Assignment 1 of CS193p

Question

I recently started following the online course on iPhone development from Stanford University on iTunes U.

I'm trying to do the homework assignments now for the first couple of lectures. I followed through the walkthrough where I built a basic calculator, but now I'm trying the first assignment and I can't seem to work it out. Having some issues:

Trying to implement these:

Add the following 4 operation buttons:
• sin : calculates the sine of the top operand on the stack.
• cos : calculates the cosine of the top operand on the stack.
• sqrt : calculates the square root of the top operand on the stack.
• π: calculates (well, conjures up) the value of π. Examples: 3 π * should put
three times the value of π into the display on your calculator, so should 3 Enter π *, 
so should π 3 *. Perhaps unexpectedly, π Enter 3 * + would result in 4 times π being 
shown. You should understand why this is the case. NOTE: This required task is to add π as 
an operation (an operation which takes no arguments off of the operand stack), not a new
way of entering an operand into the display.

My code for performOperation is this:

-(double)performOperation:(NSString *)operation
{
    double result = 0;
    double result1 = 0;
    if ([operation isEqualToString:@"+"]){
        result = [self popOperand] + [self popOperand];
    }else if ([@"*" isEqualToString:operation]){
        result = [self popOperand] * [self popOperand];
    }
    else if ([@"/" isEqualToString:operation]){
        result = [self popOperand] / [self popOperand];
    }
    else if ([@"-" isEqualToString:operation]){
        result = [self popOperand] - [self popOperand];
    }
    else if ([@"C" isEqualToString:operation])
    {
        [self.operandStack removeAllObjects];
        result = 0;
    }
    else if ([@"sin" isEqualToString:operation])
    {
       result1 = [self popOperand];
        result = sin(result1); 
    }
    else if ([@"cos" isEqualToString:operation])
    {
        result1 = [self popOperand];
        result = cos(result1);
    }
    else if ([@"sqrt" isEqualToString:operation])
    {
        result1 = [self popOperand];
        result = sqrt(result1);
    }

    [self pushOperand:result];
    return result;
}

Facing some problems such as :

1. i get inf when i type 5 enter 3 /
2. also not sure whether my code for sin,cos and sprt are correct?

Answer 1

There is one error in your division.

If you type now '2 enter 4 enter /', you will get 2 (4/2) as answer. It should be 0.5 (2/4).

Maybe this hint helps.

You could shorten your functions to 'result = sin([self popOperand]);' for instance.

Anyway when you are stuck try to use NSLog() and print interesting values in the console. Is really useful when debugging.

Answer 2

Since I'm also going through the class in iTunesU maybe this will help.

First what Arnaud said is true.

The first operand you pop should actually be in the denominator, so you'll have to store it off to the side first. (also since its the denominator you should make sure it isn't = to 0.) You should also look at your subtraction to make sure the order of operations is correct there.

getting inf from 5 enter 3 / is telling me that you missed one thing from the walk through. In operationPressed: in your CalculatorViewController are you sending the enterPressed if we are in the middle of entering a number?

- (IBAction)operationPressed:(UIButton *)sender {
    if (self.userIsInTheMiddleOfEnteringANumber) {
        [self enterPressed];
    }
    double result = [self.brain performOperation:sender.currentTitle];
    NSString *resultString = [NSString stringWithFormat:@"%g",result];
    self.display.text = resultString;
    self.history.text = [CalculatorBrain descriptionOfProgram:self.brain.program];
}

Answer 3

I think what you are missing here is this point:

"This required task is to add π as an operation"

Consider the following case- 3π* should produce 9.42 as the answer. According to your performOperation: , when '*' is encountered, the operand stack should be as

• 3.14
• 3

The required task is you add π as an operation. So what that π operation needs to do is push the appropriate value onto the stack and nothing more. As for the other functions, give it a shot and check with the scientific calculator to see if you implemented it right.