-[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance - objective-c

Question

I've got an Mutable NSArray storing object types (ids - NSNumber, NSString) that will store digits (1,2,3,4, etc), operations (+,-,/,*, etc.), and variables (x,y,z, etc.). I have the variables and related values stored in an NSDictionary, keys are NSStrings equal to x,y,z with NSNumber values 5,5,2 respectively. I want to replace my variable in my NSArray with the actual value stored in my NSDictionary. I keep getting the following error when I attempt to replace the object. Please help.
-[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance
NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:
[NSNumber numberWithDouble:5],#"x",
[NSNumber numberWithDouble:5],#"y",
[NSNumber numberWithDouble:2],#"z",
nil];
+ (double)runProgram:(id)program
usingVariableValues:(NSDictionary *)variableValues;
{
// introspection - ensure program is NSArray and variableValues is NSDictionary
if ([program isKindOfClass:[NSArray class]] && [variableValues isKindOfClass: [NSDictionary class]])
{
// array for program stack
NSMutableArray *stack = [program copy];
// Loop to replace variables with actual values in NSDictionary
for (NSUInteger i = 0; i < [stack count]; i++)
{
NSLog(#"object at index = %#", [stack objectAtIndex:i]);
if ([[stack objectAtIndex:i] isKindOfClass:[NSString class]] && [[stack objectAtIndex:i] isEqualToString: #"x"])
{
// replace variable with value in corresponding value in dictionary
NSNumber *numberKeyValue = [variableValues objectForKey:#"x"];
[stack replaceObjectAtIndex:i withObject:numberKeyValue];
}
}
return 0
}

Answer 1

NSArray's copy method always returns an immutable array, regardless of whether the original was mutable or immutable. If you want a mutable copy, you need to use mutableCopy. (It's a Cocoa convention. NSString, NSDictionary, and generally any class that has mutable and immutable variants will work the same way.)