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1 Strings Cheng-Chin Chiang

2 Strings Strings 一串儲存在連續記憶體之字元串表示法 : 以雙引號圍起 This is a book, I love programming, 字串須有一結束字元 \0 (NULL) 在字串尾,NULL 在 C++ 內為一個內定常數值 H i t h e r e! \0

3 An Array Type of Strings Character Array ( 字元陣列 ) 沿用 C 語言的陣列型態, 陣列元素為字元, 因此常被稱為 C-String 例如 : char str[10]; // 一個可以放 10 個字元的陣列 char message[12]; // 一個可以放 12 個字元的陣列 char message[12]= Hi there! // 宣告和初始化一次完成 char mystring[ ]= abc // 利用初始化來決定陣列大小 (=4) char mystring[ ]={ a, b, c } // 和上例不一樣, 那裏不一樣? 宣告技巧 : H i t h e r e! \0 Char ArrayName[Maximum_C-String_Size+1];

4 An Array Type of Strings Character Array ( 字元陣列 ) 沿用 C 語言的陣列型態, 陣列元素為字元, 因此常被稱為 C-string 例如 : char str[10]; // 一個可以放 10 個字元的陣列 char message[12]; // 一個可以放 12 個字元的陣列 char message[12]= Hi there! // 宣告和初始化一次完成 H i t h e r e! \0 char mystring[ ]= abc // 利用初始化來決定陣列大小 (3 or 4?) char mystring[ ]={ a, b, c } // 和上例不一樣, 那裏不一樣? 宣告技巧 :

5 Pointers to Character Array 指標 (pointer) 經過動態記憶體配置 (dynamic memory allocation) 後, 也可儲存陣列資料, 所以也可以用來儲存一個字元陣列例如 : char *s; // 需配置記憶體, 大小 = 字串最長長度 +1 s = new char[5]; char *str= ABCDE ; delete [] str; // 宣告配置初始化三工作一體成型

6 String Array 字元二維陣列 : char s[4][6]={ Black, White, Red, Green }; 佔用 24 Bytes B l a c k \0 1 W h i t e \0 2 R e d \0 3 G r e e n \0

7 String Array 指標陣列 : char *s[4]={ Black, White, Red, Green }; 佔用 22 Bytes char *s[ ]={ Black, White, Red, Green }; B l a c k \0 1 W h i t e \0 2 R e d \0 3 G r e e n \0

8 = and == with C-strings C-strings 特殊之處無法直接指定 (assign) 或比較 (compare) char astring[10]; astring = "Hello"; // ILLEGAL! ( = 只能用在宣告的初始化!) 若要做指定, 必須呼叫字串函數內建函數 ( 定義在標頭檔 <cstring>) cpy 意為拷貝 (copy) 拷貝時電腦不負責檢查陣列大小是否足夠! 程式設計師的責任若要做比較, 必須呼叫字串函數 char astring[10] = "Hello"; char anotherstring[10] = "Goodbye"; astring == anotherstring; // NOT allowed! 回傳值 >0 cstringa>cstringb =0 cstringa==cstringb <0 cstringa<cstringb

9 C-string Functions: strlen() 取得字串長度 ( 不包含 NULL) char mystring[10] = "dobedo"; cout << strlen(mystring); 傳回 6

10 C-string Functions: strcat() 字串串接 char stringvar[20] = "The rain"; strcat(stringvar, "in Spain"); 結果 : stringvar 變成 The rainin Spain ( 記得要為 stringvar 備好足夠的記憶體空間 )

11 Other Predefined Functions in <cstring>

12 Other Predefined Functions in <cstring>

13 例題 :Using strcpy and strncpy 11 int main() 12 { 13 char x[] = "Happy Birthday to You"; // string length char y[ 25 ]; 15 char z[ 15 ]; strcpy( y, x ); // copy contents of x into y cout << "The string in array x is: " << x 20 << "\nthe string in array y is: " << y << '\n'; // copy first 14 characters of x into z 23 strncpy( z, x, 14 ); // does not copy null character 24 z[ 14 ] = '\0'; // append '\0' to z's contents cout << "The string in array z is: " << z << endl; 27 return 0; // indicates successful termination 28 } // end main The string in array x is: Happy Birthday to You The string in array y is: Happy Birthday to You The string in array z is: Happy Birthday

