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Pointers Contd.

Pointers Contd. 8. ptr. -5. ptr2. 8. ptr. -5. ptr2. Causing a Memory Leak. int *ptr = new int; *ptr = 8; int *ptr2 = new int; *ptr2 = -5;. ptr = ptr2;. 8. ptr. 8. ptr. NULL. ptr2. -5. ptr2. Leaving a Dangling Pointer. int *ptr = new int; *ptr = 8; int *ptr2 = new int;

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Pointers Contd.

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  1. Pointers Contd.

  2. 8 ptr -5 ptr2 8 ptr -5 ptr2 Causing a Memory Leak int *ptr = new int; *ptr = 8; int *ptr2 = new int; *ptr2 = -5; ptr = ptr2;

  3. 8 ptr 8 ptr NULL ptr2 -5 ptr2 Leaving a Dangling Pointer int *ptr = new int; *ptr = 8; int *ptr2 = new int; *ptr2 = -5; ptr = ptr2; delete ptr2; ptr2 = NULL;

  4. array array array array idx=3 3.4 2.6 5.7 Dynamic Array Example Sort an array of float numbers from standard input. The first element is the size of the array. For example: 5 3.4 2.6 5.7 100.4 43.5 sortFromInput( ) { float *array; int size, idx; cin >> size; array = new float[size]; for (idx=0; idx<size; idx++) cin >> array[idx]; Sort(array, size); OutputSortedArray(array, size); delete[] array; }

  5. Pointer Arithmetic • Int *p; • p = new int[10] • p[i]; 0 <= i <= 9 • What are *p, *p++ etc..

  6. Algorithm Efficiency and Sorting Algorithms

  7. Measuring the Efficiency of Algorithms • Analysis of algorithms is the area of computer science that provides tools for contrasting the efficiency of different methods of solution. • Concerns with significant differences

  8. How To Do Comparison? • Implement the algorithms in C++ and run the programs • How are the algorithms coded? • What computer should you use? • What data should the programs use? • Analyze algorithms independent of implementations

  9. The Execution Time of Algorithms • Count the number of basic operations of an algorithm • Read (get), write (put), compare, assignment, jump, arithmetic operations (increment, decrement, add, subtract, multiply, divide), shift, open, close, logical operations (not/complement, AND, OR, XOR), …

  10. The Execution Time of Algorithms • Counting an algorithm’s operations int sum = item[0]; int j = 1; while (j < n) { sum += item[j]; ++j; } <- 1 assignment <- 1 assignment <- n comparisons <- n-1 plus/assignments <- n-1 plus/assignments Total: 3n operations

  11. Algorithm Growth Rates • Measure an algorithm’s time requirement as a function of the problem size • Number of elements in an array Algorithm A requires n2/5 time units Algorithm B requires 5n time units Algorithm efficiency is a concern for large problems only

  12. Common Growth-Rate Functions - I

  13. Common Growth-Rate Functions - II

  14. Big O Notation Algorithm A is order f(n)-denoted O(f(n))-if constants k and n0 exist such that A requires <= k*f(n) time units to solve a problem of size n>=n0 • n2/5 • O(n2): k=1/5, n0=0 • 5*n • O(n): k=5, n0=0

  15. More Examples • How about n2-3n+10? • O(n2) if there exist k and n0 such that • kn2≥ n2-3n+10 for all n ≥ n0 • 3n2 ≥ n2-3n+10 for all n ≥ 2; so k=3, n0=2

  16. Properties of big-Oh • Ignore low-order terms • O(n3+4n2+3n)=O(n3) • Ignore multiplicative constant • O(5n3)=O(n3) • Combine growth-rate functions • O(f(n)) + O(g(n)) = O(f(n)+g(n))

  17. Worst-case vs. Average-case Analyses • An algorithm can require different times to solve different problems of the same size. • Worst-case analysis (find the maximum number of operations an algorithm can execute in all situations) • is easier to calculate and is more common • Average-case (enumerate all possible situations, find the time of each of the m possible cases, total and dive by m) • is harder to compute but yields a more realistic expected behavior

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