Welcome to Lesson 13!

• Quiz 5 on Thursday
  
• Return Quiz 4
  
• Reminder: Friday is the Veteran's Day holiday. No lab on Friday!
  

Review Questions

• What are two problems with the following loop? What will happen if you run the code as it is now?
  

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#include <iostream>
using namespace std;

int main() {
    int number = 0;

    cout << "Enter a number and I will show its"
         << " bar graph.\nEnter your number: ";
    cin >> number;

    cout << "\nBar graph:\n";
    for (int i = 0; i < number; i++) {
        cout << '*';
    }
    cout << endl;

    return 0;
}

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#include <iostream>
using namespace std;

int main() {
    cout << "Enter the number to sum to: ";
    int number = 0;
    cin >> number;
    int sum = 0;
    for (int i = 1; i <= number; i++) {
        sum = sum + i; //using the counter as a value to add to my sum 
    }
    cout << "Total sum = " << sum << endl;

    return 0;
}

for (int i = 10; i <= 15; i++)

{

    if (i == 11 || i == 13 || i== 15)

        cout << "*";

    else

        cout << "!";

}

• Find a partner. Then, open up a text editor (Not CodeBlocks) and create a new document called tracing.doc or tracing.txt.
• In the text editor write both your names at the top of the program.
• Then, type what the output of the above for loop would be to the console.
• Note, you should do this without running the code. 
• Then, change the statement i <=15 to be i < 15.
• Draw the output of the for loop with this alteration.
• How is it different?
• The instructor will be going over the examples so you can verify your answers and understanding.
• Don't forget to upload your text document before the end of class.

More Loopy Errors

• A common problem with counting loops is the off-by-one error
• Finding the correct upper and lower bounds can be confusing
  • Should you start at 0 or 1?
  • Are you counting up to a number (<), or up to and including a number (<=)?
• To understand the counting loop you need to count iterations.
• For instance, the following loop executes b - a times:

  for (int i = a; i < b; i++)

• However, the following loop is executed b - a + 1 times:

  for (int i = a; i <= b; i++)

• The "+1" is the source of many errors
• For example, to sum the numbers from 1 to 5, what is wrong with:

  #include <iostream>
  using namespace std;
  
  int main()
  {
      int sum = 0;
      for (int count = 0; count < 5; count++)
      {
          sum += count;
      }
      cout << sum << endl;
  
      return 0;
  }
  

• One way to visualize the "+1" error is by looking at a fence

Image source.

• How many posts do you need for a fence with three sections?
• It is easy to be "off-by-one" with problems like this
• Forgetting to count the last value is sometimes called a fence-post error

Debugging Technique: Tracing Variables

• One good way to discover loopy errors is to display the variables that are part of the loop
• Tracing variables means watching their values change as the program executes
• You can insert temporary output statements in your program to watch the variables, like:

  cout << "count=" << count << ", sum=" << sum << endl;

Activity 13.2: ASCII (10 pts)

In this exercise we continue exploring some uses of counter-controlled loops.

Background

Recall that a char data type is stored by the computer as a number using the ASCII code (ASCII Table). Since a char is stored as an int by the computer, C++ lets you cast an int to a char.

int count = 65;
cout << (char) count << endl; //What does the (char) do?

This casting feature lets us construct our own ASCII table.

Specifications

• Find a partner for pair programming.
  
• Copy and paste the starter code into a file called ascii.cpp
  

• Alter the for-loop so that it counts from 32 to 127
• Inside the for-loop, alter the cout statement as follows:

• Compile and run your modified program and make sure your output looks like:

  (some output not shown)
  65      A
  66      B
  67      C
  (more output not shown)
  

• When you are finished, play around with the starting and ending values of your for loop.
• Can you find the ASCII value for any unusual characters, such as the 4 ranks of playing cards, smiley faces and musical notes.
• It turns out that each of these symbols has its own ASCII value in C++!
  
• Submit the final version of your file to Catalyst when you are finished.

Nested Loops

• Some applications require loops within loops, called "nested" loops
• For example, you may use a nested loop to print a table of values
• The following example shows a simple table created with nested loops
• Let's follow the execution sequence before checking the result

Example of Nested Loops

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#include <iostream>
using namespace std;

int main()
{
    for (int outer = 1; outer < 4; outer++)
    {
        for (int inner = 1; inner < 4; inner++)
        {
            cout << outer << " " << inner << endl;
        }
    }
}

Tracing the Variables

• To understand any looping application, you need to trace the loop by hand
• To trace the loop, write the variables used in the loop as headings across a page
• On the first line under the headings, write the initial values
• Execute the loop 3-5 times and record the values of the variables each time through the loop
• Pay special attention when entering the loop the first time and when ending the loop
• You can slightly modify the computations if it helps to test the loop
• Below is an annotated trace of the variables for the inner and outer loops
• Note that the variable outer changes only after the inner loop is finished

Memory		Screen
outer	inner
1	1	1 1
	2	1 2
	3	1 3
	4 (end of loop)
2	1	2 1
	2	2 2
	3	2 3
	4 (end of loop)
3	1	3 1
	2	3 2
	3	3 3
	4 (end of loop)
4 (end of loop)

• By analogy, nested loops are like an odometer on a car
• The inner loop is like the digit to the right on an odometer
• The numbers to the right loop completely before the number to the left increments by one

• Another example of a nested loop is to draw a square using stars ("*")
• Remember our bar graph example with the for loop?

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#include <iostream>
using namespace std;

int main() {
    cout << "I will print squares for you!\n\n";
    cout << "Enter the width of the square: ";
    int size;
    cin >> size;

    for (int row = 1; row <= size; row++)
    {
        for (int col = 1; col <= size; col++)
        {
            cout << "*";
        }
        cout << endl;
    }

    return 0;
}

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for (int row = 1; row <= 5; row++)
{
    for (int col = 1; col <= row; col++)
    {
        if (col == 1 || row == col || row == 5)
        {
            cout << "*";
        }
        else
        {
            cout << ".";
        }
    }
    cout << endl;
}