Previous Lecture lect15 Next Lecture

lect15, Tue 03/05

Strings, Using Linked Lists

HW #5 due Tuesday 3/12!

In class, we mainly talked about linked lists and walked through an example of creating a struct for a playing Card and player’s Hand.

The notes below about string functions are meant to help with Lab 08, and are easier to understand on your own.

Dot operator review

If you do not include the parentheses, you’ll get the following compiler error:

box.cpp:17:38: error: member reference type 'Box *' is a
      pointer; did you mean to use '->'?
    cout << "Box width = " << *boxptr.w;
                               ~~~~~~^
                                     ->
box.cpp:17:31: error: indirection requires pointer operand
      ('int' invalid)
    cout << "Box width = " << *boxptr.w;
                              ^~~~~~~~~

The complete example for the above structs:

#include <iostream>
using namespace std;

struct Box{
            int w;
            int h;
        };
int main() {

    Box b1;
    b1.w = 42;
    b1.h = 16;
    cout << "Box width = " << b1.w;
    cout << " height = " << b1.h << endl;

    Box* boxptr = &b1;
    cout << "Box width = " << (*boxptr).w;
    cout << " height = " << (*boxptr).h << endl;

    return 0;
}

Compilation and output:

$ g++ box.cpp -o box
$ ./box
Box width = 42 height = 16
Box width = 42 height = 16

Linked List Example: Cards

Strings

Examples of declaring C-strings

// Declare a char array capable of holding 10 elements
char s1[10];

// Declare and initialize a C-string "CS16"
char s2[10] = {'C', 'S', '1', '6', '\0'};

// Shortcut to initialize a C-string. Automatically inserts
// the null character.
char s3[10] = "CS16";

// Initialize a C-string using a pointer referencing the
// start of the char array (null character is inserted).
char* s4 = "CS16";

// Null string (just contains the null character)
char* s5 = "";

cout << s2 << endl;	// CS16
cout << s3 << endl;	// CS16
cout << s4 << endl;	// CS16 (not a memory address!)
cout << *s4 << endl;	// C (dereferences the first element

Example of indexing C-strings

cout << s2[1] << endl;		// S
cout << s2 + 1 << endl;		// S16 (reads until null char)
cout << *(s2 + 1) << endl;	// S (same as [1])
cout << *s2 + 1 << endl;	// 68 ('C' == 67, + 1 == 68)

Size of a C-string (strlen)

char s6[] = {'a', 'b', '\0'};

cout << strlen(s2) << endl;	// 4
cout << strlen(s6) << endl;	// 2

Example of not including the ‘\0’ character

char s7[] = {'a', 'b'};
cout << strlen(s7) << endl;	// 7
cout << s7 << endl;		// ab??... Undefined behavior

Example of writing our own C-string length function

int length(char* s) {
	char temp = *s;
	int counter = 0;
	while (temp != '\0') {
		cout << temp << endl;
		counter++;
		temp = *(s + counter);
	}
	return counter;
}

int main() {
	cout << length(s3) << endl; // 4
	cout << length(s7) << endl; // undefined, no null character
}

C-String comparisons (strcmp)

char name1[] = {'n','a','m','e','\0'};
char name2[] = {'n','a','m','e','\0'};
char name3[] = {'N','A','M','E','\0'};

if (name1 == name2) {
	cout << "It's true!" << endl;
} else {
	cout << "It's false!" << endl; 	// prints this - comparing addresses
}

-----

if (strcmp(name1, name2) == 0) {
	cout << "It's true!" << endl; 	// prints this – checks for equality
} else {
	cout << "It's false!" << endl;
}

-----

if (strcmp(name2, name3) == 0) {
	cout << "It's true!" << endl;
} else {
	cout << "It's false!" << endl;	// prints this – compares ASCII values
}

In general, if strcmp(s1, s2) is ...
	* == 0, then s1 == s2 
	* >= 0, then s1 >= s2
	* <= 0, then s1 <= s2

C-string references

char s1[] = {'a','\0'};
char s2[] = {'b','\0'};

s1 = s2; // ERROR!

s1[0] = 'Z';
cout << s1 << endl; 	// Z

strcpy(s1, "Z");
cout << s1 << endl;	// Z. s1 is replaced with 'Z','\0'
			// anything after '\0' doesn't matter.

char s1[10] = {'a','\0'};
strcpy(s1, "this is a long string");
cout << s1 << endl;	// undefined behavior, could overflow contents of memory

C-string concatenation example

char s1[20] = "abc";
char s2[] = "123";

strcat(s1, ", it's as easy as ");
strcat(s1, s2);

cout << s1 << endl; 	// "abc, it's as easy as 123"
cout << s2 << endl;	// "123"

C++ Strings

Example illustrating (the same) string size

string s1 = "";
cout << sizeof(s1) << endl;
string s2 = "abcd";
cout << sizeof(s2) << endl;
string s3 = "abcdabcdabcdabcdabcdabcdabcdabcdabcdabcd";
cout << sizeof(s3) << endl;

Concatenation example

string name = "Chris";
name += " Gaucho"; // + concatenates characters to a string
string name2 = "Chris Gaucho";

if (name == name2) {// Checks for equivalent characters
	cout << "The names are equal." << endl; // prints this
}
else {
	cout << "The names are not equal." << endl;
}

Examples of using C++ strings

string x = "Hello there";
cout << x.length() << endl; // or .size()
cout << x[1] << endl;
cout << x.at(1) << endl;

string y = "";
if (y.empty())
	cout << "The string " << y << " is empty" << endl; // prints this
else
	cout << "The string " << y << " is not empty" << endl;

y += "Hi";
if (y.empty())
	cout << "The string " << y << " is empty" << endl;
else
	cout << "The string " << y << " is not empty" << endl; // prints this

cout << x.substr(1,3) << endl; // returns a string starting at index 1 of the
                               // character array and returns 3 characters (as
                               // defined in the substr parameters).Returns "ell"
cout << x.substr(1,100) << endl; // “ello there” (OK to read more characters than the
                                 // string contents. Will return the entire string.)
cout << x.substr(100,5) << endl; // ERROR

// isalpha
// returns true if the character is in the alphabet, false otherwise.

cout << isalpha('A') << endl; // 1
cout << isalpha('j') << endl; // 1
cout << isalpha(' ') << endl; // 0
cout << isalpha('&') << endl; // 0
cout << isalpha('-') << endl; // 0

// erase
string s = "CS16";
s.erase(1,2); // removes 2 characters starting at index 1
cout << s << endl; // C6
s = "CS16";
s.erase(4,1); // removes 1 characters starting at index 4
cout << s << endl; // CS16
s = "CS16";
s.erase(0,3); // removes 3 characters starting at index 0
cout << s << endl; // 6

// tolower
// returns the lower-case ASCII representation of a character
cout << tolower('A') << endl; // 97 == 'a'
cout << tolower('c') << endl; // 99 == 'c'
cout << tolower('%') << endl; // 37 == '%'

// toupper
// returns the upper-case ASCII representation of a character

// find
// returns the index of the 1st occurrence of a character in the string.
// returns string::npos if it doesn't exist.
// Note: string::npos == -1 when converted to an int type

s = "CS1616";
cout << s.find('c') << endl; // 18446744073709551615 (unsigned int type)
cout << (int) s.find('c') << endl; // -1
cout << s.find('S') << endl; // 1
cout << s.find('6') << endl; // 3 (1st occurrence)