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HKUST Summer Programming Course 2008

HKUST Summer Programming Course 2008. Arrays, Character Strings and Standard Type String ~ Collection of Objects. Overview. Motivation Introduction to Array Array Subscripting and Element Manipulation Array Initialization Constant Arrays Multi-Dimensional Arrays

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HKUST Summer Programming Course 2008

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  1. HKUST SummerProgramming Course 2008 Arrays, Character Strings and Standard Type String ~ Collection of Objects

  2. Overview • Motivation • Introduction to Array • Array Subscripting and Element Manipulation • Array Initialization • Constant Arrays • Multi-Dimensional Arrays • Unsized Array Initialization • Representing Strings as Character Strings • Functions to Manipulate C-Strings • C++ String Type

  3. Arrays, Character Strings and Standard Type String Motivation

  4. Motivation • Suppose you need to find the maximum value of 5 int values, Value1, Value2, …, Value5. • The following code segment computes that value. int MaximumSoFar = Value1; if(Value2 > MaximumSoFar) MaximumSoFar = Value2; if(Value3 > MaximumSoFar) MaximumSoFar = Value3; if(Value4 > MaximumSoFar) MaximumSoFar = Value4; if(Value5 > MaximumSoFar) MaximumSoFar = Value5;

  5. Motivation • Notice that you need to have a separate if statement for each integer variable, because each variable is totally independent of other variables, although the variables are similar. • Now suppose that you need the maximum value from a group of 500 integer values Value1 through Value500. • Because the variables are independent, you cannot simply introduce iteration into the solution. • Using the approach as above, with a separate if statement for each integer variable, this resulting very large code segment would be both clumsy and error prone. • So, what should we do? Array!

  6. Arrays, Character Strings and Standard Type String Introduction to Array

  7. Arrays • An array is a collection of data elements that are of the same type (e.g., a collection of integers, characters, doubles). SYNTAX: <type> <arrayname>[<size>]; • The array elements are all values of the type <type>. • The size of the array is indicated by <size>, the number of elements in the array. • <size> must be an intconstant or a constant expression. • It must be a compile-time deductible integral constant

  8. Example • Single-dimensional arrays are essentially lists of information of the same type that are stored in contiguous memory locations in index order. • For example // array of 5 // un-initialized // ints int A[5]; Memory Address Variables in memory Variables Name A[0] 2000 A[1] 2004 A[2] 2008 A[3] 2012 A[4] 2016 Memory

  9. Arrays, Character Strings and Standard Type String Array Subscripting and Element Manipulation

  10. Array Subscripting • Arrays provides a good way to name a collection, and to reference its individual elements using [ ] operator. • Suppose, we declare a collection of 5 integers int A[5]; // array of 5 ints To access an individual element we must apply a subscript with index to array A, • In C++, first element of array has index 0  A[0]. • Second element of array has index 1  A[1], and so on. • Last element has an index one less than the size of the array  A[4].

  11. Example - Array Subscripting // array of 5 un-initialized ints int A[5]; A[2]=1; int x=A[2]; Memory Address Variables in memory Variables Name A[0] 2000 A[1] 2004 1 2008 A[2] 2012 A[3] 2016 A[4] Memory

  12. Inputting a List of Values to Arrays #include <iostream> using namespace std; int main(){ const int MaxSize = 10000; int A[MaxSize]; int n = 0; int CurrentInput; while(n< MaxSize && cin>>CurrentInput){ A[n] = CurrentInput; n++; } // List A of n elements has already been set int i; for (i=0; i<n; i++) cout << A[i] << " "; cout << endl; return 0; }

  13. Example – Bad Subscripting • C++ does not provide an automatic way for ensuring the proper subscripts are used. • For example, the following code segment does not generate an error message. int B[5]; B[-5] = 1; • Instead of an error message, it modifies another memory location (5 integers before the array B[0]). • If that location is used to store data, the program may result in incorrect result. • If that location is used to store code, the program may result in runtime-error (core dump). • Many other possible errors……

