CS31: Introduction to Computer Science I Discussion 1A 5/14/2010 Sungwon Yang [email protected] swyang.

CS31: Introduction to Computer Science I

Discussion 1A 5/14/2010

Sungwon [email protected]

www.cs.ucla.edu/~swyang

Quick Review• What did we learn last week?

• C-strings• string type can be used only in c++• C-string is an array of characters• it must end with zero byte/null character : ‘\0’

– indicates the end of the string

• C-string input/output• cout prints out characters until it meets null character• cin.getline reads one line

– need to specify the maximum length of the input string– null character will be automatically inserted

• converting C-strings to C++ strings• simple ‘=‘ operator works• not vice versa

• C-string functions : #include <cstring> • strlen(s)

– returns the length of s, not counting ‘\0’

• strcpy(t, s)– copies the string s to t

• strcat(t, s)– concatenate (append) s to the end of t

• strcmp(t, s)– compare t and s– return 0 if they are equal– return something greater than 0 if t > s– return something less than 0 if t < s

Pointers• where are variables stored?– how can computers know where they are??

int main (){ int a = 10; double d = 5.5; char c = ‘a’; bool b = true; }

10

⁞

⁞

5.5

⁞

a

⁞

true

001FFD08

⁞

⁞

002FDC68

⁞

002FF128

⁞

004DEF00

Pointers

• Variables are stored in memory locations• The memory location of a variable is called the

address of the variable• The address specifies a place, not a value• Pointer variables store the address!

pointers

int main (){ int a = 10; int b = 100;

int* ptr1; int *ptr2;

ptr1 = &a; ptr2 = &b;

int sum = *ptr1 + *ptr2;

cout << ptr1 << " : " << ptr2 << endl; cout << &ptr1 << " : " << &ptr2 << endl; cout << a << " : " << b << endl; cout << *ptr1 << " : " << *ptr2 << endl; cout << sum << endl;}

001AF73C : 001AF738001AF744 : 001AF74010 : 10010 : 100110

pointers

• multiple pointers can point to the same variable

int main (){ int* ptr1; int *ptr2;

int a = 10;

ptr1 = &a; ptr2 = ptr1;

cout << *ptr1 << endl; cout << *ptr2 << endl;}

1010

quick questions

• what will be displayed?int main (){ int a = 10; int *ptr = &a; int *p = ptr;

cout << a << endl; cout << *&a << endl; cout << *ptr << endl; cout << *p << endl;}

10101010

int main (){ int* ptr1; int *ptr2;

int a = 10; int b = 100;

ptr1 = &a; ptr2 = &b;

a = *ptr2+2*a; b = *&b*b;

cout << b+*ptr1 << endl;}

10120

pointers and arrays

• The name of an array is a pointer to its first element(item)

int main (){ int arr[10] = {0,1,2,3,4,5,6,7,8,9}; cout << arr << endl; cout << &arr[0] << endl;}

22FE38DF22FE38DF

array element access

• when an array is created, items are allocated in consecutive memory spaces– if we know the beginning address, we know the locations of all the

items.

int main (){ int i = 5; int arr[10] = {10,1,2,3,4,5,6,7,8,9}; cout << *arr << endl; cout << *(arr+ 1) << endl; cout << *(arr + 2) << endl; cout << *arr + 3 << endl; cout << *arr + 4 << endl; cout << *(arr + i) << endl; cout << *arr+ i+1 << endl;}

10121314516

Pointers and functions

• Pointer is one type of variable– stores addresses

• Pointers can be function parameters– Value pointer parameters– Reference pointer parameters

• Pointers can be function return values– Value returned is a pointer

value parameter• passing pointer by value

– cannot change the address outside– but, can change the values pointers point to

void func(int* arg1, int* arg2){ int* temp; temp = arg1; arg1 = arg2; arg2 = temp;

cout << *arg1 <<" : "<< *arg2 << endl;}

int main (){ int a = 10, b = 100; int *ptr1 = &a, *ptr2 = &b; func(ptr1, ptr2); cout << *ptr1 <<" : "<< *ptr2 << endl;}

100 : 1010 : 100

void func(int* arg1, int* arg2){ *arg1 = 100; *arg2 = 10; cout << *arg1 <<" : "<< *arg2 << endl;}

int main (){ int a = 10, b = 100; int *ptr1 = &a, *ptr2 = &b; func(ptr1, ptr2); cout << *ptr1 <<" : "<< *ptr2 << endl;}

100 : 10100 : 10

reference parameter• passing pointer by reference– can change the address outside

void func(int* &arg1, int* &arg2){ int* temp; temp = arg1; arg1 = arg2; arg2 = temp;

cout << *arg1 <<" : "<< *arg2 << endl;}

int main (){ int a = 10, b = 100; int *ptr1 = &a, *ptr2 = &b; func(ptr1, ptr2); cout << *ptr1 <<" : "<< *ptr2 << endl;}

100 : 10100 : 10

Project #5• Plain Text File Reformatter– reformats a plain text file into neatly arranged paragraphs

with a particular maximum allowed line length– int reformat(istream& inf, ostream& outf, int lineLength);

• an already-opened input source you will read from (probably a file the caller opened).

