7. String

Today’s Outline

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• Recap: char
• C string basics
• Reading and printing C strings
• Common string functions
• Safety of string functions

Recap: char

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cstring vs std::string

• cstring - inherited from the C lang
  • #include <cstring>
• string - class defined in std lib
  • #include <string>
  • Class and object

C String Basics

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C String: '\\0'

• A C string is a char array terminated by '\\0'
• Used to distinguish a C String from an ordinary char array
• '\\0' == null

char str[20] = "Hello World"; // '\\0' will be added at the last index

C String: Declaration and Initialization

// With initialization
char studentID[] = "51234567";
char HKID[] = "a123456(7)";
 
// Without initialization
char studentID[8+1];
char HKID[10+1];
 
// Cannot initialize after the declaration
char name[10];
name = "john"; // error

C String: Storage

char s1[] = "Hello World"; // s1 = 20
char s2[] = "cs2311";      // s2 = 32

Passing String to Functions

int count(char s[100], char c) {
    int frequency=0;
    int i=0;
    while (s[i] != '\\0') {
        if (s[i] == c) {
            frequency++;
        }
        i++;
    }
    return frequency;
}
 
int main() {
    char T[100] = “hello world”;
    char t = ‘a’;
    cout << count(T, t) << endl;
    return 0;
}

Reading and Printing C strings

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int main() {
    char word[5];
    // we can use 'cin' to initialize the string after declaration
    cin >> word;
    cout << word;
    return 0;
}

Printing C Strings

int main() {
    char s1[] = "abc";
    char s2[] = "def";
    s1[3] = '+';
    cout << s1 << endl << s2 << endl;
    return 0;
    // abc+def
    // def
}

Reading C Strings

char s1[20], s2[20];
// 'cin' will terminate when whitespace is encountered
// whitespace = space, teb, newline, ...
cin >> s1;  // input = "hello world" / s1 = "hello\\0"
cin >> s2;  // buffer = "world"      / s2 = "world\\0"
cout << s1; // will print "hello"
cout << s2; // will print "world"

Reading a Line: get() Loop

char s[20];
int i = 0;
char c;
do {
    cin.get(c); // input = "hello world" / c = 'h'
    str[i++] = c;
} while (c != '\\n' && i < 20);

Reading a Line: getline()

char s[20];
cin.getline(s, 20);

int main() {
    char s[5];
    int i = 0;
    while (true) {
        cin.getline(s, 5); // will accept 4 characters + eofbit flag set
        cin.clear();       // reset flag
        cout << i++ << ": " << s << endl;
    }
    return 0;
    // 0: 1234
    // 1: 5
}

Common cstring Functions

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strlen()

char s[20] = "Hello World!";
int len = strlen(s);
int siz = sizeof(s);
cout << len << endl; // 12
cout << siz << endl; // 20

• strlen(str) - returns the number of chars
  • '\\0' does NOT count

strcpy()

char s1[6];
strcpy(s1, "hello");
char s2[6];
strcpy(s2, s1);
s2[0] = 'c';
cout << s1 << endl; // hello
cout << s2 << endl; // cello

• strcpy(dst, src) - copies src into dst

strcat()

char s[13];
strcpy(s, "hello ");
strcat(s, "world!");
cout << s; // "hello world!

• strcat(dst, src) - concatenates src into dst

strcmp()

cout << strcmp("abc",  "abc") << endl; // 0
cout << strcmp("abc", "abcd") << endl; // -1
cout << strcmp("abcd", "abc") << endl; // 1
cout << strcmp("abc",  "abd") << endl; // -1

• strcmp(s1, s2) - compare s1 and s2 until encountering different characters
  • < 0 if s1 < s2 in alphabet
  • > 0 if s1 > s2 in alphabet
  • 0 if s1 == s2

Other String Functions

• strncpy(dst, src, n)
• strncat(dst, src, n)
• strncmp(str1, str2, n)
• strchr(str, ch) / strrchr(str, ch)
• strstr(haystack, needle)
• strspn(str, accept)
• strcspn(str, reject)

Safety of String Functions

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char s1[14];
strcpy(s1, "hello, world!");
char s2[6];
strcpy(s2, s1); // s2 = "hello," / other program memory = " world!\\0"

Additional Notes

• strcpy and strcat are considered unsafe, as they don’t check memory boundary
• Use strcpy_s and strcat_s instead