Computer Science I

Lab 3.0 - Conditionals

School of Computing
College of Engineering
University of Nebraska-Lincoln

This lab introduces conditionals control structures using the C programming language.

Prior to Lab

• Read and familiarize yourself with this handout.
• Read the required chapters(s) of the textbook as outlined in the course schedule.

Peer Programming Pair-Up

For students in the online section: you may complete the lab on your own if you wish or you may team up with a partner of your choosing, or, you may consult with a lab instructor to get teamed up online (via Zoom).

For students in the on campus section: your lab instructor will team you up with a partner.

To encourage collaboration and a team environment, labs are be structured in a peer programming setup. At the start of each lab, you will be randomly paired up with another student (conflicts such as absences will be dealt with by the lab instructor). One of you will be designated the driver and the other the navigator.

The navigator will be responsible for reading the instructions and telling the driver what to do next. The driver will be in charge of the keyboard and workstation. Both driver and navigator are responsible for suggesting fixes and solutions together. Neither the navigator nor the driver is "in charge." Beyond your immediate pairing, you are encouraged to help and interact and with other pairs in the lab.

Each week you should alternate: if you were a driver last week, be a navigator next, etc. Resolve any issues (you were both drivers last week) within your pair. Ask the lab instructor to resolve issues only when you cannot come to a consensus.

Because of the peer programming setup of labs, it is absolutely essential that you complete any pre-lab activities and familiarize yourself with the handouts prior to coming to lab. Failure to do so will negatively impact your ability to collaborate and work with others which may mean that you will not be able to complete the lab.

Lab Objectives & Topics

At the end of this lab you should be familiar with the following

• How basic control flow works
• When and how to use if, if-else, and if-else-if conditional statements in a program

Background

The default control flow in a typical program is sequential. That is, each statement is executed one after the other. However, we often have to make decisions based on program state or variable values. This is known as conditional control flow. C, like many programming languages, provides several control structures to alter the flow of control in a program based on the truth-value of some conditional statement.

An if-statement can be used to conditionally execute a block of code. If the condition in an if-statement evaluates to true the code block is executed, otherwise it is not.

if(x > 0) {
  printf("x is positive!\n");
  printf("the value of sqrt(x) = %f\n", sqrt(x));
}

An if-else statement can be used to execute one of two mutually exclusive code blocks. If the condition evaluates to true, then the first block is executed. Otherwise, if the condition is false, the second code block is executed.

if(x > 0) {
  printf("x is positive!\n");
  printf("the value of sqrt(x) = %f\n", sqrt(x));
} else {
  printf("x is not positive, cannot compute its square root\n");
}

An if-else-if statement can be used to execute one of several mutually exclusive statements. It also requires more than one conditional statement to determine which code block should be executed. The control flow of an if-else-if statement is such that the first condition that evaluates to true is executed. All subsequent statements are skipped. If no condition holds true, then the else code block is executed. However, much like the difference between an if-statement and an if-else statement, the final else block is optional.

if(x > 0) {
  printf("x is positive!\n");
  printf("the value of sqrt(x) = %f\n", sqrt(x));
} else if(x == 0) {
  printf("x is zero, its square root is zero\n");
} else {
  printf("x is not positive, its square root is complex:\n");
  printf("%f\n", sqrt(x*-1.0));
}

Yet another type of conditional control statement is a switch statement. In a switch statement, a single variable's value is used to determine which, among several provided cases gets executed.

switch(classLevel) {
  case 1:
    printf("Freshman");
    break;
  case 2:
    printf("Sophomore");
    break;
  case 3:
    printf("Junior");
    break;
  case 4:
    printf("Senior");
    break;
  default:
    printf("ERROR: Unknown value!");
    break;
}

In C, the switch statement can only be used on integer variables (or char variables since they are integers). Using switch statements can also be error prone. If you omit a break statement at the end of a case it results in fall through and unexpected results.

Compound Statements

Many logical statements require the use of logical operators that can be used to form more complex, compound statements. The three logical operators that we'll focus on are as follows.

Operator	C Syntax	Example
Negation	!	!(x > 0)
And	&&	(x != 0) && (y < 10)
Or	||	(x < 0) || (x >= 10)

• Negation: this is a unary operator (an operator that only acts on one operand) that flips the truth value of the statement or variable that it is applied to so that true becomes false and vice versa.

