DFS algorithm implementation at Objective C - objective-c

Question

I am trying to implement the Objective C realization of this algorithm. Here the implementation of it:
#implementation DFSAlgorithm
-(void)dfs:(Graph*)g andStartingPosition:(int)s{
[self performDFS:g andPosition:s];
}
-(void)markedArrayInit:(int)capacity{
//0 is for unmarked vertices
//1 is form marked ones
self.marked=[[NSMutableArray alloc]initWithCapacity:capacity];
for(int i=0;i<[self.marked count];i++)
[self.marked replaceObjectAtIndex:i withObject:[NSNumber numberWithInt:0]];
}
-(void)performDFS:(Graph *)g andPosition:(int)v{
[self markedArrayInit:(int)[g numberOfVertices]];
[self.marked replaceObjectAtIndex:v withObject:[NSNumber numberWithInt:1]];
for (NSNumber *vertex in [g.vertices objectAtIndex:v]){
if(1==[self isMarked:v atGraph:g]){
NSLog(#"%d",(int)vertex);
[self performDFS:g andPosition:(int)vertex];
}
}
}
-(int)isMarked:(int)v atGraph:(Graph *)g{
return [self.marked objectAtIndex:v];
}
#end
However, I don't understand why the following error occurs:
[__NSArrayM replaceObjectAtIndex:withObject:]: index 0 beyond bounds for empty array'
How can I correctly initialize the marked array?
Thank you.

Answer 1

An NSMutableArray is created empty, the capacity value you pass is just a hint to the implementation about how large you expect the array to become.
Therefore replaceObjectAtIndex:withObject: does not work for you, as the array is empty you have no objects to replace.
Instead just use addObject: capacity times.

Answer 2

In your markedArrayInit method you create empty mutable array and reserve memory for it to hold at least capasity number of items. But you do not actually add anything to it (for loop in that method does not actually do anything at all). To fix your problem you can add enough number of items in for loop:
for (int i=0;i< initWithCapacity:capacity;i++)
[self.marked addObject: #0];
}
Edit:
Your implementation has several other problems:
you initialize marked array on each call to performDFS:andPosition:, and call that method recursively. You should move initialization to dfs:andStartingPosition: method
In isMarked:atGraph: method you return object from array, not the numeric value it holds - so it will never be 1, you might want to replace it with the following implementation (Note that method name implies we return some boolean value, not an integer that we'll need to interpret later):
-(BOOL)isMarked:(int)v atGraph:(Graph *)g {
return [self.marked[v] intValue] == 1;
}
...
if([self isMarked:v atGraph:g]){
...
}
There're better data structures to store indices of marked nodes, e.g. NSSet or NSIndexSet

Answer 3

You try to replace not existing object inside array. In markedArrayInit use addObject: method from NSMutableArray. [self.marked count] is always 0 in for cycle.