cs31: introduction to computer science i discussion 1a 4/16/2010 sungwon yang [email protected]...

CS31: Introduction to Computer Science I

Discussion 1A 4/16/2010

Sungwon [email protected]

www.cs.ucla.edu/~swyang

mailto:[email protected]

Quick Review• What did we learn last week?

– Arithmetic Expressions• Addition, Subtraction, Multiplication, Division, Modulo• Evaluation precedence: left -> right, parenthesis• Abbreviation: +=, -=, *=, /=, %=• Increment & decrement: ++, --

– Boolean Expressions• bool type: true or false • >, <, >=, <=, ==, !=, &&, ||

– IF statement• Conditional statements

– IF - ELSE statement• Can choose one between two alternatives

– IF - ELSE IF – ELSE statement• Can choose one among multiple alternatives

– Nested IF statement• Embed IF statements in IF statement

– String variables• variable for “text”• getline (cin, variable);• cin.ignore(100000, '\n');

– Variables vs. Constants

WHILE Loops

• Repeat statements while condition holdswhile (Condition){ statements; statements; statements; …}

int yourInput = 0; cin >> yourInput;while (yourInput > 0){ cout << yourInput << endl; yourInput--;}cout << “The End” << endl;

5

54321The End

DO-WHILE Loops• Repeat statement while condition holds– Execute the statements once then test conditiondo{ statements; statements; statements; …} while (Condition);

int yourInput = 0; cin >> yourInput;do{ cout << yourInput << endl; yourInput--;} while (yourInput > 0);cout << “The End” << endl;

5

54321The End

while vs. do-whileint main(){ int yourInput = 0, sum = 0; cin >> yourInput; while (yourInput > 0) {

yourInput--; sum += yourInput;

} cout << sum << endl;}

int main(){ int yourInput = 0, sum = 0; cin >> yourInput; do {

yourInput--; sum += yourInput;

} while (yourInput > 0); cout << sum << endl;}

5

0

10 10

0 -1

Infinite loops

• Your loop will never end – Make sure that it ends at some point

int main (){

int x = 0, y = 0, sum = 0;while ( sum != 100 ){

sum = x + y;x += 1;y += 2;cout << sum << endl;

}}

int main (){

int x = 0, y = 0, sum = 0;while ( sum < 100 ){

sum = x + y;x += 1;y += 2;cout << sum << endl;

}}

FOR Loopsfor ( initialization ; condition ; update ){ statements; statements;

…}

The order of execution1.Initialization2.Check the conditionIf true

3. Run the body (statements)4. Do the update Go back to 2

If falseExit the for loop

Initialization in for loop

• Can declare initialization variable inside for loop– It is different from variables declared elsewhere

int i = 0;for ( i = 0 ; i < 5 ; i++ ){ cout << i << endl;} cout << i << endl;

int i = 0;for (int i = 0 ; i < 5 ; i++ ){ cout << i << end;} cout << i << endl;

012345

012340

Nested loop• Embed loops into loopsint main(){

for (int i=0 ; i < 10 ; i++) {

cout << i << ':';

for (int j=0 ; j < 10 ; j++){

cout << j << ',';}

cout << endl;}

}

0:0,1,2,3,4,5,6,7,8,9,1:0,1,2,3,4,5,6,7,8,9,2:0,1,2,3,4,5,6,7,8,9,3:0,1,2,3,4,5,6,7,8,9,4:0,1,2,3,4,5,6,7,8,9,5:0,1,2,3,4,5,6,7,8,9,6:0,1,2,3,4,5,6,7,8,9,7:0,1,2,3,4,5,6,7,8,9,8:0,1,2,3,4,5,6,7,8,9,9:0,1,2,3,4,5,6,7,8,9,

FOR to WHILE Conversion

• while loop <-> for loop

for ( initialization ; condition ; update ){ statements; statements;

…}

initializationWhile ( condition ){ statements; statements;

… update }

Quick question

int main(){

int result = 0;for ( int i = 10; i >= 0; i--){

result += i;}cout << result << endl;

}

55

int main(){

int result = 0;for ( int i = 32; i > 1; i /= 2){


}

62

Functions

• Sub-programs in your program• Simplify multiple operations which are basically same

