Identifier Naming & Arithmetic Operators

Identifier Naming Guidelines

• Compliers may only recognize the first 31 characters of an identifier name
• Avoid using identifier names that only differ by case, as they can cause issues in bug detection
• Use meaningful identifiers for clarity
• Use all uppercase for constant macros, as defined using #define
  • KMS_PER_MILE serves as a constant
  • KmsPerMile or Kms_Per_Mile is a variable

Invalid Identifiers

• 1Letter is invalid because it begins with a number
• double and int are invalid because they are reserved words
• TWO*FOUR, joe’s, and age# are invalid because they contain disallowed characters like *, backtick, and #
• Age-of-cat is invalid because it uses a hyphen instead of an underscore

Valid Identifiers

• Age_of_person, taxRateY2k, and PrintHeading are valid

Arithmetic Operators: Shortcut Assignment

• Shortcut assignment operators combine an operation with an assignment
• Instead of writing a = a + 1, you can write a += 1

Arithmetic Operators: Prefix Form

• Prefix increment and decrement operators increment or decrement the variable before returning the resulting value

int a, b;
a = b = 10;
printf("%d\n", ++a); /* Prints 11 */
printf("%d\n", a);   /* Prints 11 */
printf("%d\n", --b);   /* Prints 9 */
printf("%d\n", b);   /* Prints 9 */

• If ++ comes before the variable, it increments before determining the result

Arithmetic Operators: Postfix Form

• Postfix increment and decrement operators return the original value of the variable before incrementing or decrementing it

int a, b;
a = b = 10;
printf("%d\n", a++);  /* Prints 10 */
printf("%d\n", a);    /* Prints 11 */
printf("%d\n", b--);  /* Prints 10 */
printf("%d\n", b);    /* Prints 9 */

Relational Operators

Relational Operator Example

Logical Operators

• && represents the AND operator
• || represents the OR operator
• ! represents the NOT operator

Logical Operator: AND

• AND returns true only when both operands are true

• Example: Salary < 100 && Age >= 35

Logical Operator: OR

• OR returns true if at least one operand is true

| Operand1 | Operand2 | Operand1 || Operand2 | | :------- | :------- | :----------------------- | | false| false| false| | false| true | true | | true | false| true | | true | true | true |

Logical Operator: NOT

• NOT negates the operand

Logical Operator Example

int age, height, weight;
age = 25;
height = 70;
weight = 145;

Operator Precedence and Associativity

Math Library (math.h)

• The C math library offers predefined math functions
• A function serves as a subprogram for tasks, accepting zero or more inputs (parameters) and producing zero or one output (return value)
• Include the math library using #include <math.h>
• To use function sqrt, use y = sqrt(x);

Common Math Functions

• sin(x)
• cos(x)
• tan(x)
• sqrt(x)
• pow(x, y)
• abs(x)
• log(x)
• log10(x)
• exp(x)
• fabs(x)
• floor(x)
• ceil(x)

Math Library Example: abs()

• abs(int): Returns the absolute value of an integer

int n = -1234, x;
x = abs(n); // x will be 1234

Math Library Example: floor()

• floor(double): Rounds down to the nearest integer

double n = 123.54, x;
x = floor(n); // x will be 123.0

Math Library Example: ceil()

• ceil(double): Rounds up to the nearest integer

double n = 123.54, x;
x = ceil(n); // x will be 124.0

Math Library Example: pow()

• pow(double x, double y): Calculates x to the power of y

double x = 3.0, y = 2.0, result;
result = pow(x, y); // result will be 9.0

Math Library Example: sqrt()

• sqrt(double): Calculates the positive square root

double n = 4, x;
x = sqrt(n); // x will be 2.0

Math Library Example: exp()

• exp(double): Calculates the exponential e to the power of x

double n = 4, x;
x = exp(n); // x will be e^4

Sample Problem: Quadratic Equation Roots

• Goal: Obtain the roots of a quadratic equation given its coefficients
• Equation: ax² + bx + c = 0
• Roots are determined as (-b ± √(b² - 4ac)) / (2a)

disc = pow(b, 2) - 4 * a * c;
root_1 = (-b + sqrt(disc)) / (2 * a);
root_2 = (-b - sqrt(disc)) / (2 * a);

Control Structures

• These control the flow of execution in programs
• Three primary control structures exist: sequential, selection, and repetition

Control Structures: Sequential Flow

• Statements are executed in the order they appear
• A compound statement or code block groups statements within { and }, dictating sequential flow

{
 Statement_1;
 Statement_2;
 Statement_3;
}

• The function body is a compound statement

C Control Structures: Selection

• Includes statements if, if...else, and switch

C Control Structures: Repetition

• Includes statements for, while, and do...while

Selection Control Structure

• Is a control structure that chooses among alternative program statements based on a condition
• A condition is an expression that is false (0) or true (1)