• an already-opened output destination you will write to (probably either cout or an output file the caller created)

• an int with the desired maximum line length – return value

• 0 if is successful (i.e., neither of the following problems occurs)• 1 if any input word portion is longer than the maximum line length• 2 if the desired maximum line length is less than 1

definition of an word

• A word is a sequence of non-whitespace characters.– The <cctype> function isspace tells you if a character is a

whitespace character

• A word can be viewed as a sequence of one or more word portions

Word Word portions

Sungwon Sungwon

so-called so- | called

come-as-you-are come- | as- | you- | are

so--called so- | - | called

so- so-

-so - | so

reformatting rules

• Fit as many word portions in an output line as you can without exceeding the line length. Output lines may well end up being different lengths, giving a "ragged-right" effect

It•always•does•seem•to•me•that•I•am doing•more•work•than•I•should•do.••It•is not•that•I•object•to•the•work,•mind•you; I•like•work:•it•fascinates•me.••I•can sit•and•look•at•it•for•hours.••I•love•to keep•it•by•me:•the•idea•of•getting•rid of•it•nearly•breaks•my•heart.

reformatting rules• Words in an output line must normally be separated by one blank. • if the last character of an output word is a period, question mark, or exclamation point, it

must be separated from any following word on that output line by two blanks. • The last word on an output line must not be followed by any blanks. • The first word portion on an output line must not be preceded by any blanks. • Blanks must not appear on an output line within any word. • The last line of output must end with a newline, and there must be no output lines (not even

empty ones) after the last word of the last paragraph.

It•always•does•seem•to•me•that•I•am doing•more•work•than•I•should•do.••It•is not•that•I•object•to•the•work,•mind•you; I•like•work:•it•fascinates•me.••I•can sit•and•look•at•it•for•hours.••I•love•to keep•it•by•me:•the•idea•of•getting•rid of•it•nearly•breaks•my•heart.

reformatting rules• If a word portion is longer than the line length, as much as will fit must be on an output line

by itself. • The rest of that word portion must begin the next output line (and, of course, is subject to

similar splitting if it's too long). • If this situation ever occurs, your function continues reformatting, but it must eventually

return 1 instead of 0 to its caller. • Notice that this is the only situation where a word is allowed to be split across lines other

than at a hyphen.

abcdefghij klmnopqrst uvwxyz

function returns 1

reformatting rules• A paragraph break is indicated in the input by one or more consecutive lines that contain no

words (i.e., are empty or contain only whitespace characters). • The first word in the input following a paragraph break will be the first word of a new

paragraph in the output. • If a paragraph has already been output, the new paragraph must be separated from the one

that precedes it by an empty line (i.e., a line with no characters other than the terminating newline). The very first output paragraph must not be preceded by an empty line

I•love•to•keep•it•by•me:•the•idea•of getting•rid•of•it•nearly•breaks•my heart.

You•cannot•give•me•too•much•work;•to accumulate•work•has•almost•become•a passion•with•me:

more rules• Your reformat function and any functions you write that it calls directly or

indirectly must not use any std::string objects. If you need to use a string, use a C string.

• Your function may assume that no input line will be 200 or more characters long.

• This project is doable without assuming any upper limit for the output line length (the third parameter of reformat) — rather than storing the parts of an output line in a C string to be written later, you instead write them as soon as you can. However, some people may not figure out how to do that, so we'll give you a choice. If the third parameter of reformat is greater than 500, your implementation of reformat must either: – return 2 without writing any output – reformat the text using the indicated line length, returning 0 or 1 as

appropriate (i.e., your algorithm doesn't impose any upper limit on the output line length). Implementing this choice correctly is worth 5 bonus points.

File I/O

• #include <iostream> #include <fstream> – ifstream• input file stream type• this object will be associated with a particular file

– ofstream• output file stream type • this object will be associated with a particular file

Advice from Professor

• Break down the function into two major tasks– one fetches next word from the input text file• identifies the next interest is a word, paragraph break,

or the end of file• if it is a word, get the word

– the other one writes the fetched word according to the defined format• will it be the first word in a line?• will it be the last word in a line?• does it end with period/question mark/exclamation?

getting next word

• basically deliver a single word to the reformat function– read single character in text file

• .get() function

– need code that skips redundant spaces• isspace() function

– maybe helpful to provide additional information along with the word• is this word preceded by a newline?• is this word followed by a newline?

sample codebool getWord(istream& inf, char outWord[], int& wordLen){ char c; int index = 0; do {

if ( ! inf.get(c)) return false; } while (isspace(c));

do {

outWord[index] = c; index++;

if ( ! inf.get(c)) break;

} while( ! isspace(c)); outWord[index] = '\0'; wordLen = index; return true;}

int main (){ ifstream inf("input.txt"); char word[200]; int wordLen; while (getWord(inf, word, wordLen))

{ cout << word << " : ";

cout << wordLen << endl;}

}

Hello everyone, my name is Sungwon Yang.Nice to meet you!

Hello : 5everyone, : 9my : 2name : 4is : 2Sungwon : 7Yang. : 5Nice : 4to : 2meet : 4you! : 4

sample code

• it eliminates all spaces/newlines– need to add some code that identifies line breaks

Hello everyone, my name is Sungwon Yang.

Nice to meet you!

Hello : 5everyone, : 9my : 2name : 4is : 2Sungwon : 7Yang. : 5Nice : 4to : 2meet : 4you! : 4

writing text in file

• it is very similar to cout objectint main (){ ofstream outText("result.txt");

char space = ' '; char newline = '\n'; char word1[10] = "Hello,"; char word2[10] = "how"; char word3[10] = "are"; char word4[10] = "you?";

outText << word1 << newline; outText << word2 << space << word3 << space << word4;}

Hello,how are you?