cs193p fall 2011 Assignment 2 Task 1 solution - iPhone Application Developnment

Hi,
I am having pretty hard time understanding the assignment 2 of the iPhone Application Development fall 2011. Starting point for this Assignment is right after you watch The Lecture 4- Views. You can also watch Xcode and Source Code Management because the same code is entered again.  After you made modifications to the Calculator Brain you should update your app so that all of the things from Assignment 1 work here as well.

Lets get started. I will be honest here. I read the assignment few times and I didn't understood it completely. I had few questions. One of them is: how should a user enter a variable inside the Calculator's program? I was looking at the screenshots and I saw three new buttons for variables A,B and X? Does this mean there will be only three variables? After reading whole assignment few more times I think that other variables are possible via the external API. You can probably send as many as you want using the NSDictionary and variableValues from the task one. So I stopped asking myself all those questions and start programming. If you have a different explanation feel free to share it.



TASK 1:
Read the whole text of the task 1 of the Assignment 2 in the PDF file. This is the short version an some stuff is left out.
1. Add the capability to your CalculatorBrain to accept variables as operands (in
addition to still accepting doubles as operands).  You will need new public API in
your CalculatorBrain to support this.
You must add a new version of  the runProgram: class method with the following
signature ...
+ (double)runProgram:(id)program
 usingVariableValues:(NSDictionary *)variableValues;
In addition, create another class method to get all the names of  the variables used in a
given program (returned as an NSSet of  NSString objects) ...
+ (NSSet *)variablesUsedInProgram:(id)program;
If  the program has no variables return nil from this method (not an empty set).

SOLUTION: I will start out with variablesUsedInProgram: It looks easier to me. Under hint number 5 you will find you should use create helper function that checks if something is operation or not. So I am going to create that first.

+(BOOL)isOperation:(NSString *)operation{
    NSSet *supportedOperations = [[NSSet alloc] 
       initWithObjects:@"+",@"-",@"*",@"/",@"sin",@"cos",@"sqrt",@"π", nil];
    return [supportedOperations containsObject:operation]
}

The idea behind VariablesUsedInProgram: is this. First I check if my program is an array. This is just a safety measure.  I go through all the elements of the program. I check if they are strings. If they are I check if they are I check if teha are operation. If they are not I add them to my mutable set. if the set is empty I return nil just as the assignment instructed.

+(NSSet *)variablesUsedInProgram:(id)program{
    NSMutableSet *_variablesUsedInProgram = [[NSMutableSet alloc] init];
    if([program isKindOfClass:[NSArray class]]){
        for (id element in program){
            if ([element isKindOfClass:[NSString class]]){
                if(![self isOperation:element]){
                    [_variablesUsedInProgram addObject:element];
                }
            }
        }
    }
    if ([_variablesUsedInProgram count]==0) {return nil;}
    else {return _variablesUsedInProgram;}
}

I had a bit of help with this one. I have came u with this solution thanks to 8vius from Google Groups. Thanks man.

Now for the  runProgram:usingVariableValues: class method. First step is to check if the program is an array and to create mutable copy. Same as in the original runProgram method.  then I inspect every element on the stack. If it is an operation just skip it. Else if it is a variable from dictionary replace variable with number. Else replace a variable name with zero, because you don't know what is the value of the variable. Then just let the recursion do the math.

+(double) runProgram:(id)program usingVariableValues:(NSDictionary *)variableValues{
    NSMutableArray *stack;
    int i=0;
    
    if([program isKindOfClass:[NSArray class]]){
        stack=[program mutableCopy];
    }
    for (i=0;i<stack.count;i++){
        if ([[stack objectAtIndex:i] isKindOfClass:[NSString class]]){
            for (id key in variableValues){
                if ([self isOperation:[stack objectAtIndex:i]]) {
                   //leave it alone, it is an operation 
                }else if([[stack objectAtIndex:i] isEqual:key]){
                    [stack replaceObjectAtIndex:i withObject:[variableValues objectForKey:key]];
                } else {
                    [stack replaceObjectAtIndex:i withObject:[NSNumber numberWithInt:0]];
                }
            }
        }
    }
    return [self popOperandOffStack:stack];
}

Solutions to the rest of the tasks form Assignment2 are soon to follow. As always I am here to learn so comment. This is my solution and it doesn't have to be correct so if you spot some bugs or know smarter way of writing this code let me know in the comments below.