Conditions

• Operators: Relational and Logical
• < Less than
• > Greater than
• <= Less than or equal
• >= Greater than or equal
• == Equal
• != Not equal
• ! Not
• && AND
• || OR

if statement

• Is a selection of whether or not to execute a statement, which may be a single statement or a block

if Syntax

if (Expression)
 Statement

• The Statement can be a single, null, or block statement

Terminating Programs with if

int number;
printf("Enter a non-zero number ");
scanf("%d", &number);

if (number == 0) {
 printf("Bad input. Program terminated ");
 return; // or exit(1);
}
// otherwise continue processing

Writing English Conditions in C

• Ensure the C condition is logically equivalent to the English statement.
• Example

//valid
(x > z) && (y > z)
//invalid
x && y > z

• "x and y are greater than z"

Example if Statement

• Increase price by adding taxRate times price to the original price if taxCode is 'T'

if (taxCode == 'T')
 price = price + taxRate * price;

if-else Statement

• A selection of whether to execute statement A or statement B. Either can be a single statement or a block

if-else Syntax

if (Expression)
 Statement A
else
 Statement B

• Both Statement A and Statement B can be single, null, or block statements

if-else Alternatives

if (condition) {
 compound_statement_1 // if condition is true
} else {
 compound_statement_2 // if condition is false
}

Example if-else Statement

• If A is strictly between 0 and 5, set B equal to 1/A; otherwise, set B equal to A.

if (A > 0 && A < 5)
 B = 1 / A;
else
 B = A;

Example if-else Statement: Area & Circumference of a Circle

• Calculate area and circumference of a circle for non-negative radius

if (radius >= 0) {
 area = 2 * 3.14 * pow(radius, 2);
 cir = 2 * 3.14 * radius;
 printf("area=%f, circumference=%f \n", area, cir);
} else
 printf("ERROR! Negative values for radius are not allowed.\n");

Note on else Braces

• In some cases, braces for the else block can be omitted

Braces for else in an if-else Statement

• Braces can only be omitted when each clause contains a single statement

if (lastInitial <= 'K')
 volume = 1;
else
 volume = 2;
printf("Look it up in volume # %d of the phone book", volume);

Nested if Statements

• Used to code decisions with multiple alternatives.

if (x > 0)
 num_pos = num_pos + 1;
else
 if (x < 0)
 num_neg = num_neg + 1;
 else
 num_zero = num_zero + 1;

Comparison of Nested if and Sequences of ifs

• A sequence of if statements may be used instead of a nested if, but are less efficient
• All conditions are always tested in a sequence of ifs
• In a nested if, only the first condition is tested when x is positive

Multi-way Branching

if (Expression1)
    Statement1
else if (Expression2)
    Statement2
else if (ExpressionN)
    StatementN
else
    Statement N+1

• Exactly 1 of these statements will be executed

Example of Nested if Statements

if (creditsEarned >= 90)
 printf ("Fourth year student ");
else if (creditsEarned >= 60)
 printf ("Third year student ");
else if (creditsEarned >= 30)
 printf ("Second year student ");
else
 printf ("First year student ");

Example of describing an int value

• Display one word to describe the int value of the number as “Positive,” “Negative,” or “Zero”

if (number > 0)
 printf ("Positive");
else if (number < 0)
 printf ("Negative");
else
 printf ("Zero");

Nested if Statements Example

• Pollution index classification

if (index < 35)
 printf ("Pleasant");
else if (index <= 60)
 printf ("Unpleasant");
else
 printf ("Health Hazard");

Example: Nested If Statement (with char grade)

float score;
Char grade;
scanf("%f", &score);
if(score>=0 && score= 85 )
  grade='A';
 else if ( score >= 75 )
  grade='B';
 else if ( score >= 65 )
  grade='c';
 else if ( score >= 50 )
  grade='D';
 else
  grade='F';
 printf("grade is %c", grade);
else printf("ERROR! Wrong score.");

Common if Statement Errors

• No parentheses around the condition causes a syntax error

if crsr_or_frgt == 'C' // Error
 printf("Cruiser\n");

• A semicolon after the condition is interpreted as doing nothing if the condition is true.

