Decisions in programs

Allow programs to follow different courses of action or 
execute different tasks depending on data values.

Flow-diagrams of decisions


		








Psuedocode

   If x is not zero		            if amount is positive
       Multiply product by x			process credit
				            else
						Process debit


1.   If angle measurement is in degrees
         Convert angle measurement to radians
2. Calculate height of projectile when it hits target.

1. if yard width is less than house width or yard 
      height is less than house width
           Display an error message and exit program
2. Calculate cutting time.

1. if coins value Is greater than 10 dollars
           Display a message "good saver"
   else
           Display a message " save more"

C provides two language constructs:

If statement and switch statement

If (condition)					If (condition)
{						{
     true statements			     		true statements
}						}
else
{ 
    false statements
}


condition has the form :

    expression rel-op expression      

expression is a variable, literal (constant), or 
general expression

rel-op is a symbol or symbol pair: <, <=, >, >= ("relational operators)
			     ==, != (not equal)  ("equality" operators)



A condition evaluates to true (non-zero in C) or false 
(zero in C).

(x < y),  (y + z >= 0),  (y != 0)

For x = 5, y = 12, z = -15, what are the values of the 3 
conditions above?

Other operators that can appear in conditions:

   Logical operators - && (and), || (or), and ! (not)


Evaluate:
	(x < y || y != 0)     ((y + z >= 0) && (y != 0))
       ( y != 0 && z <= y)

       ( !(x < y) && (y + z >= 0))
	( !((x < y) && (y + z >= 0)))

nested if statements can model decisions with several 
alternatives.

if student receives an exam score of 90 or higher
   Set grade to A
else if student receives an exam score of 80 or higher
   Set grade to B
else if student receives an exam score of 70 or higher
   Set grade to C
else if student receives an exam score of 60 or higher
   Set grade to D
else
   Set grade to F

if (score >= A_SCORE)
   grade = 'A';
else if (score >= B_SCORE) 
   grade = 'B';
else if (score >= C_SCORE)	
   grade = 'C';
else if (score >= D_SCORE)	
   grade = 'D';
else
   grade = 'F';

Notice that (score >= B_SCORE) could also be 
written as (score >= B_SCORE && score < A_SCORE) 
This condition contains an unnecessary comparison: score < A_SCORE

Write this as function.

_____  assign_grade(__________)
{
     _____  grade;   /* local variable */

     /* put if statement here */

     return _______;
}



Problem: Write a program that reads a student's exam 
score and determines the student's grade. Use 
function assign_grade and a display function that 
displays the score, grade, and exam scale. Also, use 
an instruct function to display the results.

Data requirements:

Inputs
_________
Outputs
_________

Algorithm:
1. Display user instructions
2.
3.
4.

Function instruct
Input arguments:		output:

Function assign_grade
Input arguments:          output:


Function display
Input arguments:          output:


How to use this function:

#define A_SCORE 90
#define B_SCORE 80
#define C_SCORE 70
#define D_SCORE 60

______ assign_grade(__________);

void instruct();

void display(______, _______);

void main()
{
     int score;
     char letter_grade;

    /* display instructions */
    _______(_____);

    printf("Enter student's exam score:");
    scanf("%d", &score);

    /* find letter grade */
    _______ = assign_grade(_______);

   /* display result */
   display(______, _______);
}


void instruct()
{
    printf("This is a simple grading program.\n");
    printf("You enter the student's score and \n");
    printf(" the program displays the grade."\n\n");
}

void display(int s, char l_g)
{
	printf("Student got a score of %d on the exam\n", 
          _____);
	printf("This is a grade of %c\n\n", _______);

	printf("Scale is A: %d and above\n",
		  _____);
	printf("         B: %d up to %d\n",
                _____, ______);
   	printf("         C: %d up to %d\n",
                _____, ______);
  	printf("         D: %d up to %d\n",
                _____, ______);
   	printf("         F: less than %d\n",
                _____);
}


void assign_grade(int score)
{
   char grade; 
   /* put if statement here. */
   if (______ >= A_SCORE)
      ______ = 'A';
   ...

    return ______;
}


This is a simple grading program.
You enter the student's score and
the program displays the grade.

Enter student's exam score:75
Student got a score of 75 on the exam
This is a grade of C

Scale is A: 90 and above
         B: 80 up to 90
         C: 70 up to 80
         D: 60 up to 70
         F: less than 60





Putting if statements in functions.

// Calculates hourly rate of pay for an employee
double calc_gross( double hours, double rate)
{
    double max_hours = 100.0;
    double max_reg_hours = 40.0;
    double overtime_rate = 1.5;
    double gross;

    if (hours <= 0.0)
    {
       gross = ERROR;     /* constant that indicates error */
       printf("error - negative value for hours\n");
   }
   else if (hours <= max_reg_hours)
       gross = hours * rate;
   else if (hours <= max_hours)
       gross = max_reg_hours * rate + 
          (hours - max_reg_hours) * overtime_rate * rate;
   else
   {
       gross = ERROR;
       printf("error - hours is too large\n");
   }

   return gross;
}

To call this function in a main function:

#define ERROR -1
double calc_gross(double, double);

int main(void)
{
    double hours, pay_rate;    /* inputs */
    double gross_pay, net_pay;   /* outputs */
   ...

   /* assume hours and pay_rate have been read --
         calculate gross_pay */

    gross_pay = calc_gross(hours, pay_rate);

   /* calculate net pay if gross_pay was calculated */
   if (gross_pay == ERROR)
       return ERROR;
   else
   {
       net_pay = calc_net(gross_pay);
       printf("Gross pay is $%f and net pay is $%f\n", 
               gross_pay, net_pay);
   }

   return 0;
}  /* end of main */