1. Incremental Programming
Topics
Review of Incremental Programming
Example of Incremental Programming
Reading
None.

2. Incremental Programming Review
It is best not to take the “big bang” approach to coding.
Write your code in incomplete but working pieces.
For example, for your projects,
Don’t write the whole program at once.
Just write enough to display the user prompt on the screen.
Get that part working first (compile and run).
Next, write the part that gets the value from the user, and then just print it out.

3. Increment Programming Review (con’t)
Get that working (compile and run).
Next, change the code so that you use the value in a calculation and print out the answer.
Continue this process until you have the final version.
Get the final version working.
Bottom line: Always have a working version of your program!

4. Example of Incremental Programming
Problem:
Write an interactive program that allows the user to calculate the interest accrued on a savings account. The interest is compounded annually.
The user must supply the principal amount, the interest rate, and the number of years over which to compute the interest.

5. Rough Algorithm Print explanation of the program
Get <principal> from user
Get <interest rate> from user
Get <number of years> from user
<amount> = <principal>
While (<number of years> > 0 )
amount = amount + (amount X <interest rate>)
<number of years> = <number of year> + 1
End_while
<interest accrued> = <amount> - <principal>
Display report

6. Report Design Interest rate : 7.0000 % Period : 20 years
Principal at start of period :
Interest accrued : Total amount at end of period :

7. Version #1 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
int main ( ) {
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
return 0; }

8. Output #1
This program computes the interest accrued in an account that compounds interest annually. You will need to enter the amount of the principal, the interest rate and the number of years.

9. Version #2 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
int main ( ) {
float principal, rate ;
int years ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”) ;
printf (“compounds interest annually. You will need to enter the amount\n”) ;
printf (“of the principal, the interest rate and the number of years.\n\n”) ;
/* Get input from user */
printf (“Enter the principal amount : “) ;
scanf (“%f”, &principal) ;
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate) ;
printf (“Enter the number of years : “) ;
scanf (“%d”, &years) ;
printf (“\nprincipal = %f, rate = %f, years = %d\n”, principal, rate, years ) ;
return 0 ; }

10. Output #2
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
principal = , rate = , years = 20

11. Version #3 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
int main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get input from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
printf (“Enter the number of years : “);
scanf (“%d”, &years);

12. Version #3 (con’t)
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < 1 ; i++ ) {
amount += amount * rate ; }
/* Calculate accrued interest */
interest = amount - principal ;
printf (“\nprincipal = %f, rate = %f, years = %d\n”, principal, rate, years ) ;
printf (“amount = %f, interest = %f\n”);
return 0 ;

13. Output #3
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
principal = , rate = , years = 20
amount = , interest =

14. Version #4 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
int main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get input from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
printf (“Enter the number of years : “);
scanf (“%d”, &years);

15. Version #4 (con’t)
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < 2 ; i++ ) {
amount += amount * rate ; }
/* Calculate accrued interest */
interest = amount - principal ;
printf (“\nprincipal = %f, rate = %f, years = %d\n”, principal, rate, years ) ;
printf (“amount = %f, interest = %f\n”);
return 0 ;

16. Output #4
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
principal = , rate = , years = 20
amount = , interest =

17. Version #5 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
int main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get input from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
printf (“Enter the number of years : “);
scanf (“%d”, &years);

18. Version #5 (con’t)
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < years ; i++ ) {
amount += amount * rate ; }
/* Calculate accrued interest */
interest = amount - principal ;
printf (“\nprincipal = %f, rate = %f, years = %d\n”, principal, rate, years ) ;
printf (“amount = %f, interest = %f\n”);
return 0 ;

19. Output #5
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
principal = , rate = , years = 20
amount = , interest =

20. Final Version /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
int main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get input from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
printf (“Enter the number of years : “);
scanf (“%d”, &years);

21. Final Version (con’t)
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < years ; i++ ) {
amount += amount * rate ; }
/* Calculate accrued interest */
interest = amount - principal ;
/* Print report */
printf (“Interest rate : %.4f %%\n”, 100 * rate ) ;
printf (“ Period : %d years\n\n”, years ) ;
printf (“ Principal at start of period : %9.2f”, principal );
printf (“ Interest accrued : %9.2f”, interest );
printf (“Total amount at end of period : %9.2f”, amount);
return 0 ;

22. Final Output
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
Interest rate : %
Period : 20 years
Principal at start of period :
Interest accrued :
Total amount at end of period :

23. No “Big Bang,” Please!
We now have a working program that we are convinced is logically correct and that solves the problem.
Would we have this much confidence in our program if we had used the “big bang” approach? I doubt it!

24. Improvements to Interest
We now have a working program that we are convinced that is correct and solves the problem.
Our program is not robust, since none of the input provided by the user is checked for validity.
We need to decide what is valid.

25. Improvements to Interest
What range of values will we allow for the principal ? Positive values less than or equal to 100,000.
How about the interest rate ? Positive values less than 1.
And the numbers of years ? Years between 1 and 100.