if (crsr_or_frgt == 'C'); // Error
 printf("Cruiser\n");

switch statement

• Usually used instead of nested if for selecting multiple alternatives
• Useful when selection is based on a single variable or simple expression
• The controlling expression is of type integer or character, not floating-pointing
• Example: Determine a student's grade given their score

switch Statement Selection Flow

graph TD
    A[expression] --> B{case value1}
    B -- true --> C[statements1]
    C --> D[break]
    B -- false --> E{case value2}
    E -- true --> F[slatements2]
    F --> G[break]
    E -- false --> H{case valuen}
    H -- true --> I[statementsn]
    I --> J[break]
    H -- false --> K[default]
    K --> L[statements]

switch Syntax

switch (controlling expression) {
 case value1: statements 1;
  break;
 ...
 case valueN: statements n;
  break;
 default: statements;
}

Example: switch Statement with char Type

#include 
int main(void) {
    char classID;
    printf("Enter class id [b, c, d, or f]: ");
    scanf("%c", &classID);
    switch (classID) {
        case 'B':
        case 'b':
            printf("Battleship\n");
            break;
        case 'C':
        case 'c':
            printf("Cruiser\n");
            break;
        case 'D':
        case 'd':
            printf("Destroyer\n");
            break;
        case 'F':
        case 'f':
            printf("Frigate\n");
            break;
        default:
            printf("Unknown ship class %c\n", classID);
    }
    return 0;
}

Explanation of the switch Example

• The value of the variable class is compared to each case in a top-down approach
• Execution stops when the first case is found
• Execution starts and continues until a break statement is encountered, or the switch statement ends
• If no case is matched, the default case (if provided) is executed

switch Statement Example 2

char color;
printf("Enter a color: ");
scanf("%c", &color);

switch (color) {
    case 'R':
        printf("red\n");
        break;
    case 'B':
        printf("blue\n");
        break;
    case 'Y':
        printf("yellow\n");
        break;
}

switch Statement Example 3

char grade;
printf("Enter your letter grade: ");
scanf("%c", &grade);

switch (grade) {
    case 'A':
        printf(" Excellent Job");
        break;
    case 'B':
        printf(" Very Good “);
        break;
    case 'C':
        printf(" Not bad ");
        break;
    case 'F':
        printf("Failing");
        break;
    default:
        printf(" Wrong Grade");
}

switch Statement Example 4

• Program asks for brightness of a light bulb (Watts) & prints expected lifetime

int bright;
printf("Enter the bulb brightness: ");
scanf("%d", &bright);
switch (bright) {
    case 25:
        printf(" Expected Lifetime is 2500 hours");
        break;
    case 40:
    case 60:
        printf("Expected Lifetime is 1000 hours“);
        break;
    case 75:
    case 100:
        printf("Expected Lifetime is 750 hours ");
        break;
    default:
        printf("Wrong Input ");
}

Key Reminders for Switch Statements

• Statements following a case label can be one or more C statements without braces
• Strings cannot be used as case labels
• double type values cannot be case labels
• Forgetting the break statement is a common error, leading to fall-through into the next alternative
• Forgetting the closing brace of the switch statement body is easy to do

Nested-if versus switch

• Advantages of if:
  • More general than a switch,
  • Operates on ranges of values (e.g., x 100
  • Can't compare doubles
• Advantages of switch:
  • Switch is more readable
• Use the switch whenever there are ten or fewer case labels

switch versus if-else

• Example 5: Prompt user for two integer values, to choose a calculation

 USING IF-ELSE STATEMENT
#include
void main()
{ int x,y,result; char op;
  printf(“please enter the first number:”);     scanf(“%d”,&x);
  printf(“please enter the second number:”); scanf(“%d”,&y);
  printf(“please enter an operation(+ - * /):”); scanf(“%c”,&op);
  if (op==‘+’)
             result=x+y;
    else if (op==‘-’)
             result=x-y;
    else if (op==‘*’)
             result=x*y;
    else if (op==‘/’)
            result=x/y;
    printf(“result= %d:”, result);
}

  USING SWITCH STATEMENT
#include
void main()
{
  int x, y, result;
  char op;
    printf(“please enter the first number:”);    scanf(“%d”,&x);
    printf(“please enter the second number:”); scanf(“%d”,&y);
    printf(“please enter an operation(+ - * /):”);scanf(“%c”,&p);
    switch(op)
     {
       case ‘+’: result=x+y; break;
       case ‘-’: result=x-y; break;
       case ‘*’: result=x*y; break;
       case ‘/’: result=x/y; break;
      }
    printf(“result= %d:”,result);
}

Common Programming Errors

• Consider the Statement if (0 <= x <= 4)
  • This is always evaluated to true in C
  • First 0 > You must use == for comparison

More Common Errors:

• Parenthesizing the condition.
• The { and } if they are needed
• In switch statements, make sure the controlling expression and case labels are of the same permitted type.
• Remember to include the default case for switch statements.
• Don't forget your { and } for the switch statement.
• Avoid forgetting the break statements!!!