• Logical And: this is a binary operator that is applied to two logical expressions and evaluates to true if and only if both expressions are true

• Logical Or: this is a binary operator that is applied to two logical expressions and evaluates to true if at least one (or both) of the expressions are true

Logical operators are necessary for more complex statements such as checking for ranges of variable values. For example, to check if a variable x lies in the range [0, 10], you would need to use the logical and operator:

if( 0 <= x && x <= 10 ) {
  printf("x is in the range [0, 10]\n");
}

Activities

We have provided partially completed programs for each of the following activities. You will need to clone the Lab 03 project from Github using the URL: https://github.com/cbourke/CSCE155-C-Lab03. Refer to previous labs for a step-by-step process.

Tax Program

The federal income tax for a married couple filing jointly for 2021 is determined by the rules indicated in the following table.

If the AGI is over	But not over	Tax is:	Of the amount over
$0	$19,900	10%	$0
$19,901	$81,050	$1,990 + 12%	$19,900
$81,051	$172,750	$9,328 + 22%	$81,050
$172,751	$329,850	$29,502 + 24%	$172,750
$329,851	$418,850	$67,206 + 32%	$329,850
$418,851	$628,300	$95,686 + 35%	$418,850
$628,301	--	$168,933.50 + 37%	$628,300

In addition, the total tax is reduced by $2,000 for each child that a couple has but cannot make the tax liability less than zero (for the purposes of this exercise).

We have provided a partially completed program that reads in a user's Adjusted Gross Income (AGI) and prompts whether or not they have any children. If they do it prompts them for how many. Complete the program by computing the user's total tax based on the user's AGI the number of children they have.

Some example input/output results can be found in the following table. For example, if the user has an adjusted gross income of $150,000 and has 4 children, then their tax would be calculated as follows.

• Their AGI falls in the 3rd tax bracket and so would be $9,328 + 22% of the amount over $81,050 or

  $9,328 + 22% * ($150,000 - $81,050) = $24,497.00

• With 4 children, they have a $8,000 tax credit giving a new total of $16,497.00

AGI	Number of kids	Tax Total
$4,000	1	$0.00
$20,000	0	$2,002.00
$120,000	3	$11,897.00
$150,000	4	$16,497.00
$250,000	0	$48,042.00
$500,000	5	$114,088.50

Calculator Program

In this activity, you will implement a simple menu-based command line calculator. A partially completed program has been provided to you, calculator.c. The program prompts the user for two operands (values) and one of several choices for an operation to be performed on them. Your program should process the input and display the result of the chosen operation. Take care with the following edge cases:

• For division, you should check if the divisor, b is zero (division by zero is not defined). If it is, an appropriate error message should be output instead of a result and must include the keyword ERROR.

• For the logarithm operation, you should use the math library's log function, which is the natural logarithm (base e). To change to another base, use the formula:

  $$\log_a{(b)} = \frac{\ln{(b)}}{\ln{(a)}}$$

  In addition, you should check that both operands are positive. If one or both are not positive, output an appropriate error message.

Reminder: when compiling on CSE you may need to direct gcc that it needs to link the math library by giving it the -lm (link math) flag; for example:

gcc -lm calculator.c

On the CS50 IDE (ubuntu) you would include the flag at the end:

gcc calculator.c -lm

Complete the program and thoroughly test it.

Leap Years

Nearly every 4 years is a leap year in the Gregorian calendar. In a leap year, there are 366 days (adding February 29th) instead of the usual 365. A year is a leap year if it is divisible by 4. However, every year that is divisible by 100 is not a leap year unless it is divisible by 400. 2000, 2004, 2008 were leaps years but 2001, 2002, 2003 were not. 1900 was not a leap year; though it was divisible by 4 it was divisible by 100 but not 400.

We've provided a partially completed program, leapYear.c that tests whether or not various years are leap years.

1. Implement a conditional statement inside the isLeapYear() function to determine if the given year is a leap year or not. Return true (1) if it is, false (0) if it is not.

2. Compile and run your program: we've provided 3 hard-coded test cases. Fix any errors in your program until they all pass.

3. Using the provided code as an example, at least 3 more test cases to your program. Repeat your compile/run/test until they all pass.

Handin/Grader Instructions

1. Hand in your completed files:

   • taxes.c

   • calculator.c

   • leapYear.c

   through the online handin system. Be sure your program passes all tests to get credit.

2. Even if you worked with a partner, you both should turn in all files.

Advanced Activities (Optional)

1. Another conditional operator is the ternary if-then-else operator. It is often used to choose between two values. For example: int min = ( (a < b) ? a : b ); The syntax, X ? Y : Z is as follows: X is any conditional statement; if it evaluates to true, then the expression takes on the value Y; otherwise it takes on the value Z. Modify your programs to use this ternary operator where appropriate.

2. Change the calculator program as follows: add a menu option so that the user has the option to quit; then add a loop so that the program continues to print the menu. As long as the user performs an operation, it should continue until the user selects the quit option.