CH9 CStrings

備註

本文為 2021-Fall 學期旁聽台大資管系孔令傑教授開授的 Programming Design 所記錄的課程筆記。課程內容程式碼可以參閱我的 Github repo: C++ Programming-Design-2021-Fall

Characters

char

• Use one byte (–128 to 127) to store English letters, numbers, symbols, and special characters(但不能儲存中文符號)
• It is also an integer! 字元本身儲存的是一個整數，再經由ASCII code encode成我們看到的符號，因此每個char其實都可以轉成int，也可以最整數運算，參考9_1.cpp, 9_2.cpp

The C++ standard library <cctype> contains some useful functions for processing characters.

int islower(int c)
int isupper(int c)
int tolower(int c)
int toupper(int c)
int isalpha(int c)
int isdigit(int c)
int isalnum(int c)
int isprint(int c)
int isspace(int c)
int ispunct(int c)

C strings

String

在C++中Strings有兩種形式:

• C strings as character arrays, A character array can be initialized as a usual array.
• C++ strings as objects. (後面章節會提)

A special way to input a string

在C++中char arrays被用來作為string，因此很多opration都會為了char arrays被overloaded，exp: cin >>, cout <<
對一般array來說沒辦法一次輸入或輸出多個元素但char array可以

char str[10]; 
cin >> str; // if we type "abcde" 
cout << str[0]; // a 
cout << str[2]; // c

int values[5] = {0}; 
cout << values; // an address 
char str[10]; 
cin >> str; // if we type "abcde" 
cout << str; // abcde

The null character

When we use cin >> to input a string, a null character \0 will be appended at the end automatically.

• \0 is an escape sequence. It marks the end of a string.
• When you declare a character array of length n, you can store a string of length at most n – 1.
• A C string may be initialized with a double quotation
  • char s[100] = "abc"

char a[100] = "abcde FGH";
cout << a << "\n"; // abcde FGH 
char b[100] = "abcde\0FGH";  
cout << b << "\n"; // abcd

One may also initialize a C string by assigning multiple characters.

• char s[100] = c;
• No null character will be appended (though uninitialized values will be initialized to 0, which is the null character).
• = is overloaded for “a C string” and “some characters” in different ways.

Comparisons

Will a null character be appended?

char s[10] = "abc";  // Yes 
char s[100] = {'a', 'b', 'c'};  // No
cin >> s;  // Yes 
cin >> s[0];  // No

String assignments

用""來 assign string 只能用在宣告時，宣告後char array variable存的就是address了

char s[100]; 
s = "this is a string"; // compilation error!

How to explain the outputs of this program?

char c[100] = {0};  // (1)
cin >> c;  // (2) "123456789" 
cin >> c; // (3) "abcde"; 
cout << c << "\n"; // "abcde" 
c[5] = '*'; 
cout << c << "\n"; // (4) "abcde*789"

// (1) 0000000……0
// (2) 123456789\0
// (3) abcde\0789\0
// (4) abcde*789\0

Array boundary

C++ does not check array boundary!

char a[5] = {0}; 
cin >> a; // "123456789" 
cout << a; // "123456789" or an error

A strange case

並不是因為空白鍵被當作C strings的結尾，而是因為cin

char a1[100] = {0}; 
cin >> a1; // "this is a string" 
cout << a1; // "this"

char a2[100] = {'a', 'b', ' ', 'c', '\0', 'e'}; 
cout << a2; // ab c

cin >> vs. cin.getline()

cin >> splits the input stream into char pieces according to white spaces. The same thing happens for the newline character and tab.

To input a string with white spaces, use cin.getline().

char a[100] = {0};
char b[100] = {0};
cin >> a >> b; // this is
cout << a << "\n"; // this
cout << b << "\n"; // is

char a[100];
cin.getline(a, 100); // Hi, it's me
cout << a << "\n"; // Hi, it's me

String literals and character pointers

A character pointer may also be initialized as a string literal.

char* p = "12345"; 
cout << p + 2 << "\n"; // 345

• When we do so, the system allocates space storing “12345”.
• That space is read-only.
• p stores the address of that space.

char a[100] = {0}; 
a = "123"; // compilation error

char* p; 
p = "abc"; // okay

char a[100] = "12345";  
char* p = a;  
p = "abc"; // does not affect a  
cout << p << "\n"; // abc  
cout << a << "\n"; // 12345


char a[100] = "12345";
char* p = a;
p = "abc";
cout << p << "\n";
cin >> p; // run-time error 因為character pointer為read-only

Main function arguments

我們可以在main function傳入arguments

#include<iostream> 
using namespace std;
int main(int argc, char* argv[]) 
{
    for(int i = 0; i < argc; i++)
        cout << argv[i] << "\n"; 
    return 0;
}

• argv[0] is the name of the executable file.
• argv[i] is the ith string passed into main.

C string processing functions

The C++ standard library <cstring> contains many useful pointer-based string processing functions.

• Query and searching strlen, strchr, strstr.
• Comparison: strcmp, strncmp.
• Concatenation: strcat, strncat.
• Copying: strcpy, strncpy.
• Splitting: strtok.

The C++ standard library <cstdlib> contains some more.

• String-number conversion: atoi, atof, itoa.
• Look for detailed explanations by yourselves!

String length query

char* p = "12345"; 
cout << strlen(p) << "\n";  // 5
char a[100] = "1234567890";  
cout << strlen(a) << "\n";  // 10
cout << sizeof(a) << "\n";  // 100(陣列空間大小)
cout << sizeof(a + 2) << "\n";  // 8(單獨一個記憶體位置，被解讀成pointer的size)

Searching in a string

To find the location of a character in a string (or conclude that it does not exist in the string), use strchr.

It returns the address of the first occurrence of the character. If the character does not exist, it returns nullptr.

• The returned address may be used to modify the given string.
• The returned address may be used as the starting location of a “new string” to search for the next occurrence of the character.

char* strchr(char* str, int character);

Searching for a substring

If we want to search for a substring, we use strstr.
This returns the address of the first occurrence of str2 in str1.

char* strstr(char* str1, const char* str2);

String-number conversion

In <cstdlib>, two functions converts a character array into a number:

• For atoi, str should contain only digits
• For atof, str may contain at most one ‘.’

int atoi(const char* str);  
double atof(const char* str);

A function converts a number into a character array

char* itoa(int value, char* str, int base);

String comparisons

Strings may also be compared alphabetically strcmp compares the entire strings. strncmp does up to num characters.

int strcmp (const char* str1, const char* str2); 
int strncmp(const char* str1, const char* str2, unsigned int num);

They returns 0 if the two strings are identical, a negative number if str1 is in front of str2, and a positive number if str2 is in front of str1.

String copying

It copies the string at source into the array at dest, including the terminating null character in source. It returns dest.

char* strcpy(char* dest, const char* source);

char a[100] = "watermelon";  //(1)
char b[100] = "orange";  //(2)
cout << a << "\n"; 
strcpy(a, b);  //(3)
cout << a << "\n";

//(1) a watermelon\0
//(2) b orange\0
//(3) a orange\0lon\0

String concatenation

A similar task is to concatenate two strings. This copies source to the end of dest. The \0 of dest is replaced by the first character of source. The \0 of source is also copied. It returns dest.

• The destination must be an array (static or dynamic), not just a pointer.
• A programmer must make sure that there is enough space of the modification.

char* strcat(char* dest, const char* source);

Reference

• IM 1003 Programming Design