LAB: FOR LOOPS IN C++

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2 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 2 Introduction This lab will provide students with an introduction to the fundamentals of conditional repetition in the C++ programming language. The focus of this lab is on how to solve complex problems using the C++ for statement. At the end of this module, students will Be able to solve simple problems using C++ for statement Understand the structure of C++ for statement These exercises have two different types of answers. Some questions ask for answers to specific questions; you should collect these answers in a Microsoft Word document. Other exercises ask you to write code; for these exercises, structure your storage drive in the following manner. There should be a palms folder, and within that folder there should be a LabFor folder. All coding exercises (and related folders) for this lesson should be placed in the LabFor folder. As always, be sure to properly document your code. Consult the C++ Coding Guidelines document for proper coding standards. Use good design principles and design the solution before attempting to write code. All programs should make use of multiple functions besides main() (use the input/process/output cycle as your guide). Think about how each problem actually involves more than one subproblem. At a minimum, you must construct functions aligned with the input-process-output steps. Let s begin! Part #1: Counter-Driven Repetition in C++ The idea of repetition, repeating a set of statements until a condition becomes false, has many applications in computer programming. As a result, C++ provides us with several constructs we can use to do repetitive programming. We have see one such construct in (the while statement) and we will now focus on the for construct. The for construct allows us to do something called counter-driven repetition in a more compact manner. While loops provide for conditional repetition within a program, allowing you to specify a statement or sequence of statements that you want executed as long as some condition holds. Since a while loop is controlled by a condition, it is usually the case that the number of repetitions cannot be predicted ahead of time. In contrast, there are many problems involving repetition where the number of repetitions is known ahead of time. In these cases, some action must be carried out a certain number of times in order to complete a task. For example, suppose you wanted to sum the numbers from one to ten. You could do this by using a counter variable. A counter is a variable that keeps track of the number of occurrences

3 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 3 of some event. A common use of counters is to keep track of the number of times a loop has executed, and so control the number of repetitions. For example, in the code below, we use a counter to ensure we only add the numbers from one to ten, inclusive. int count = 1, sum = 0; while (count <= 10) sum = sum + count; count = count + 1 The variable count is initialized to 1, signifying that no summation has been made yet. During each pass through the loop, the current value of the count variable is added to the value of sum. Afterwards, the value of count is incremented accordingly. When the count finally reaches 11, the loop test will fail and the loop will terminate. For Loops The above code is an example of a counter-driven loop, one whose number of repetitions is controlled by a counter. Counter driven loops always involve three components: 1. Initialization of the counter variable to a starting value. 2. Testing the counter variable for a terminating condition. 3. Modifying the value of the counter variable to ensure an eventual end to the loop. Since counter-driven loops of this form are common, C++ provides a variant of while loops that captures this pattern more succinctly. The general form of a for loop is: for ( /*initialize counter*/ ; /*test counter*/ ; /*modify counter*/) // statements to execute... As this general form shows, a for loop combines all of the components of a counter-driven loop, making the loop more compact and easier to read. In particular, the steps involving the loop counter: initialization, testing, and modification, are all isolated on one line, separated by semicolons. Since these statements control the behavior of the loop, listing them together makes it easier to grasp the overall behavior of the loop. Separating the loop control statements from the actual code to be executed in the body of the loop also makes it easier to focus on the actions performed by the loop.

4 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 4 In all other respects, a for loop behaves precisely the same as its while loop counterpart. In particular, the initialization of the loop counter (the first component of the for loop header) is executed first. Then, the loop test (the second component) is evaluated, and if the test succeeds then the body of the loop is executed. After the loop body is executed, the loop counter is incremented (the third component) and the loop test is reevaluated, possibly re-executing the loop body as before. As an example, the code below that uses a for loop to sum the numbers from one to ten: int sum = 0; for (int count = 1; count <= 10; count = count + 1) sum = sum + count; Notice in the above example the use of int count = 1 as the initialization component. When we want to only use a variable within a for loop, we can create the variable in this manner. The variable is created when the loop begins and is destroyed when the loop ends; i.e. the variable count would not be accessible in code following the loop. Watch and listen to the For Loops in C++ podcast below:

5 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 5 Part #2: Applications of For Loops We previously examined the the while statement as a general-purpose loop structure. These loops can be used for any type of conditional repetition needed by our programs. However, alternative repetition constructs are available for specific purposes. Recall that the purpose of the for construct is to allow us to perform counter-driven repetition in a more compact manner. In the While Loops in C++ lab, the program CompoundInterest was one of the examples used to illustrate the use of a while statement. This program involves the use of counter-driven repetition. If we swap out the while statement for a for statement, the code in main would look like the following: int main() double interest_rate = 0.0; double balance = 0.0; // stores the interest rate // stores the user balance // prompt the user and input two values cout << "Enter a starting balance and interest rate, " << "separated by a space: "; cin >> balance >> interest_rate; // set up the output for two digits of precision... cout.setf(ios::fixed); cout.precision(2); for (int year = 1; year <= 10; year++) // compute the new balance for this year... balance = balance + (balance * interest_rate); // show the balance each year... cout << setw(4) << year << setw(15) << balance << endl; // return "success" to the operating system... return 0; Ultimately the code in this version is more compact. Since we have separated the loop control statements from the actual code to be executed in the loop body, the purpose of the loop can be made more apparent to the casual reader: to compute a new balance and to output that new value.

6 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 6 For Loops and Grade Calculators Suppose we wanted to write a C++ program to compute student grades. Our program would take one input at the beginning: Inputs Data Type Description Student ID string Exactly four characters Next, our program would input exactly 11 of the following values: Inputs Data Type Description Exam Score int An exam score [0-100] Once the input was finished, our program would calculate the total score by throwing out the lowest score and computing the students grade as a percentage. One possible dialog between the program and the user would look like the following: Enter the student ID: 11AB Enter Exam Score #0: 80 Enter Exam Score #1: 82 Enter Exam Score #2: 87 Enter Exam Score #3: 74 Enter Exam Score #4: 68 Enter Exam Score #5: 89 Enter Exam Score #6: 80 Enter Exam Score #7: 92 Enter Exam Score #8: 83 Enter Exam Score #9: 81 Enter Exam Score #10: 84 The lowest score was removed (68). Student 11AB has a total score of 832 (83.20 percent, B). This problem is well-suited to the use of for loops. The problem involves a repeated set of instructions (the instructions to provide input) which differ only in terms of the prompt: each user prompt includes a number (#0, #1, etc) which increases by 1 each time. The repeated prompting of the user has a defined starting point (Exam Score #0) and a defined ending point (Exam Score #10). The succession of prompts is an example of counter-driven repetition. Exercise 1: Create a new C++ Project called GradeCalculator. Add a new C++ code file called GradeCalculator.cpp to the project, and construct the program described above. A few notes as you construct the program:

7 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 7 The percentage score must be output with two digits of precision. Letter grades should follow the following scale: [ )% = F, [ )% = D, [ )% = C, [ )% = B, and [ ]% = A. Be sure to validate all input. Repetition and conditional constructs must be used where appropriate. Compile and Build your program to verify it works as expected. Enter in several different tests cases to verify your program s logic. For Loops and Population Growth Two major factors that affect population growth are birth rates and death rates. These rates can be measured and can be used to predict how the population s size will change over time. To find a change in population size over a fixed time interval, use the formula: Where ΔN Δt = (B D) Variable ΔN Δt B D Meaning The change ( ) in population size The change ( ) in time (fixed interval) The number of births The number of deaths This equation can be modified to represent the average number of births and deaths per individual during the time interval. The variable b represents the per capita birth rate (the number of offspring per unit of time for an average individual), where similarly, d is the per capita death rate. Using N to represent population size, the equation becomes ΔN Δt = (bn dn) To identify the difference in per capita birth and death rates this equation becomes: r = (b d) When r = 0, the birth rates and death rates are equal, and this is defined as Zero Population growth (ZPG). Combining the previous two equations gives the overall equation:

8 ΔN Δt = rn PALMS MODULE 2 LAB: FOR LOOPS IN C++ 8 If population growth is experienced under ideal conditions, the growth is referred to as exponential population growth because the curve of population size vs. time is a j-shaped curve that corresponds to the graph of an exponential function. Exponential growth appears in populations that have been drastically reduced and are now rebuilding their population. In the next exercise, we will use a modified form of this logic to simulate population growth. Exercise 2: Create a new C++ Project called PopulationGrowth. Add a new C++ code file called PopulationGrowth.cpp to the project. In this exercise you will write a program to compute the growth rates of a population of Lycans over time. Your program should take the following inputs from the user: Variable Meaning Range/Valid Values P Initial population size [1-1000] b Per capita birth rate [ ) d Per capita death rate [ ) All input values should be validated according to the rules above. Once the input is finished, perform your calculations for the next 50 years and display them to the screen. Your output report should have two columns of information: Year and Population Size. Remember that the first year is considered Year 0, not Year 1. Be aware that it is possible for the population to go extinct (i.e. have a population 0.005); at that point you must stop processing, close the report, and inform the user. The population in any year is a function of two things: the current population (i.e. last year s population) and the difference in per capita birth and death rates (r). The following equations may be handy: Difference in per capita birth and death rates: r = b d Calculating the new population for years 1-49: This Year s Population = (Last Year s Population) + (r (Last Year s Population)) Remember that the population at year 0 is the initial population. Your numerical output should be precise to two decimal places on the right of the decimal for all real-number data.

9 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 9 Compile and Build your program to verify it works as expected. Two sample output dialogs are provided. The first involves a continuously growing population and is provided below: Enter the initial population size (1-1000): 100 Enter the Birth Rate [ ): 0.5 Enter the Death Rate [ ): 0.4 YEAR POPULATION

10 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 10 The second case involves extinction and is shown below: Enter the initial population size (1-1000): 10 Enter the Birth Rate [ ): 0.4 Enter the Death Rate [ ): 0.6 YEAR POPULATION The population has gone extinct Part 3: The Game of Roulette The idea of Monte Carlo Methods involves processes whereby we can predict the behavior (on average) of a random event. The end result of the application of this methodology is a numerical probability (likelihood) that a given event will occur. For example, suppose we have an American roulette wheel. This version of a roulette wheel has 38 numbers on it (1-36, 0, and 00). The "slots" for each number are each colored according to the following chart:

11 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 11 Even or Odd Numbers......In Range......Has Color Odd 1-9 Red Odd Red Even Red Even Red N/A 0 Green N/A 00 Green All slots falling outside of these values are colored Black. Exercise 3: Create a C++ program which simulates "spins" of the roulette wheel. Allow the user to specify how many spins to perform, and print out a summary of how many times the colors Red, Green, and Black are "winners", i.e. the spin lands on these colors. Perform some tests of your own and watch the results. Your output should consist of three things: The total number of spins; The number of times each of Red, Green, and Black "win"; The winning percentage for each color. To simulate a die roll you can use the spin() function, as shown below. Note that we are using the value 37 to represent 00. You will need to write this function into your code:

12 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 12 ///////////////////////////////////////////////////////////////////////////// // // FUNCTION NAME: spin // PARAMETERS: None // RETURN TYPE: int // PURPOSE: Returns a random integer in the range // //////////////////////////////////////////////////////////////////////////// int spin() static bool initialized = false; if (!initialized) // Set the seed value for generating random numbers. srand((unsigned)time(0)); // set the flag initialized = true; // This computes a random integer between 0 and 37 using the // Standard C++ rand() function. // Get the random number. int rand_num = rand(); // Convert the number to a number in the range [0 37]... rand_num = (rand_num % 38); return rand_num; Part 4: Vocabulary Words You will be assessed on your knowledge of the following terms and concepts as well as the C++ concepts discussed in this document. As a result, you are encouraged to review these terms in the preceding document. Boolean true or false. Conditions/Conditional Expressions expressions which evaluate to one of two possible values: true or false. Conditional Repetition the repeated execution of a statement or a sequence of statements.

13 PALMS MODULE 2 LAB: FOR LOOPS IN C++ 13 Counter-Driven Repetition (Loop) a loop whose number of repetitions is controlled by a counter. These loops always involve three components: initialization of the counter variable to a starting value; testing the counter variable for a terminating condition; and modifying the value of the counter variable to ensure an eventual end to the loop. Counter Variable A variable (usually an integer) which keeps track of the number of loop iterations. Infinite Loop Occurs when a conditional expression never becomes false, causing a loop to repeat forever. It appears to the user as a non-responsive or hanging program.. Iterations the cycles of a loop; in other words, the number of times the loop body is repeated. Loop In C++, a construct that repeats a series of statements. Loop Body the portion of a loop inside.

LAB: WHILE LOOPS IN C++

LAB: WHILE LOOPS IN C++ MODULE 2 JEFFREY A. STONE and TRICIA K. CLARK COPYRIGHT 2014 VERSION 4.0 PALMS MODULE 2 LAB: WHILE LOOPS IN C++ 2 Introduction This lab will provide students with an introduction