14 例題 :Concatenating Strings with strcat and strncat 11 int main() 12 { 13 char s1[ 20 ] = "Happy "; // length 6 14 char s2[] = "New Year "; // length 9 15 char s3[ 40 ] = ""; cout << "s1 = " << s1 << "\ns2 = " << s2; 19 strcat( s1, s2 ); // concatenate s2 to s1 (length 15) 21 cout << "\n\nafter strcat(s1, s2):\ns1 = " << s1 << "\ns2 = " << s2; 23 // concatenate first 6 characters of s1 to s3 24 strncat( s3, s1, 6 ); // places '\0' after last character cout << "\n\nafter strncat(s3, s1, 6):\ns1 = " << s1 27 << "\ns3 = " << s3; strcat( s3, s1 ); // concatenate s1 to s3 30 cout << "\n\nafter strcat(s3, s1):\ns1 = " << s1 31 << "\ns3 = " << s3 << endl; 32 return 0; // indicates successful termination 33 } // end main

15 例題 :Concatenating Strings with strcat and strncat s1 = Happy s2 = New Year After strcat(s1, s2): s1 = Happy New Year s2 = New Year After strncat(s3, s1, 6): s1 = Happy New Year s3 = Happy After strcat(s3, s1): s1 = Happy New Year s3 = Happy Happy New Year

16 例題 :Comparing Strings with strcmp and strncmp 14 int main() 15 { 16 char *s1 = "Happy New Year"; 17 char *s2 = "Happy New Year"; 18 char *s3 = "Happy Holidays"; cout << "s1 = " << s1 << "\ns2 = " << s2 << "\ns3 = " << s3 21 << "\n\nstrcmp(s1, s2) = " << setw( 2 ) << strcmp( s1, s2 ) 22 << "\nstrcmp(s1, s3) = " << setw( 2 ) << strcmp( s1, s3 ) 23 << "\nstrcmp(s3, s1) = " << setw( 2 ) << strcmp( s3, s1 ); cout << "\n\nstrncmp(s1, s3, 6) = " << setw( 2 ) 26 << strncmp( s1, s3, 6 ) << "\nstrncmp(s1, s3, 7) = " << setw( 2 ) 27 << strncmp( s1, s3, 7 ) << "\nstrncmp(s3, s1, 7) = " << setw( 2 ) 28 << strncmp( s3, s1, 7 ) << endl; 29 return 0; // indicates successful termination 30 } // end main

17 例題 :Comparing Strings with strcmp and strncmp s1 = Happy New Year s2 = Happy New Year s3 = Happy Holidays strcmp(s1, s2) = 0 strcmp(s1, s3) = 1 strcmp(s3, s1) = -1 strncmp(s1, s3, 6) = 0 strncmp(s1, s3, 7) = 1 strncmp(s3, s1, 7) = -1

18 例題 :Tokenizing a String with strtok 10 int main() 11 { 12 char sentence[] = "This is a sentence with 7 tokens"; 13 char *tokenptr; cout << "The string to be tokenized is:\n" << sentence 16 << "\n\nthe tokens are:\n\n"; // begin tokenization of sentence 19 tokenptr = strtok( sentence, " " ); // continue tokenizing sentence until tokenptr becomes NULL 22 while ( tokenptr!= NULL ) 23 { 24 cout << tokenptr << '\n'; 25 tokenptr = strtok( NULL, " " ); // get next token 26 } // end while 28 cout << "\nafter strtok, sentence = " << sentence << endl; 29 return 0; // indicates successful termination 30 } // end main

19 例題 : Tokenizing a String with strtok The string to be tokenized is: This is a sentence with 7 tokens The tokens are: This is a sentence with 7 tokens After strtok, sentence = This

20 C-string Arguments and Parameters 因為 c-string parameter 是陣列 C-strings 事宜 call by reference 的方式傳遞其值可被呼叫函數的內部指令改變如果想避免被修改, 可以 const 宣告函數引數一般陣列需要把元素個數當作參數傳遞,C-strings 可有彈性傳遞元素個素不傳遞, 找出結束字元即可算出元素個數

