1. CS31: Introduction to Computer Science I Discussion 1A 4/9/2010 Sungwon Yang swyang@cs.ucla.edu

2. Quick Review What did we learn last week? – What is programming translator – Basic grammar of C++ #include, int main(), case sensitive, semi-colon, etc… – Print out on the screen cout function, endl – Get user input from keyboard: cin function – Variables int and double type Identifiers – Comments // /* ~~~~~~ */

3. Some tips from the last class What if want to use “” inside your text – Hello “Sungwon” !! – cout << “Hello “Sungwon” !!” << endl; – cout << “Hello \”Sungwon\” !!” << endl;

4. Expressions Arithmetic expressions (x, y, z variables) – Addition: z = x + y; – Subtraction: z = x – y; – Multiplication: z = x * y; – Division: z = x / y; – Modulo(remainder): z = x % y; The evaluation precedence is similar to algebra – Multiplication, division, modulo  addition, subtraction You can use parenthesis to yield precedence – x + y * z – ( x + y ) * z Number(variable) types determine the type of results – If numbers(variables) are int(double) type: the result is int(double) cout << 5 / 2 << endl;  2 cout << 5.0 / 2.0 << endl;  2.5 – If at least one number(variable) is double: the result is double cout << 5 / 2.0 << endl;  2.5 – What if assign the result to a variable? int x = 5 / 2.0;cout << x << endl;  2

5. More arithmetic expressions Equivalent expressions – x = x + 5;x += 5; – x -= 5;x *= 5;x /= 5; – z = z * (x + 2);z *= x + 2; Increment & decrement – x = x + 1;x += 1;x++;++x; – y = y – 1;y -= 1;y--;--y; ++ and -- behave differently depending on their locations – x = 5; y = ++x; cout << y << endl; x = 5; y = x++; cout << y << endl;

6. Quick questions What will the output be? int x = 5, y = 11, z = 3; cout <<x + y / z<< endl; 8 int x = 9, y = 5, z = 3; cout <<x % y * z<< endl; 12 int x = 9, y = 5, z = 3; cout <<z * x % y<< endl; 2 int x = 1, y = 2, z = 3; cout <<x++ * y * --z << endl; 4 int x = 1, y = 2, z = 3; x += (y + x) * z; cout << x << endl; 10

7. Boolean Expressions bool Type Variable – True (1) or False (0) bool test = true; bool test = 1; Bool test = 0; bool test = 1 < 2; bool test = x < 5; – If x is 3, true – If x is 10, false

8. Boolean Expressions Expression that evaluates to true or false SymbolMeaning >Greater than >=Greater than or equal to <Less than <=Less than or equal to ==Equal to !=Not equal to &&AND ||OR Wrong symbols: =>, =<, !< We know the difference between = and == !

9. Boolean Expressions Valid Expressions Invalid expressions x = y x 5 5 < x < 10 x >= y x == y (x – y) < 10 x > y && y < z

10. Quick questions int x = 5, y = 6, z = 7; x >= y false x == y false ( x – y ) > 10 false x < y && y < ztrue (( x != y) || ( x > y )) && ( y == z) false ( x != y) || (( x > y ) && ( y == z )) true

11. If statements Syntax … if (Boolean Expression) { Yes Statement … } Statement … int x=5, y=1; if ( x > y ) { cout << x << endl; } cout << y << endl; int x=1, y=5; if ( x > y ) { cout << x << endl; } cout << y << endl;

12. If-else statements Choose between two alternative statements Syntax if (Boolean Expression) { Yes Statement … } else { No Statements … } if ( x > y ) { cout << x << endl; … } else { cout << y << endl; … }

13. If-else if-else statements Choose among multiple alternative statements Syntax if (Boolean Expression) { Statement … } else if (Boolean Expression) { Statement … } else if (Boolean Expression) { Statement … } else { Statement } if ( x > y ) { cout << x << endl; } else if ( x == y ) { cout << y << endl; } else if ( x <= y ) { cout << x-y << endl; } else { cout << x+y << endl; }

14. Nested if-statement If-statement inside if-statement if (Boolean Expression) { Statement … if (Boolean Expression) { statement … } if ( x > y ) { cout << y << endl; if ( x > z ) { cout << z << endl; }

15. If-statement You can omit braces if your statement is single line if ( x > y ) { cout << y << endl; if ( x > z ) { cout << z << endl; } if ( x > y ) { cout << y << endl; if ( x > z ) cout << z << endl; } if ( x > y ) cout << y << endl; if ( x > z ) cout << z << endl;

16. Quick questions #include using namespace std; int main() { int age = 30, height = 170, weight = 70, result = 0; if ( height / weight != 2 ) result += 2; else result++; if ( age >= 30 ) { ++result; if ( height < 150 || weight == 70 ) { result *= 5; if ( result > 10 ) cout << result << endl; else if ( result < 10 ) cout << --result << endl; else cout << ++result << endl; }

17. String A variable type for “text” #include using namespace std; int main() { string str = "Hello! Sungwon"; cout << "Hello! Sungwon" << endl; cout << str << endl; }

18. Get a line of “text” from user getline( cin, variable) Space, tab, and enter are handled as text #include using namespace std; int main() { string str = “Hello”; string myName = “”; cout << “What is your name?" << endl; getline ( cin, myName); cout << str << endl;; cout << myName << endl; }

19. Checking string if ( string == “text” ) If ( string == “” ) If ( sting != “” ) #include using namespace std; int main() { string myName ; cout << “What is your name?" << endl; getline ( cin, myName); if ( myName != "" ) cout << "Hello " << myName << endl; else cout << "Please Enter Your Name" << endl; }

20. cin.ignore(); getline ( cin, variable ) after cin >> variable int main() { cout << "How many place settings would you like to buy? "; int numberOfSettings; cin >> numberOfSettings; cout << "In which style? "; string style; getline(cin, style); } int main() { cout << "How many place settings would you like to buy? "; int numberOfSettings; cin >> numberOfSettings; cin.ignore(100000, '\n'); cout << "In which style? "; string style; getline(cin, style); }

21. Set the digit after decimal points #include using namespace std; int main() { double x = 10.0; cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(2); cout << x << endl; } #include using namespace std; int main() { double x = 10.0; cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(0); cout << x << endl; } 10.00 10.

22. Variables vs Constants We can assign new values into variables – Values of variable can be changed at any time – But, we can declare values cannot be changed int main() { int x = 0; cout << x << endl; x = 10; cout << x << endl; x *= 5; cout << x << endl; } int main() { const double PI = 3.14159; cout << PI << endl; PI = 3.5;// this is error }