  14. Array Element Manipulation #include <iostream> using namespace std; int main(){ int A[5], i=3, j=2, k=4; A[0]=1; A[1]=5; A[i]=2; A[j]=A[i]+1; A[j+1]=A[i]+A[0]; A[A[j]]=12; // assume the next input value is 3 cin >> A[k]; return 0; } Memory Address Variables in memory Variables Name A[0] 1 2000 5 A[1] 2004 3 2008 A[2] 12 2012 A[3] 3 2016 A[4] Memory

  15. Arrays, Character Strings and Standard Type String Array Initialization

  16. Array Initialization • Array elements are un-initialized unless there is explicit initialization. • Elements in the array can be initialized in the following manner. int A[5] = {5,4,3,2,1}; cout << "A[3]= " << A[3]; A[3] = -1; cout << "A[3]= " << A[3]; Memory Address Variablesin memory Variables Name A[0] 5 2000 4 A[1] 2004 3 2008 A[2] 2 -1 2012 A[3] 1 2016 A[4] Memory

  17. Example Definitions • Suppose const int N = 20; const int M = 40; const int MaxStringSize = 80; const int MaxListSize = 1000; • Then the following are all legal array definitions. int A[10]; // array of 10 ints char B[MaxStringSize]; // array of 80 chars double C[M*N]; // array of 800 doubles int Values[MaxListSize]; // array of 1000 ints • How about this? Is it valid? int i=5; int A[i]; // INVALID, since size must be an int // constant or a constant expression

  18. Arrays, Character Strings and Standard Type String Constant Arrays

  19. Constant Arrays • As in other type definitions, the modifier const can be applied in an array definition. • As usual, after applying the initialization, the element is treated as constant. • For example, const int A[2] = { 10, 1000 }; With the const definition in effect, the following statements are invalid. A[0] = 20; // illegal cin >> A[1]; // illegal

  20. Arrays, Character Strings and Standard Type String Multi-Dimensional Arrays

  21. Two-Dimensional Arrays • In addition to defining one-dimensional arrays, it is also possible to define multi-dimensional arrays of data elements. • The simplest form of the multi-dimensional array is two-dimensional array. • For example, // 2-D array of 30 uninitialized ints int A[3][10]; 0 1 2 3 4 5 6 7 8 9 -- -- -- -- -- -- -- -- -- -- 0 -- -- -- -- -- -- -- -- -- -- A 1 -- -- -- -- -- -- -- -- -- -- 2

  22. Two-Dimensional Arrays • To access element in the array, we can use subscript operator, for example: // 2-D array of 30 uninitialized chars char A[3][10]; A[1][2] = ‘a’; 0 1 2 3 4 5 6 7 8 9 -- -- -- -- -- -- -- -- -- -- 0 -- -- ‘a’ -- -- -- -- -- -- -- A 1 -- -- -- -- -- -- -- -- -- -- 2

  23. Example #include <iostream> using namespace std; int main(){ int num[3][4]; for(int i=0; i<3; i++){ for(int j=0; j<4; j++){ num[i][j] = (i*4)+j+1; // now print them out cout << num[i][j] << “ ”; } cout << endl; } return 0; } num[i][j] 0 1 2 3 1 2 3 4 0 5 6 7 8 1 9 10 11 12 2

  24. Two-Dimensional Array Initialization • Two-dimensional arrays can be initialized the same as single-dimension ones. • For example, the following initializes sqrs with the numbers 1 through 10 and their squares. int sqrs[10][2] = { 1,1, 2,4, 3,9, 4,16, 5,25, 6,36, 7,49, 8,64, 9,81, 10,100 }; • Alternatively, you can write the preceding declaration as follows. int sqrs[10][2] = { {1,1}, {2,4}, {3,9}, {4,16}, {5,25}, {6,36}, {7,49}, {8,64}, {9,81}, {10,100} };

  25. Multi-Dimensional Arrays • C++ allows array of more than two dimensions. • The exact limit is determined by the compiler. • The general form of a multi-dimensional array declaration is as follows. SYNTAX: <type> <arrayname>[<size1>][<size2>][<size3>]…[<sizeN>]; • Example: // un-initialized 3 dimensional array int A[3][4][5];

  26. Multi-Dimensional Arrays • Arrays of more than three dimensions are not often used. • Most cases a maximum of 3 dimensions will suffice for programming. • Actually, single dimension C++ arrays are the commonest. • In multi-dimensional arrays, it takes the computer time to compute each index. This means accessing an element in a multi-dimensional array can be slower than accessing an element in a single-dimensional array.

