Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x117540> was mutated while being enumerated

Question

Im gettin the "Collection was mutates while beign enumerated" exception when my code is like this as follows:

NSString *poiStr = nil;
    int SuccessValue=0;
    NSMutableArray* poisArray=[[NSMutableArray alloc]init];
    for (id<MKAnnotation> annotation in mainMapView.annotations)
    {
        MKAnnotationView* anView = [mainMapView viewForAnnotation: annotation];
        if(anView)
        {

            POI* locationPOI = (POI*)[anView annotation];
            printf("\n Location poi shape groupId:%s=====%s",[locationPOI.shapeSpaceGroupId UTF8String],[poiStr UTF8String]);
            if((poiStr == nil) || ([poiStr isEqualToString:locationPOI.shapeSpaceGroupId]))
            {

                poiStr = locationPOI.shapeSpaceGroupId;
                [poisArray addObject:locationPOI];
                SuccessValue++;
            }
            printf("\n successValue:%d",SuccessValue);
            if(SuccessValue==2)
            {
                POI* poi=[poisArray objectAtIndex:0];
                [mainMapView removeAnnotation:poi];
                SuccessValue--;
            }

        }      

    }

Can anyone please suggest me what's the wrong in the code and how to solve the issues.

Thanks to all, Madan.

Answer 1

Basically you are modifying the list in the loop you are iterating over it. The line that causes the problem is:

[mainMapView removeAnnotation:poi];

as you are iterating over mainMapView.annotations. A possible solution is to accumulate the elements you want to delete in a different list and remove them after the loop.

Based on your code, a possible solution would be:

     NSString *poiStr = nil;
    int SuccessValue=0;
    NSMutableArray* poisArray=[[NSMutableArray alloc]init];
    NSMutableArray *to_delete = [[NSMutableArray alloc] init];
    for (id<MKAnnotation> annotation in mainMapView.annotations)
    {
        MKAnnotationView* anView = [mainMapView viewForAnnotation: annotation];
        if(anView)
        {

            POI* locationPOI = (POI*)[anView annotation];
            printf("\n Location poi shape groupId:%s=====%s",[locationPOI.shapeSpaceGroupId UTF8String],[poiStr UTF8String]);
            if((poiStr == nil) || ([poiStr isEqualToString:locationPOI.shapeSpaceGroupId]))
            {

                poiStr = locationPOI.shapeSpaceGroupId;
                [poisArray addObject:locationPOI];
                SuccessValue++;
            }
            printf("\n successValue:%d",SuccessValue);
            if(SuccessValue==2)
            {
                POI* poi=[poisArray objectAtIndex:0];
                //[mainMapView removeAnnotation:poi];
                [to_delete addObject:poi];
                SuccessValue--;
            }

        }      

    }
    for (id<MKAnnotation> annotation in to_delete)
           [mainMapView removeAnnotation:poi];

    [to_delete release];