21 Input/Output of Character Arrays 除了可用陣列方式搭配迴圈逐一元素輸出入 ( 勿忘加上結束字元 ) 外, 也可使用簡易輸出入例如 : char st[5]; for (n=0;n<5;n++) { cout << st[n]; cin>>st[n]; } char str[10]= My god! ; char message[12]; cout << str; cin >> message; 使用字元陣列儲存字串時切勿覆蓋掉結束字元

22 String/Numeric Conversion Functions 須 #include <cstdlib> FUNCTION PARAMETER ACTION atoi C-string converts C-string to an int value, returns the value atol C-string converts C-string to a long value, returns the value atof C-string converts C-string to a double value, returns the value itoa int,c-string, int converts 1 st int parameter to a C-string, stores it in 2 nd parameter. 3 rd parameter is base of converted value

23 String/Numeric Conversion Functions int inum; long lnum; double dnum; char intchar[10]; inum = atoi("1234"); // puts 1234 in inum lnum = atol("5678"); // puts 5678 in lnum dnum = atof("35.7"); // puts 35.7 in dnum itoa(inum, intchar, 8); // puts the string // "2322" (base 8 for ) in intchar

24 String/Numeric Conversion Functions - Notes 如果 C-string 中含有非數字字元, 回傳結果就變成未定義可能狀況一 : 以非數字字元前的數字字元作為回傳結果可能狀況二 : 回傳 0 itoa 函數不做陣列長度檢查, 請自備足夠的長度空間來儲存回傳的字串

25 Output of C-strings cout 之直接輸出 : 字串之輸出 cout << message; cout.put() 之輸出 : 單一字元之輸出 cout.put(message[5]); cout.write() 之輸出 : 功用為輸出字串前 n 個字元, 並不因碰到字串之結束字元 \0 而結束 cout.write(message, 21);

26 Input of C-strings cin 之直接輸入 cin.getline() 之輸入 : 整行輸入或限定字數輸入 cin.get() 之輸入 : 逐字輸入或限定字數輸入

27 Direct C-string Input 使用 extraction operator >> 注意事項 Tab, space, line breaks 會被跳過輸入讀取多個變數時會以空白和跳行作為分隔字串變數須事先備好足夠的記憶體空間 (C++ 不會警告陣列長度不夠 ) char a[80], b[80]; cout << "Enter input: "; cin >> a >> b; cout << a << b << "END OF OUTPUT\n"; 結果 : Enter input: Do be do to you! DobeEND OF OUTPUT

28 C-string Input with getline() cin.getline() 讀資料時只有在輸入字串後遇到 Enter 鍵 ( \n ) 後才取得資料交給字串變數, 並捨去 \n 後在字串尾加上 \0 cin.getline(str,num_of_char); 如要以 getline 來輸入 20 個字元之字串 ( 第 21 個字元為結束字元 ), 就用 cin.getline(str,21); 此敘述會將整行字串放入 name 內, 但輸入字元需小於 20 如果在 20 各字元內遇到 \n, 則停止讀取 \n 的後續字元

29 C-string Input with get() cin.get() : int get(); // 讀取下一字元, 傳回整數型態 istream & get(char&); 讀取單一字元放入參數內 get() 可用來讀取 space, \n 等特殊字元 char c1, c2, c3, c4; cin.get(c1); cin.get(c2); cin.get(c3); cin.get(c4); cout<<c1<<c2<<c3; 結果 : istream & get(char*, int len, char = \n ); 讀取 (len-1) 個字元, 當中時若碰到輸入鍵 ( \n ) 就結束 ( \n 還留在 input stream 中 ) AB CD AB C

30 例題 : 以 get() 檢查輸入行的結束 cout << Enter a line of input:\n ; char symbol; do { cin.get(symbol); cout <<symbol; } while (symbol!= n ); cout << That s all.\n ;

31 例題 : 以 get() 參數輸入後輸出 #include <iostream> using namespace std; const int SIZE=21; int main( ){ int age; char sport[size]; cout << 輸入年齡 :"; cin<<age; cout <<" 喜好運動名 :"; cin.get(sport,size); cout << \n << age << 歲的你, 喜歡輸入年齡 :18 喜好運動名 : 18 歲的你, 喜歡我也很喜歡 <<sport<<, 我也很喜歡 \n"; return 0; } Why 運動名 sport 無法輸入? 原因乃在讀取年齡後輸入鍵 (Enter)( \n ) 保留在 input stream 內, 因此下一輸入 cin.get(sport,20) 就讀到 \n ( 別忘了 get() 不會跳過 \n 和空格 ), 所以造成 sport 得到一 NULL 字元而已

