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# CS31: Introduction to Computer Science I

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

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## CS31: Introduction to Computer Science I

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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 • Wrong symbols: =>, =<, !< • We know the difference between = and == !

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

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

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

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 <iostream> 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 < string> #include <iostream> #include <string> 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 <iostream> #include <string> 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 <iostream> #include <string> 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 <iostream> using namespace std; int main() { double x = 10.0; cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(2); cout << x << endl; } #include <iostream> using namespace std; int main() { double x = 10.0; cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(0); cout << x << endl; } 10. 10.00

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 }

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