1. Announcements First Midterm
November 7th on Tuesday next week at 19:40 (110 minutes)
Exam classes:
if (LastName <= "Camat")
     cout << " FENS G077 " << endl;
else if ("Canbakal" <= LastName <= "Gungor")
     cout << " FMAN 1099 " << endl;
else if ("Gurcan" <= LastName <= "Kose")
     cout << " FASS G062 " << endl;
else if ("Kulaoglu" <= LastName <= "Onder")
     cout << " FASS G022 " << endl;
else if ("Oskay" <= LastName <= "Ozyazgan")
     cout << " FENS L035 " << endl;
else if ("Pala" <= LastName <= "Sonmez")
     cout << " FENS G032 " << endl;
else if ("Sulun" <= LastName <= "Turkoglu")
     cout << " FENS G035 " << endl;
else if ("Ucak" <= LastName)
     cout << " FENS L045 " << endl;
This week recitations, we will solve sample midterm questions.
Detailed about the midterm is sent with previous years’ exams.
Homework 4 is due on Wednesday, November 8th next week

2. for loop compared with while
<initialization>
while (<test>) {
<statement1>;
...
<statementN>;
<update> }
for (<initialization>;
<test>;
<update> ) {
<statement1>;    <statementN>; }
Let’s compare for loop with the while loop.
for loop merges the 3 pieces (initialization, condition and update) of a while loop into one line. This makes loops easier to write and read in one line.

3. Example Rewrite the while loop of main of primes.cpp using for
k = low; while (k <= high) {
if (IsPrime(k)) {
cout << k << endl; numPrimes += 1; } k += 1; }
for (k = low; k <= high; k += 1) {
if (IsPrime(k)) {
cout << k << endl; numPrimes += 1; } }

4. Shorthand for increment/decrement
Lots of code requires incrementing a variable by one
Three methods, using = and +, using +=, and using ++
effectively they are same
num = num + 1;
num += 1;
num++; // post increment
It is also possible to write ++num;
pre-increment
These differ on when the increment is performed, but this difference doesn’t matter when used as an abbreviation for the statement n += 1; in a single statement
Similarly there are post-decrement (and pre-decrement)
num = num - 1; num -= 1; num--;

5. The do-while loop
Similar to while loop, but the test is after the execution of the loop body
The while loop may never execute, do-while loop executes at least once
<initialization> do {
<statement1>;    ...   <statementN>;
<update>
} while (<condition>);
Example: Prompt for a number between 0 and 100, loop until such a number is entered (user should enter at least one number)
cout << "enter number in range [0..100] ";
cin >> num;
} while (num < 0 || num > 100 );
Don’t forget

6. Priming Priming: reading an initial value before the loop
do not get confused with prime numbers; this is something else
Problem: enter numbers, add them up, stop when -1 entered
int sum = 0;
int num;
cin >> num; // prime the loop
while (num != -1)
{ sum += num;
cin >> num; }
cout << "total = " << sum << end;
Code duplication exists here: input (and perhaps prompt) code is repeated before the loop and in the loop

7. Pseudo infinite solution using break
To avoid repeating code, include it in the body of the loop only, use a test to break out of the loop
break statement exits (inner-most) loop
I don’t prefer this kind of loops (I’d prefer code duplication)
Because the loop condition is not clear, hence prevents readability
Try not to use break in this course
Save it for later when you develop really complicated loops
int sum = 0;
int num;
while (true) // seemingly infinite loop
{ cin >> num;
if (num == -1)
{ break; // get out of loop }
sum += num;
cout << "total = " << sum << end;

8. Fence Post Problem
The problem that occurs when one or more operations of the loop body are executed one less then the others.
Example: Display integers between 1 and 10 separated by comma
1,2,3,4,5,6,7,8,9,10
no comma after 10; no comma before 1.
for (n=1; n <= 10; n++)
{ cout << n << ",";
} Problem: comma after 10
for (n=1; n < 10; n++) }
cout << n; No problem, but code duplicates
Think of other solutions! (see page 175 of Tapestry)

9. Downward-counting loop
Calculate n to the power of m: nm=nxnx…xn
Example: 25=2x2x2x2x2=32
int power = 1;
int n, m;
cin >> n >> m;
for (int i = m; i <= 1; i--) {
power = power * n; }

10. Nested loops Sometimes one loop occurs in another
Generating 2-dimensional tabular data
multiplication table
Sorting vectors (which will be studied much later)
Display some geometric figures using character * (or any other character)
display rectangles, triangles
Although other loops can be nested as well, most of the time, for loops are used in nested manner

11. Nested loops - Example
Write a function to display a rectangle of stars (height and width are parameters)
e.g. if height is 4 and width is 7, the output should look like
*******
for (i=1; i<= height; i++) {
for (j=1; j<=width; j++) // inner loop prints 1 line of stars
cout << "*"; }
cout << endl; // end of line is put to the end of each line
See drawfigures.cpp for the complete function and its use in main

12. Nested loops - Example
Write a function to display a perpendicular isosceles triangle of stars (perpendicular side length is parameter)
e.g. if side length is 6 , the output should look like * **
***
****
*****
******
for (i=1; i <= side; i++) {
for (j=1; j<=i; j++) // inner loop prints 1 line of stars
cout << "*"; }
cout << endl; // end of line is put to the end of each line
See drawfigures.cpp for the complete function and its use in main

13. Same loop: downward-counting
for (i=1; i <= side; i++) {
for (j=1; j<=i; j++)
cout << "*"; }
cout << endl;
for (i=1; i <= side; i++) {
for (j=i; j>=1; j--)
cout << "*"; }
cout << endl;

14. Drawfigures – Other Considerations
What about having a function to display a line of stars (number of stars is a parameter)
useful for both rectangle and triangle
void PrintLine (int numstars)
// pre: numstars > 0
// post: displays numstars stars in one line {
int i;
for (i=1; i<= numstars; i++)
{ cout << "*"; }
cout << endl; // end of line is put to the end of the line
in rectangle function, inner loop is replaced by a function call
for (i=1; i<=height ; i++)
PrintLine(width);
use of PrintLine in triangle function is similar

15. Example – Multiplication Table
On ith line print, i*1, i*2, i*3, ... , i*i
Total number of lines is an input. Display lines starting with 1.
See multiply.cpp
#include <iostream>
#include <iomanip> // for setw
using namespace std;
int main() {
int i,k,numlines;
const int WIDTH = 4;
cin >> numlines;
for (i=1; i <= numlines; i++)
for (k=1; k <= i; k++)
cout << setw(WIDTH) << i*k; }
cout << endl;
return 0;

16. Constants Sometimes very useful
provides self documentation
re-use the same value across the program
avoid accidental value changes
like variables, but their value is assigned at declaration and can never change afterwards
declared by using const before the type name (any type is OK)
const double PI = ;
const string thisclass = "CS201"
const int WIDTH = 4;
later you can use their value
cout << (PI * 4 * 4);
but cannot change their value
PI = 3.14; causes a syntax error

17. Formatting Output
We use stream manipulator setw to specify the total number of spaces that the next output will use
setw(field length)
written in cout and affects only the next output value
not the whole cout line
output is displayed using field length spaces in right justified manner (any empty space is on the left)
defined in header file <iomanip>, so you have to have #include <iomanip>
Example
cout << setw(9) << "cs201";
output shown is four blanks and cs201

18. Example using robot class (see rectangularscan.cpp)
Write a program in which the robot starts at 0,0 and searches a rectangular space that covers n*n cells
n is input (in the example below, n is 8)
during this journey the robot should pick or put things on the cells so that all visited cells occupy one thing