ECE 264 Object-Oriented Software Development

ECE 264Object-Oriented

Software Development

Instructor: Dr. Honggang WangSpring 2013

Lecture 14: strings and pointer

Lecture outline Announcements / reminders

Project groups: e-mail Dr. Wang by Thur., March 28 Each group consists of 4-5 students Those who don’t choose a group will be randomly assigned Can e-mail me with “sub-group”; I’ll fill rest of group

Lab 7 will posted today Today

Review: Arrays & vectors Other container classes Strings Pointer

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Review: Arrays Constant size list of items of same type Can initialize using comma-separated list:

int n[] = {10, 20, 30, 40, 50}; Can access individual elements using []

cout << n[1]; would print 20 Can pass arrays to functions

void printArray(int arr[], int size); Pitfalls

Indexing past array boundaries Array name is a pointer passed by reference

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Review: Vectors Vectors allow programmer to create “arrays”

that: Are dynamically resizable Can be assigned to one another Can be compared for equality Contain easier generic boundary checking

Can access vectors like arrays: v[0] Can also use vector functions

Examples:vector <double> list; //empty vectorvector<string> wordList(n); //capacity:n strings

//vector of 8 integers, each initialized to 0vector<int> intList(8,0);

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Review: Vector methods Common member functions:

bool empty(): true if vector contains no values void pop_back(): deletes last element in vector

Does not actually return the element Gives an error if vector is empty

void push_back(element): add element to end of vector

void resize(int): changes the size of vector size_t size(): returns the size of vector <el> at(<el>): allows you to insert element in

vector, but also provides boundary checking <el>: type of elements stored in the vector (e.g. int, double)

void clear(): removes all elements from vector

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Additional container classes C++ standard template library contains other

useful containers list: doubly-linked list

Allows for efficient element insertion Can also push/pop front of list; sort, merge, reverse lists

queue: FIFO queue Has simple push/pop operations

priority_queue: sorted queue with largest element first

stack: LIFO stack Can access top of stack

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7

Standard Library Class string

Class string Header <string>, namespace std We’ve seen:

Initialization: string s1( "hi" ); Input/output (as in cout << s1) Assignment: s1 = "hi";

Can also use: Relational operators: ==, !=, >=, >, <=, <

Perform char-by-char comparison using ASCII values Concatenation: +=

E.g.: s1 += “lly” s1 = “hilly”

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Standard Library Class string (Cont.)

Class string (Cont.) Substring member function substr

s1.substr( 0, 14 ); Starts at location 0, gets 14 characters

s1.substr( 15 ); Substring beginning at location 15, to the end

Overloaded [] Access one character No range checking (if subscript invalid)

Member function at Accesses one character

Example s1.at( 10 );

Has bounds checking, throws an exception if subscript is invalid

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Example: Strings & functionsint main(){ string s1( "happy" ); string s2( " birthday" ); string s3;

// test overloaded equality and relational operators cout << "s1 is \"" << s1 << "\"; s2 is \"" << s2 << "\"; s3 is \"" << s3 << '\"' << "\n\nThe results of comparing s2 and s1:" << "\ns2 == s1 yields " << ( s2 == s1 ? "true" : "false" ) << "\ns2 != s1 yields " << ( s2 != s1 ? "true" : "false" ) << "\ns2 > s1 yields " << ( s2 > s1 ? "true" : "false" ) << "\ns2 < s1 yields " << ( s2 < s1 ? "true" : "false" ) << "\ns2 >= s1 yields " << ( s2 >= s1 ? "true" : "false" ) << "\ns2 <= s1 yields " << ( s2 <= s1 ? "true" : "false" );

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Example (cont.)

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Output from previous slide:

s1 is “happy”; s2 is “ birthday”; s3 is “”

The results of comparing s1 and s2:

s2 == s1 yields false

s2 != s1 yields true

s2 > s1 yields false

s2 < s1 yields true

s2 >= s1 yields false

s2 <= s1 yields true

Example (cont.) // test string member function empty cout << "\n\nTesting s3.empty():" << endl;

if ( s3.empty() ) { cout << "s3 is empty; assigning s1 to s3;" << endl; s3 = s1; // assign s1 to s3 cout << "s3 is \"" << s3 << "\""; } // end if

// test overloaded string concatenation operator cout << "\n\ns1 += s2 yields s1 = "; s1 += s2; // test overloaded concatenation cout << s1;

// test concatenation operator with C-style string cout << "\n\ns1 += \" to you\" yields" << endl; s1 += " to you"; cout << "s1 = " << s1 << "\n\n";

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Example (cont.)