  27. Arrays, Character Strings and Standard Type String Unsized Array Initialization

  28. Unsized Array Initialization • If, in an array initialization statement, the size of the array is not specified, the C++ compiler automatically creates an array that is big enough to hold all the initializes present. • For example: int A[] = { 1, 2, 3, 4, 5, 6, 7, 8 }; Then, compiler will create an array of size 8 to hold all the initial values. 0 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 A

  29. Unsized Array Initialization sqrs • Unsized array initializations are not restricted to one-dimensional arrays. For multi-dimensional arrays, you must specify allbut the leftmost dimension. • For example: int sqrs[][2] = { {1,1}, {2,4}, {3,9}, {4,16}, {5,25}, {6,36}, {7,49}, {8,64}, {9,81}, {10,100} }; Then, compiler will create an two-dimensional array of size 10x2 to hold all the initial values. 0 1 1 1 0 2 4 1 3 9 2 4 16 3 5 25 4 6 36 5 7 49 6 8 64 7 9 81 8 10 100 9

  30. Arrays, Character Strings and Standard Type String Representing Strings as Character Strings

  31. Representing Strings as Character Strings • C++ provides another initialization style for char arrays to support the representation of character strings. • For example, char letters[] = “Computer Sci.”; The first 13 elements of the array – letters[0] through letters[12] – get their initialization character from the corresponding position in the initialization string, i.e., • letters[0] gets the first character, i.e. ‘C’ • letters[1] gets the second character, i.e. ‘o’ … and so on.

  32. Representing Strings as Character Strings • The last array element letters[13] is initialized to the null character ‘\0’. • Actually the way that we initialize the array letters above is the same as writing, char letters[] = {‘C’,‘o’,‘m’,‘p’,‘u’,‘t’,‘e’, ‘r’,‘ ’,‘S’,‘c’,‘i’,‘.’,’\0’}; • The size of the array is 14, instead of 13, since C++ representation for character strings includes a null character at the end of the string. • Null character (‘\0’) is used to mark the end of a character string. • Its ASCII code is 0

  33. String VS. Character • A character is simply stored in a 8-byte memory location • A string is stored as a continuous character array, terminated by the null character • Example: “a” and ‘a’ are different • “a” is represented as an array of size 2 • ‘a’ is simply stored in one memory location • Therefore, you cannot directly compare them • (“a” == ‘a’) // ERROR a \0 a

  34. Character String – Escape Sequence • In a program text, we can only type a “printable character” (A-Z, a-z, 0-9, punctuations, etc.) • A character string needs some special characters. • Newline (to instruct the program print the next character on a new line). • Null character, etc. • These characters cannot be typed as usual in C++. • Newline is represented as ‘\n’ • Tab can be represented as ‘\t’ • Null character is represented as ‘\0’ • Double quote is represented as ‘\”’ • Forward slash is represented as ‘\\’

  35. Character String – Escape Sequence • Note that each escape sequence is represented (typed) as two characters in the program text. • But they are stored as ONE character (can be stored in a single char variable).

  36. How to determine the length of a character string? int main( ){ char message[ ] = “Hello World”; int len = 0; for (int i=0; message[i]; i++, len++) ; cout << “length of \”” << message << “\”: “ << len << endl; return 0; } • length of “Hello World”: 11 • Comma is used to separate several expressions. • The return value is the last (the rightmost) expression.

