北京大学计算概论课件第十二讲 数组.pptVIP

Lesson 12 Arrays 2007.11.7 Objectives: About the Fibonacci sequence One-Dimensional Arrays Array Initialization Arrays as Function Arguments Case Study: Computing Averages and Standard Deviations Two-Dimensional Arrays Fibonacci Numbers One pair rabbit give birth to a new pair baby rabbits each year The baby rabbits need to wait one year to bear new baby fn+1 = fn + fn-1 f0 = 1 f1 = 1 Fibonacci Numbers in Nature Compute the Fibonacci numbers recursively int fib(int n) { if (n = 2) return 1; else return fib(n-1) + fib(n-2); } Time complexity of the recursive algorithm The growth rate of the computing time with the problem size Use iterative algorithm Use the Golden Ratio: (1+ sqr(5))/2 F(n) = round ( pow (1+sqr(5)) / 2, n) / sqr(5) ) By recursive powering By recursive powering (cont.) Arrays: Introduction Atomic variable: variable whose value cannot be further subdivided into a built-in data type Aggregate data type: data type with two main characteristics Its values can be decomposed into individual data elements, each of which is either atomic or another Aggregate data type It provides an access scheme for locating individual data elements within the data structure Name + space + access scheme Arrays A set of data items with same type Stored in consecutive memory Fixed in size throughout program —— not dynamic Create a one-dimensional array #define NUMELS 5 int grades[NUMELS]; To access an element Format: arrayname[ index value ] First element at position 0 Use a subscripted variable Subscripted variables can be used anywhere scalar variables are valid grades[0] = 98; grades[1] = grades[0] - 11; Any expression that evaluates an integer may be used as a subscript #define NUMELS 5 total = 0; for (i = 0; i NUMELS; i++) total = total + grades[i]; Input and Output of Array Values Array elements can be assigned values by: using assignme