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# Unit i basic concepts of algorithms

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Algorithm Concepts

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### Unit i basic concepts of algorithms

1. 1. UNIT I BASIC CONCEPTS OF ALGORITHMS
2. 2. Algorithm is a step by step procedure, which defines a set of instructions to be executed in certain order to get the desired output. Algorithms are generally created independent of underlying languages, i.e. an algorithm can be implemented in more than one programming language. ALGORITHM
3. 3. From data structure point of view, following are some important categories of algorithms − Search − Algorithm to search an item in a data structure. Sort − Algorithm to sort items in certain order Insert − Algorithm to insert item in a data structure Update − Algorithm to update an existing item in a data structure Delete − Algorithm to delete an existing item from a data structure ALGORITHM
4. 4.  Not all procedures can be called an algorithm.  An algorithm should have the below mentioned characteristics  Unambiguous − Algorithm should be clear and unambiguous. Each of its steps (or phases), and their input/outputs should be clear and must lead to only one meaning.  Input − An algorithm should have 0 or more well defined inputs.  Output − An algorithm should have 1 or more well defined outputs, and should match the desired output.  Finiteness − Algorithms must terminate after a finite number of steps.  Feasibility − Should be feasible with the available resources.  Independent − An algorithm should have step-by-step directions which should be independent of any programming code. CHARACTERISTICS OF AN ALGORITHM
5. 5.  We design an algorithm to get solution of a given problem. A problem can be solved in more than one ways.
6. 6.  Efficiency of an algorithm can be analyzed at two different stages, before implementation and after implementation, as mentioned below −  A priori analysis − This is theoretical analysis of an algorithm. Efficiency of algorithm is measured by assuming that all other factors e.g. Processor speed, are constant and have no effect on implementation.  A posterior analysis − This is empirical analysis of an algorithm. The selected algorithm is implemented using programming language. This is then executed on target computer machine. In this analysis, actual statistics like running time and space required, are collected.  We shall learn here a priori algorithm analysis.  Algorithm analysis deals with the execution or running time of various operations involved.  Running time of an operation can be defined as no. of computer instructions executed per operation. ALGORITHM ANALYSIS
7. 7.  Suppose X is an algorithm and  n is the size of input data,  the time and space used by the Algorithm X are the two main factors which decide the efficiency of X.  Time Factor − The time is measured by counting the number of key operations such as comparisons in sorting algorithm  Space Factor − The space is measured by counting the maximum memory space required by the algorithm.  The complexity of an algorithm f(n) gives the running time and / or storage space required by the algorithm in terms of n as the size of input data. 1. ALGORITHM COMPLEXITY
8. 8.  Space complexity of an algorithm represents the amount of memory space required by the algorithm in its life cycle.  Space required by an algorithm = Fixed Part + Variable Part  A fixed part that is a space required to store certain data and variables, that are independent of the size of the problem. Eg: Simple variables & constant used, program size etc.  A variable part is a space required by variables, whose size depends on the size of the problem. Eg. Dynamic memory allocation, recursion stack space etc. 2. SPACE COMPLEXITY
9. 9.  Space complexity S(P) of any algorithm P is, S(P) = C + SP(I) Where C is the fixed part and S(I) is the variable part of the algorithm which depends on instance characteristic I. Example:  Algorithm: SUM(A, B)  Step 1 - START  Step 2 - C ← A + B + 10  Step 3 – Stop 2. SPACE COMPLEXITY Here we have three variables A, B and C and one constant. Hence S(P) = 1+3. Now space depends on data types of given variables and constant types and it will be multiplied accordingly.
10. 10.  Time Complexity of an algorithm represents the amount of time required by the algorithm to run to completion.  Time requirements can be defined as a numerical function T(n), where T(n) can be measured as the number of steps, provided each step consumes constant time.  Eg. Addition of two n-bit integers takes n steps.  Total computational time is T(n) = c*n,  where c is the time taken for addition of two bits.  Here, we observe that T(n) grows linearly as input size increases. 3. TIME COMPLEXITY
11. 11.  Asymptotic analysis of an algorithm, refers to defining the mathematical bound/framing of its run-time performance.  Using asymptotic analysis, we can very well conclude the best case, average case and worst case scenario of an algorithm.  Asymptotic analysis are input bound i.e., if there's no input to the algorithm it is concluded to work in a constant time.  Other than the "input" all other factors are considered constant. ASYMPTOTIC ANALYSIS
12. 12.  Asymptotic analysis refers to computing the running time of any operation in mathematical units of computation.  For example, running time of one operation is computed as f(n) and may be for another operation it is computed as g(n2).  Which means first operation running time will increase linearly with the increase in n and running time of second operation will increase exponentially when n increases.  Similarly the running time of both operations will be nearly same if n is significantly small. ASYMPTOTIC ANALYSIS
