CS Class 04 Topics  Selection statement – IF  Expressions  More practice writing simple C++ programs Announcements  Read pages for next class.

Class Exercise from last Thurs. What are the final values for e, f, g, and y? #include using namespace std; int main( ) { int a=2, b=4, c=8, d=5, e, f, g; double x = 2.5, y; e = (a + b * c) / (d + 12); f = x / 2; g = c % d; y = c / d + x; }

Questions to consider What if we used double instead of int for our variables in the microwave program?  Try it  Does the output change? What if min and sec are both a double, but time is still an int?  Does it still run?  Does it always work?

Comments on writing programs Everything is a combination of Sequence, Selection, and Iteration statements Program from last time, sequence only  Read / Read / Compute / Print Other than S/S/I, need to know  Variables – storage locations – today  Functions – next month  Arrays – March

Expressions statement; Sequence statements statement; No decisions Just perform one statement after another Selection and Iteration statements Make a decision to determine what to do (Which path? Another loop?) The decision is an “expression” which evaluates to either true or false statement; truefalse statement; truefalse

CS 114 Content (details) Algorithms Definition Tracing Writing Analyzing Efficiency Recursion Searching Sorting Language Implementation (using Visual Studio.NET 2005) Environment / compilation / debugging / resources / tools Sequence input output assign File I/O formatting Selection if switch Iteration for while Basic Elements variables operators expression functions arrays objects

If statement Syntax if ( expression ) statement ; // then clause else statement ; // else clause Expression determines path to take  If TRUE, does “then” clause  If FALSE, does the “else” clause (when it exists) Example if ( a < b ) cout << a << endl; else cout << b << endl; statement; truefalse

Need to understand expressions True or False? int a, b, c; a=5; b=3; c=10; ( b == c ) ( (a+b) < c) (b == (c / 3)) ( (b > c) || (c != 10) ) ( (a <= c) && (a == b) ) == equals != not equal <less than <= less than or equal to >greater than >= greater than or equal to &&logical and (both must be true) ||logical or (one must be true) !logical not (opposite of the expr)

Modifying Our First Program #include using namespace std; int main ( ) { int min, sec; cout > min; cout > sec; int time; if (min == 0) time = sec; else time = min * 60 + sec; cout << "total time is " <<time << endl; return 0; }

Class Exercises int main ( ) { int a=5, b=10, c=15; if ( a > (c%b) ) cout << “yes” << endl; else cout << “no” << endl; if ( (a+b) == c ) cout << “UA” << endl; if ( (b/a) < (c/a) ) cout << “AU” << endl; } int main ( ) { int a=2, b=4, c=8; if ( (b>1) && (c<10) ) if ( (a>c) || (a<b) ) cout << “UA” << endl; if ( (c%b) == a ) cout << “Roll” << endl; else cout << “Tide” << endl; if ( (a != 2) && (b != 4) ) cout << “Yes” << endl; else cout << “No” << endl; }

More on selection statements Selection Statements come in three flavors  if statements  switch statements  ?: operator (we will not use this in CS 114) We will focus on if statements for now  Can use { and } to group a block of statements together if (a == b) { c = a * a; b = b + 1; } else { a = a - 1; }

Watch out! int main( ) { int a = 5; if (a = 4) cout << “yes” << endl; else cout << “no” << endl; } Expression evaluates:  True is non-zero  False is zero C++ does the following  The expression “a = 4” is an assignment stmt  assigns a the value of 4  expression itself evaluates to value assigned (4)  4 is non-zero (TRUE)   this expression is true! Output: “yes” Syntax is correct  no compile errors Logic is wrong  a semantic error

Class Exercises What does the program at the right do? int main( ) { int a, b, c; cin >> a; cin >> b; cin >> c; int num; if (a >= b) num = a; else num = b; if (num < c) num = c; cout << num << endl; }

Exercise - Put sequence & selection together Write a C++ program that:  Reads in four integers from the user  Calculates an average for these four numbers  Prints out the average and the number of inputted values that were above the average (greater than or equal to)  Example #1 Input = 1, 2, 3, 4 Output = 2.5 and 2  Example #2 Input = 1, 1, 1, 9 Output = 3 and 1  Hint: use a “counter” to keep track of how many are above average

Quick check of the solution #include using namespace std; int main( ) { int a, b, c, d; cin >> a;// read in the four values cin >> b; cin >> c; cin >> d; double avg = (a + b + c + d) / 4.0;// average should be a real number int count = 0;// this is our counter if (a >= avg) count = count + 1;// increment for each above avg if (b >= avg) count = count + 1; if (c >= avg) count = count + 1; if (d >= avg) count = count + 1; cout << avg << " " << count << endl;// print results }

End of Class 04 Read pages 50 – 73 for next time. Make sure you can do today’s exercises.