26. Improved Interest #1 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get principal from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
/* Validate principal amount */
while ( principal < 0.0 || principal > )
printf (“The principal amount must be a value between 0 and \n”) ; }

27. Improved Interest #1 (con’t)
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
printf (“Enter the number of years : “);
scanf (“%d”, &years);
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < years ; i++ ) {
amount += amount * rate ; }
/* Calculate accrued interest */
interest = amount - principal ;
/* Print report */
printf (“Interest rate : %.4f %%\n”, 100 * rate ) ;
printf (“ Period : %d years\n\n”, years ) ;
printf (“ Principal at start of period : %9.2f”, principal );
printf (“ Interest accrued : %9.2f”, interest );
printf (“Total amount at end of period : %9.2f”, amount);

28. Interest : Testing Principal Validity Check
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
The principal amount must be between 0 and
Enter the principal amount : -.01
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
Interest rate : %
Period : 20 years
Principal at start of period :
Interest accrued :
Total amount at end of period :

29. Improved Interest #2 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get valid principal amount from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
while ( principal < 0.0 || principal > )
printf (“The principal amount must be a value between 0 and \n”) ; }

30. Improved Interest #2 (con’t)
/* Get valid interest rate from user */
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
while ( rate < 0.0 || rate > 1.0 ) {
printf (“The interest rate must be between 0 and 1\n”) ; }
printf (“Enter the number of years : “);
scanf (“%d”, &years);
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < years ; i++ )
amount += amount * rate ;

31. Improved Interest #2 (con’t)
/* Calculate accrued interest */
interest = amount - principal ;
/* Print report */
printf (“Interest rate : %.4f %%\n”, 100 * rate ) ;
printf (“ Period : %d years\n\n”, years ) ;
printf (“ Principal at start of period : %9.2f”, principal );
printf (“ Interest accrued : %9.2f”, interest );
printf (“Total amount at end of period : %9.2f”, amount); }

32. Interest : Testing Interest Rate Validity Check
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) :
The interest rate must be between 0 and 1.
Enter the interest rate as a decimal (for 7% enter .07) :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20
Interest rate : %
Period : 20 years
Principal at start of period :
Interest accrued :
Total amount at end of period :

33. Improved Interest #3 /* Filename: interest.c * Author: Sue Bogar
* Date written: 11/14//99
* Description: This program computes the interest accrued in an account
* that compounds interest annually */
#include <stdio.h>
main ( ) {
float principal, rate, amount, interest ;
int years, i ;
/* Print Instructions */
printf (“This program computes the interest accrued in an account that\n”);
printf (“compounds interest annually. You will need to enter the amount\n”);
printf (“of the principal, the interest rate and the number of years.\n\n”);
/* Get valid principal amount from user */
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
while ( principal < 0.0 || principal > )
printf (“The principal amount must be a value between 0 and \n”) ; }

34. Improved Interest #3 (con’t)
/* Get valid interest rate from user */
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate);
while ( rate < 0.0 || rate > 1.0 ) {
printf (“The interest rate must be between 0 and 1\n”) ; }
/* Get valid number of years from user */
printf (“Enter the number of years : “);
scanf (“%d”, &years);
while ( years < 1 || years > 100 )
printf (“The number of years must be between 1 and 100, inclusive\n”) ;

35. Improved Interest #3 (con’t)
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < years ; i++ ) {
amount += amount * rate ; }
/* Calculate accrued interest */
interest = amount - principal ;
/* Print report */
printf (“Interest rate : %.4f %%\n”, 100 * rate ) ;
printf (“ Period : %d years\n\n”, years ) ;
printf (“ Principal at start of period : %9.2f”, principal );
printf (“ Interest accrued : %9.2f”, interest );
printf (“Total amount at end of period : %9.2f”, amount);

36. Interest : Testing Years Validity Check
This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.
Enter the principal amount :
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 101
The number of years must be between 1 and 100, inclusive.
Enter the number of years : 0
Enter the number of years : 20
Interest rate : %
Period : 20 years
Principal at start of period :
Interest accrued :
Total amount at end of period :

37. Problem : Combinatorics
Combinatorics is the art of enumerating combinations and permutations.
Find and count all positive integers with three digits or less whose digits add up to a number specified by the user.
What constraints should we put on the sum of the digits?

38. Combinatorics: Thinking through the problem
The smallest number is 0, so the input should be 0 or greater.
The largest number is 999, so the input should be 27 ( ) or less.
We could have a single for loop that goes through the integers from 0 to 999.
For each integer, we’ll need to break the integer up into its individual digits, add them up and compare that sum to num.

39. Combinatorics: Thinking through the problem (con’t)
We’ve already learned the trick used to separate an integer into its digits. It involved using integer division and modulus within a loop. Each iteration of the loop stripped one digit off of the integer.
So within the for loop, we’ll need to have another loop that separates the integer into its digits, sums them, compares the sum to num and prints and counts the integer if they are equal.
This approach will solve the problem, but it takes a lot of work at each step and will take a long time How many steps ? * 3
There must be a quicker and easier way.

40. Combinatorics: Thinking through the problem (con’t)
Instead of working with the integers 0 through 999, we could work with each of the digits instead.
We could vary just one of the digits at a time using nested for loops, like this : , 0 0 1, … 0 0 9, 0 1 0, … 9 9 9
This will eliminate the need to split the integer into its digits.
Each for loop will control one of the digits.

41. Combinatorics: Thinking through the problem (con’t)
for ( i = 0 ; i < 10 ; i++) {
for ( j = 0 ; j < 10 ; j++)
for ( k = 0 ; k < 10 ; k++)
if ( i + j + k == num )
printf ( “%d%d%d\n”, i, j, k ) ;
counter++ ; }
How long does this take ? ( 10 * 10 * 10 )

42. Algorithm : Combinatorics
Print explanation
Get num from user
Validate value of num (0 <= num <= 27)
Vary 1 digit at a time from the digit sequence 000 to the digit sequence 999
If the sum of the digits is equal to num, print the 3 digits (or less) with no space between them. Don’t print leading 0’s.
Increment the counter.
Print the count

43. Combinatorics: Incremental Programming
Write the explanation, get it running
Get num from user and validate it, get this running.
Write the nested for loops that will print all three digits when they sum to num. (has leading 0’s) and print the count. Don’t forget to initialize count to 0 before beginning. Get this running.
Modify the code to eliminate leading 0’s