13. 13. 0 20 40 60 80 100 120 1 2 3 4 5 6 7 8 9 10 Series 2 Series 3 LINEAR VS EXPONENTIAL
14. 14.  Usually, time required by an algorithm falls under three types Best Case − Minimum time required for program execution (Run Fastest among all inputs) Average Case − Average time required for program execution. Gives the necessary information about algorithm’s behavior on random input Worst Case − Maximum time required for program execution (Run slowest among all inputs)
15. 15.  Following are commonly used asymptotic notations used in calculating running time complexity of an algorithm.  Ο Notation  Ω Notation  θ Notation Big Oh Notation, Ο  The Ο(n) is the formal way to express the upper bound of an algorithm's running time.  It measures the worst case time complexity or longest amount of time an algorithm can possibly take to complete. ASYMPTOTIC NOTATIONS
16. 16.  Omega Notation, Ω  The Ω(n) is the formal way to express the lower bound of an algorithm's running time.  It measures the best case time complexity or best amount of time an algorithm can possibly take to complete.  Theta Notation, θ  The θ(n) is the formal way to express both the lower bound and upper bound of an algorithm's running time. ASYMPTOTIC NOTATIONS
17. 17.  First, we start to count the number of significant operations in a particular solution to assess its efficiency.  Then, we will express the efficiency of algorithms using growth functions.  Each operation in an algorithm (or a program) has a cost.  Each operation takes a certain of time. count = count + 1; Take a certain amount of time, but it is constant A sequence of operations: count = count + 1; Cost: c1 sum = sum + count; Cost: c2 Total Cost: c1 + c2 TO ANALYZE ALGORITHMS
18. 18. Example: Simple If-Statement Cost Times if (n < 0) c1 1 absval = -n c2 1 else absval = n; c3 1 Total Cost <= c1 + max(c2,c3) THE EXECUTION TIME OF ALGORITHMS
19. 19. Cost Times i = 1; c1 1 sum = 0; c2 1 while (i <= n) { c3 n+1 i = i + 1; c4 n sum = sum + i; c5 n } Total Cost = c1 + c2 + (n+1)*c3 + n*c4 + n*c5 The time required for this algorithm is proportional to n LOOP
20. 20. Cost Times i=1; c1 1 sum = 0; c2 1 while (i <= n) { c3 n+1 j=1; c4 n while (j <= n) { c5 n*(n+1) sum = sum + i; c6 n*n j = j + 1; c7 n*n } i = i +1; c8 n } Total Cost = c1 + c2 + (n+1)*c3 + n*c4 + n*(n+1)*c5+n*n*c6+n*n*c7+n*c8  The time required for this algorithm is proportional to n2 NESTED LOOP
21. 21.  Function abc(a,b,c)  {  Return a+b+b*c+(a+b-c)/(a+b)+4.0;  }  Problem instance : a,b,c ; One word to store each  Space needed by abc is independent of instance Sp = 0 SPACE COMPLEXITY
22. 22.  Function Sum(a,n) {  S:=0.0;  For i:=1 to n do  S:= s+a[i];  Return S;  }  Characterized by n  Space needed by a[n], n, i, S Ssum(n) >= (n+3) LOOP
23. 23. Algorithm Rsum(a,n) { If(n<=0) then return 0.0; Else return Rsum(a,n-1)+a[n]; }  Instances are characterized by n  Stack Space: Formal parameter + Local variables + Return Address  Variables : a, n, and return address (3)  Depth of recursion: n+1 SRSum(n) >= 3(n+1) RECURSION
24. 24. CENG 213 Data Structures 24 GENERAL RULES FOR ESTIMATION  Loops: The running time of a loop is at most the running time of the statements inside of that loop times the number of iterations.  Nested Loops: Running time of a nested loop containing a statement in the inner most loop is the running time of statement multiplied by the product of the sized of all loops.  Consecutive Statements: Just add the running times of those consecutive statements.  If/Else: Never more than the running time of the test plus the larger of running times of S1 and S2.
25. 25. 25 ALGORITHM GROWTH RATES  We measure an algorithm’s time requirement as a function of the problem size.  Problem size depends on the application: e.g. number of elements in a list for a sorting algorithm, the number disks for towers of hanoi.  So, for instance, we say that (if the problem size is n)  Algorithm A requires 5*n2 time units to solve a problem of size n.  Algorithm B requires 7*n time units to solve a problem of size n.  The most important thing to learn is how quickly the algorithm’s time requirement grows as a function of the problem size.  Algorithm A requires time proportional to n2.  Algorithm B requires time proportional to n.  An algorithm’s proportional time requirement is known as growth rate.  We can compare the efficiency of two algorithms by comparing their growth rates.
26. 26. 26 ALGORITHM GROWTH RATES (CONT.) Time requirements as a function of the problem size n
27. 27. 27 COMMON GROWTH RATES Function Growth Rate Name c Constant log N Logarithmic log2N Log-squared N Linear N log N N2 Quadratic N3 Cubic 2N Exponential
28. 28. 28 Figure 6.1 Running times for small inputs
29. 29. 29 Figure 6.2 Running times for moderate inputs
30. 30. #include <stdio.h> Void main() { int a, b, c, sum; printf(“Enter three numbers:”); scanf(“%d%d%d”,&a,&b,&c); sum=a+b+c; printf(“Sum=%d”,sum); }  No instance characteristics  Space required by a,b,c and sum is independent of instance  S(P)=Cp+ Sp  S(P)=4+0  S(P)=4 SPACE COMPLEXITY
31. 31. int add(int x[], int n) { int total=0,i; for(i=0;i<n;i++) total=total+x[i]; return total; }  Instance = n  Space required by total, i, n: 3  Space required by constant: 1  S(P)=Cp+ Sp  S(P)=3+1+n  S(P)=4+n SPACE COMPLEXITY
32. 32. int fact(int n) { if(n<=1) return 1; else return(n*fact(n-1)); }  Instance = Depth of recurstion=n  Space required by n, return address, return value  Space required by constant: 1  S(P)=Cp+ Sp  S(P)=4*n SPACE COMPLEXITY