  37. String Literal • It is not legal to have a line break within a string literal in C++: // this is not legal “Computer Science and Engineering, The Hong Kong University of Science and Technology.”; • However, the following is OK: “Computer Science and Engineering,” “The Hong Kong University of Science and Technology.”; • And, of course, it is legal for a string literal to contain a newline character: “Please enter an integer:\n”;

  38. Arrays, Character Strings and Standard Type String Functions to Manipulate C-Strings

  39. Functions to Manipulate C-Strings • C++ supports a wide range of functions that manipulate character strings. The most common are Note: These functions use the standard header file cstring. Make sure the destination array contains sufficient space to store the result.

  40. Arrays, Character Strings and Standard Type String C++ String Type

  41. C++ String Type • The standard C++ library provides an object string type to complement the character string used earlier: string Name1; // remember #include <string> string Name2; // and using namespace std; • A string variable may be assigned the value of a string literal or another string variable: Name1 = “Tommy”; // “Tommy” is called a string literal Name2 = Name1;

  42. String Initialization • A string variable may be initialized at the point of declaration: string word1 = “Hello”; string word2 = word1; • It is, however, not legal to assign a char or int value to a string in the declaration: string string1 = ‘N’; // illegal string string2 = 88;

  43. Strings are C++ Classes • Like the input and output streams, cin and cout, string variables in C++ are actually object of the standard string class. (More about this will be covered later) • As string is a class in C++, every string has a number of associated functions that we can use to perform operations on the string. • length(), empty(), compare(), at(), insert(), substr(), find() • Also, it supports a number of operators • +, ==, !=, >, <, >=, <=, []

  44. Function: Length of a String • The length of a string is the number of characters it contains, including whitespace characters, if any. • By calling the function length(), we can obtain the length of the string. string str = “Adam Ng”; int sLength = str.length(); // length = 7

  45. Function: Testing if a String is Empty • The boolean function empty() returns true if the string variable currently holds no characters and false otherwise. • For example, one might use this function to determine whether a read attempt actually placed any characters into the target string: string str = “”; cin >> str; if(str.empty()) cout << “Read failed” << endl; • Of course, the test is only useful if you make certain that str is empty before attempting to read something onto it.

  46. Function: String Concatenation • Two strings may be concatenated; that is, one may be appended to another: string str1 = “Hello”; string str2 = “world”; string str = str1 + “, “ + str2 + “!”; • Here, the concatenation operator (+) is used to combine several strings, variable and literal and the result is assigned to str. • The effect of the statement above is the same as: string str = “Hello, world!”; • You may use the concatenation operator to combine string variables, string literals and characters: str = str + ‘\n’; • However, you cannot use concatenation operator to combine two string literals.

  47. Function: Comparing Strings For Equality • Two strings may be compared for equality using the usual equals relational operator (==). So we can write the following: string str1 = “Hello”; string str2 = str1 + str1; string str3 = “HelloHello”; if(str2 == str3) cout << str2 << “ equals ” << str3 << endl; else cout << str2 << “ doesn’t equal ” << str3 << endl;

  48. Function: Comparing Strings For Equality • You can also use the not-equals operator (!=) with string variables: string str2 = “”; while(str2 != str3) str2 = str2 + str1; • The other relational operators (<, <=, >, >=) can also be used with C++ string variables. • According to lexicographical order (or dictionary order).

  49. Function: Lexicographic Comparison • Two strings can also be compared by using the function int compare(); • The statement str1.compare(str2) returns: • A negative value, if str1 < str2. • Zero, if str1 == str2; • A positive value, otherwise. • The same return value as strcmp(str1,str2)

  50. Example: Lexicographic Comparison • Given the strings: string TA1 = “Pakming”; string TA2 = “Pakming Cheung”; string TA3 = “adam”; string TA4 = “Andrew”; the compare function would behave as follows: int c1 = TA1.compare(TA2); // Ans: c1 < 0 int c2 = TA1.compare(TA3); // Ans: c2 < 0 int c3 = TA2.compare(TA4); // Ans: c3 > 0 int c4 = TA1.compare(TA1); // Ans: c4 == 0

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