in a graph and organizes the linked list into an array of linked list based on the first vertex of an edge struct, for example, in a struct where an node Edge has firVertex = 1, sndVertex = 2, and weight = 2, it would be sorted into the first element of an array. and another node Edge that has another firVertex = 1 would be appended onto the previous one, and etc. I have fixed it many times but it is still giving a segmentation fault when running.

Thanks for the tips, really appreciate it. Unfortunately, I cannot leave the code up since its a school assignment.

Have you stepped through your code in the debugger?

OldProgrammer
– OldProgrammer

2014-01-31 20:43:24 +00:00
Commented Jan 31, 2014 at 20:43 — OldProgrammer
– OldProgrammer, Commented Jan 31, 2014 at 20:43

IdeaHat · Accepted Answer · 2014-01-31 20:47:17Z

1

Edge* arrayList [numberVertices]

Allocates on the stack, not the heap, and goges out of scope at the end of the function.

answered Jan 31, 2014 at 20:47

IdeaHat

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Comments

hivert · Accepted Answer · 2014-01-31 20:46:57Z

0

The variable arrayList if declared as a local array. Therefore is is destroyed at the end of the function and shouldn't be returned. You should allocate it dynamically with a new:

Edge** arrayList = new Edge *[numberVertices];

By the way your compiler (if correctly configured) should warn you. Here is what g++ -Wall does:

bli.cpp: In function ‘Edge** _()’:
bli.cpp:5:9: warning: address of local variable ‘arrayList’ returned [-Wreturn-local-addr]
  Edge* arrayList[500];

Also this looks very suspicious to me:

for (int i=0; i<numberVertices; i++) { //initializes the array to NULL
   arrayList[numberVertices]=NULL;
}

you probably mean:

   arrayList[i]=NULL;

answered Jan 31, 2014 at 20:46

hivert

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Comments

Павел · Accepted Answer · 2014-01-31 20:49:41Z

0

so why don't you tried StdLib? I personally recommend to avoid using pointer. you could use references (&) instead.

and if you got segmentation fault, are you able to debug till you found the line where it fails?

and you basically will got segmentation fault because you call Edge constructor with NULL as a root, and then try to reference root->next, but root is null...

answered Jan 31, 2014 at 20:49

Павел

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3 Comments

Павел Over a year ago

it fails in both cases, you need to remove NULL from args of constructor

hysoftwareeng Over a year ago

in that case, how would I linked the node to the previous elements in the array. could you give me some tips? thanks

Павел Over a year ago

look, the problem is only with string "root->next=this" if you send NULL to constructor, root will point to nothing, 0, this is access violation. I see in "else" case you do "last->next = .." instead you could pass "last" as an arg to constructor and it will set "last->next" and in "then" case I really have no idea that should be "root", I'm not sure I understand clearly that you trying to do. I'm talking only as compiler advocate here ;) and then after all as it was said your arrayList of pointers will dissapear after you will return from function, because it is scoped to this function only

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