General

Question 1

Stack vs. Heap

Answer 1

Stack:

• linear data structure
• high-speed access
• OS manages space efficiently (memory will never become fragmented)
• memory allocated in continuous block
• issue with memory shortage
• can be implemented with array, dynamic memory and linked list
• variables are allocated on the stack in function call stack
• do not need to worry about memory allocation and deallocation of stack variables

Heap:

• hierarchical data structure
• slower than stack
• space is not used efficiently; memory can become fragmented
• memory is allocated in any random order
• memory is allocated and deallocated manually by the programmer
• can be implemented with arrays and trees

Question 2

Reference Counting vs. Garbage Collection

Answer 2

Reference Counting:

• keeps track of object reference counts
• releases objects automatically as necessary
• no background processing
• cannot cope with retain cycles

Garbage Collection:

• runtime detects unused objects and releases them at indeterminate time intevals
• can clean up retain cycles
• happens in the background but might cause other threads to be put on hold temporarily

Question 3

Stack vs. Queue

Answer 3

Stack:

• LIFO (last in first out)
• linear data structure
• item is inserted at the top with push and deleted from the top with pop
• top points to last element in the list

Queue:

• FIFO (first in fist out)
• linear data structure
• items can be inserted only at the rear and can be deleted only from the front
• insert item at the rear with enqueue, and delete item from the front with dequeue
• front and rear pointers

Question 4

Recursion vs. Iteration

Answer 4

Recursion:

• calls itself
• small amount of code
• time complexity can become exponential
• large overhead due to repeated function calls
• better choice if time complexity is not an issue and shortness of code is
• infinite repetition can lead to CPU crash

Iteration:

• loops
• larger amount of code due to repetition of a block of code
• time complexity is generally lesser
• no overhead as in recursion
• infinite iteration will lead to infinite loops which may or may not lead to system errors

Question 5

Necessity of multithreading

Answer 5

• can execute multiple parts of a program at the same time
• enables resource sharing
• increases program responsiveness
• utilizes multiprocess architecture

Question 6

How to fix a memory leak

Answer 6

-use XCode debug memory graph to detect simple retain cycles. It displays all objects currently alive. Purple exclamation mark indicate related leaked objects.

1. in the box at top projectName > device, click Edit scheme
2. select Malloc Scribble which will fill freed memory with a predefined value to make it more obvious when memory is leaked
3. select Logging>Malloc Stack>Live Allocations Only which allows Xcode to build an allocation backtrace
4. make the debug console visible
5. run the app
6. click the Memory Debugger button that looks like three joined circles in a >.
7. this will display the heap contents with purple exclamation marks beside the items in the debug navigator
   - ———————————————————————————-
   - use Allocations instrument which tracks all objects the app allocates over the course of its run
   - ———————————————————————————-
   - use Leaks instrument to check all memory and figure out the leaked objects

Question 7

How to fix a laggy list

Answer 7

• fetch data asynchronously
• update table or collection view right after data is retrieved
• load images asynchronously and cache them

Question 8

Tail Recursion

Answer 8

• a special form of recursion where last operation of a function is a recursive call
• considered better because it can be optimized by the compiler

function tailrecsum(x, running_total = 0) {
    if (x === 0) {
        return running_total;
    } else {
        return tailrecsum(x - 1, running_total + x);
    }
}

Question 9

software licensing

Answer 9

• a document that provides legally binding guidelines for the use and distribution of software
• main types are proprietary, free or open source.