Calculator

Addition

Subtraction

Multiplication

Division

Your Program

Functions

Functions• 3 operations, but basically same jobint main(){

int result = 0;for ( int i = 1; i <= 10; i++){


result = 0;for ( int i = 1; i <= 50; i++){




}

int main(){

int result = 0;result = sum ( 1 , 10 );cout << result << endl;

result = 0;result = sum ( 1 , 50 );cout << result << endl;

result = 0;result = sum ( 1 , 100 );cout << result << endl;

}

Functions

• Define a functionreturn_type function_name (argument_type_1 argument_name_1, argument_type_2 argumrnt_name_2, … ){

function_statements;function_statements;……return return_value;

}

int sum (int begin, int end ){

int result = 0;for ( int i=begin ; i <= end ; i++ ){

result += i;}return result;;

}

Function definition

int main(){

int result = 0;for ( int i = 1; i <= 10; i++){






}



result += i;}return result;

}

int main(){

int result = 0;cout << sum(1,10) << endl;

result = sum(1,50);cout << result << endl;

result = 0;cout << sum(1,100) << endl;

}

Function declaration



result += i;}return result;;

}

int main(){




}

int sum (int begin, int end); //proto type

int main(){




}

int sum (int begin, int end ) //definition{


result += i;}return result;

}

Function without arguments

• Functions can have no arguments

string askName (){

string name;cout << "What's your name?" << endl;getline (cin, name);return name;

}

int main(){

string name;name = askName();cout << "Hello!" << endl;cout << name << endl;cout << "Nice to meet you!" << endl;

}

What’s your name?

Sungwon

Hello!SungwonNice to meet you!

Void functions

• Functions can have no return values

void greeting (string name ){

cout << “Hello!” << endl;cout << name << endl;cout << “Nice to meet you!” << endl;

}

int main(){

string name;cout << “What’s your name?” << endl;getline (cin, name);greeting (name);

}

What’s your name?

Sungwon

Hello!SungwonNice to meet you!

End point of function

• return ends function• In the case of void function, the last statement

int larger (int arg1 , int arg2 ){

if ( arg1 >= arg2 )return arg1;

elsereturn arg2;

}

int main(){

int first = 0, second = 0;cout << “Enter first number” << endl;cin >> first;cout << “Enter second number” << endl;cin >> second;cout << larger(first,second) << “is larger” << endl;

}

Functions of string (Chap 9 in textbook)Operation What it does Examples

string s = “Hello”;string s2 = “!!!”;

Declares strings s and s2

s.length() or s.size()

Returns the length of s cout << s.size(); 5

s.at(i) Returns i-th characteri must be an interger between 0 and size-1(inclusive)

cout << s.at(0); Hcout << s.at(4); o

s.empty() Returns true if s is empty If (s.empty()) cout << “It is empty”;

s + s2 Concaternates two strings cout << s + s2; Hello!!!

s.substr(i,n)s.substr(i)

Takes a substring of length n, starting from the i-th character

cout << s.substr(2,2); llcout << s.substr(2); llo

s.replace( i , n , s2 ) Replace a substring of length n starting from at i with another string s2, and set s with a new string

s.replace (2,2,s2);cout << s; He!!!o

Functions of character(Chap 9 in textbook)• You need to include <cctype> library: #include <cctype>

Operations What it does

char c; declare a character variable c

isspace(c) true if c is a white space character (space, tab, newline)

isalpha(c) true if c is a letter

isalnum(c) true if c is a letter or digit

islower(c) true if c is a lowercase letter

isupper(c) true if c is a uppercase letter

tolower(c) returns the lowercase version of c, if c is a letter

toupper(c) returns the uppercase version of c, if c is a letter

Quick question

#include <iostream>#include <string>#include <cctype>using namespace std;

int main(){

string s = "hello";char c;

for (int i=0 ; i< s.length() ; i++){

c = s.at(i);c = toupper(c);cout << c;

}cout << endl;

}

HELLO

ASCII and character

• Everything is represented in 0s and 1s in computers – computers do not understand ‘a’, ‘B’, ‘3’,…– characters are stored as binary numbers

• ASCII defines the mapping between characters and integers• ‘0’ 48, ‘A’ 65

int main(){

int a = ‘A’;cout << a << endl;

}

65

int main(){

char a = 65;cout << a << endl;

}

A

cs31: introduction to computer science i discussion 1a 4/16/2010 sungwon yang [email protected]...

Documents