32 解決 cin.get() 讀取資料時留住 \n 在輸入緩衝區之困擾利用第一種格式無參數之 cin.get(); 在接下來讀取 \n, 因無參數之 get() 之功能乃讀取下一字元, 讓輸入緩衝區空無一物後, 下一輸入才能正常運作, 即 : cin.get(name,size); // 讀取 Bill Gates cin.get(); // 讀取 \n 或將上述兩列合併如下 : cin.get(name,size).get();

33 例題 : 數字輸入與 getline() 輸入 #include <iostream> //cout using namespace std; const int SIZE=21; int main( ){ char *sentence; short n; cout <<" 字串長度 ="; cin >> n; sentence = new char[n-1];// 配置記憶體 cout <<" 輸入字串 = ; cin.getline(sentence,n); cout <<" 輸入字串為 "<<sentence<<endl; delete sentence; // 釋放記憶體 return 0; } 字串長度 =30 輸入字串 = 輸入字串為為何無法輸入字串呢?sentence= \0

34 More Member Functions putback() 退回上一個讀取字元到 input stream 中 ( 下次讀取會讀到此退回字元 ) cin.putback(lastchar); peek() 傳回下個字元, 但仍然讓該字元留在 input stream 中 peekchar = cin.peek(); ignore() 最多跳過指定個數的字元, 直到特定字元出現 cin.ignore(1000, \n ); Skips at most 1000 characters until \n

35 例題 : 輸入數字和文字的判斷 ( 版本一 ) cout << "Enter a number or a word: "; c = cin.get(); if ( (c >= '0') && (c <= '9') ) { cin.putback (c); cin >> n; cout << "You have entered number " << n << endl; } else { cin.putback (c); cin >> str; cout << " You have entered word " << str << endl; }

36 例題 : 輸入數字和文字的判斷 ( 版本二 ) cout << "Enter a number or a word: "; c=cin.peek(); if ( (c >= '0') && (c <= '9') ) { cin >> n; cout << "You have entered number " << n << endl; } else { cin >> str; cout << " You have entered word " << str << endl; }

37 String Manipulation Functions 須先 #include <cctype>

38 String Manipulation Functions 須先 #include <cctype>

39 String Manipulation Functions 須先 #include <cctype>

40 Standard Class string C++ 內建的字串物件類別須 #include <string> 須 using namespace std; 可以視同一般的基本型態 ( 如 int, float, double, char 等 ) 使用可以指定 (assign) 比較 (compare) 串接 (concatenation) : string s1, s2, s3; s3 = s1 + s2; //Concatenation If (s1==s2) s3 = "Hello Mom! ; //Assignment

41 例題 :string 的使用

42 I/O with Class string 與其他基本型態相同 string s1, s2; cin >> s1; cin >> s2; 結果 : User types in: May the hair on your toes grow long and curly s1 receives value "May" s2 receives value "the"

43 getline() with Class string 一個全域性函數, 輸入完畢後回傳一個 cin 物件的 reference 與 cin 的 getline() 使用相同結果 : string line; cout << "Enter a line of input: "; getline(cin, line); cout << line << "END OF OUTPUT"; Enter a line of input: Do be do to you! Do be do to you!end OF INPUT

44 Other getline() Versions 可以指定分隔符號 string line; cout << "Enter input: "; getline(cin, line, "?"); 讀取輸入字元直到? 出現 getline() 函數回傳一個 cin 的 reference string s1, s2; getline(cin, s1) >> s2; 相當于 : (cin) >> s2;

45 Class string Processing 具備 c-strings 所有運算, 且還更多函數 ( 超過 100 個 )

46 Class string Processing

47 C-string and string Object Conversions 自動轉換 c-string 到 string object: char acstring[ ] = My C-string ; string stringvar; stringvar = acstring; // OK 不允許從 string 到 c-string acstring = stringvar; 必須使用 strcopy 進行 copy 運算 strcpy(acstring, stringvar.c_str());

新版明解C++入門編

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