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Output from previous slide:

Testing s3.empty():

s3 is empty; assigning s1 to s3;

s3 is “happy”

s1 += s2 yields s1 = happy birthday

s1 += “ to you” yields

s1 = happy birthday to you

Example (cont.) // test string member function substr cout << "The substring of s1 starting at location 0 for\n" << "14 characters, s1.substr(0, 14), is:\n" << s1.substr( 0, 14 ) << "\n\n";

// test substr "to-end-of-string" option cout << "The substring of s1 starting at\n" << "location 15, s1.substr(15), is:\n" << s1.substr( 15 ) << endl;

// test using subscript operator to create lvalue s1[ 0 ] = 'H'; s1[ 6 ] = 'B'; cout << "\ns1 after s1[0] = 'H' and s1[6] = 'B' is: " << s1 << "\n\n";

// test subscript out of range with string member function "at" cout << "Attempt to assign 'd' to s1.at( 30 ) yields:" << endl; s1.at( 30 ) = 'd'; // ERROR: subscript out of range return 0;} // end main

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Example (cont.)

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Output from previous slide:

The substring of s1 starting at location 0 for

14 characters, s1.substr(0, 14), is:

happy birthday

The substring of s1 starting at

location 15, s1.substr(15), is:

to you

s1 after s1[0] = ‘H’ and s1[6] = ‘B’ is: Happy Birthday to you

Attempt to assign ‘d’ to s1.at(30) yields

abnormal program completion

Dynamic memory allocation Up until now, allocated memory statically

Assumed we knew data size at compile time What if data size is input-dependent and

unknown until run time? In C, dynamic memory allocation handled through malloc and free

In C++, we use new and delete

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Refresher on pointers Allocators (malloc, new) return pointer to

allocated space Pointer: address of another object

We implicitly use these when we pass function arguments by reference in C++

Can get address of existing object using & Can get value of existing pointer using * Pointer declaration:

<base type>* <pointer name> Base type determines how reference is interpreted Be careful when declaring multiple pointers Be sure to initialize pointer before use

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Pointer exampleint *iPtr, i=6;

char* s, str[] = "example";

double *dPtr, d=1.25;

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iPtr

s

dPtr

6

"example"

1.25

i

str

d

Pointer assignment The assignment operator (=) is defined for

pointers of the same base type. The right operand of the assignment operator

can be any expression that evaluates to the same type as the left operand.

Example:int x, *xp, *ip;xp = &x;ip = xp;

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x

xp

ip

Arrays and pointers Array name is a pointer to first array element

Can use pointers and arrays interchangeably You can use [] to “index” a pointer Example:char myString[] = "This is a string"; char *str;str = myString;for(int i =0; str[i]; i++) //look for null

cout << str[i];

What does this print?

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Initialize the Pointer Both define the pointer student and initialize

student to the address of the first element in section:

int section[80];

int *student = section;

Is equivalent toint section[80];

int *student = &section[0];

The pointer string is initialized to point to the character a in the string "abcd".

char *string = "abcd";

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Pointer arithmetic When using pointers/arrays interchangeably, can

make use of pointer arithmetic Can’t change where array name points, but you can

change pointer If p is a pointer, p++ means “point to next element”

“Next element” determined by base type Can compare pointers

p == NULL pointer points nowhere p == q p and q point to same location

Example

int num[4] = {1,2,3,4}, *p;p = num; //same as p = &num[0];cout << *p <<endl;++p;cout << *p;

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Practice!

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int q=6;int *iPtr = &q;cout << "iPtr is " << iPtr << endl;cout << "*iPtr is " << *iPtr << endl;cout << "++*iPtr, is " << ++*iPtr << endl;cout << "q is " << q << endl;cout << "iPtr is " << iPtr << endl;cout << "*iPtr++ is " << *iPtr++ << endl;cout << "iPtr is " << iPtr << endl;cout << "q is " << q << endl;

Complete the output:iPtr is 0x7fff2f14

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Result of Practice

iPtr is 0x7fff2f14*iPtr is 6++*iPtr is 7q is 7iPtr is 0x7fff2f14*iPtr++ is 7iPtr is 0x7fff2f18q is 7

Practice #2char myString[ ] = "This is a string";char *strPtr;strPtr = myString;cout << *myString << endl; cout<<myString << endl; cout << *(myString + 1) << endl;strPtr++;cout << *++strPtr << endl;myString++; //not legal

What does this print?

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0xfff4c252strPtr

0xfff4c252

myString T h i s i s a s t r i n g \0

Practice #2 result

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T This is a string h i

Common Pointer Problems Using uninitialized pointersint *iPtr;

*iPtr = 100; iPtr has not been initialized. The value 100 will be

assigned to some memory location. Which one determines the error.

Incorrect/unintended syntax.

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Example#include <iostream>int main(){ char *aString = "What happens here?"; int len=0; while(*aString++ != '\0')

len++;std::cout << len << ": " << aString;return 0;

}

Does this compile? If not, why? If it does, what is the output? Explain

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Final notes Next time

Start dynamic allocation Midterm Survey

Acknowledgements: this lecture borrows heavily from lecture slides provided with the following texts: Deitel & Deitel, C++ How to Program, 8th ed. Etter & Ingber, Engineering Problem Solving with

C++, 2nd ed.

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