Incremental Programming Topics Review of Incremental Programming Example

Incremental Programming Topics • Review of Incremental Programming • Example of Incremental Programming Reading • None. CMSC 104, Lecture 19 1

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, o Don’t write the whole program at once. o Just write enough to display the user prompt on the screen. o Get that part working first (compile and run). o Next, write the part that gets the value from the user, and then just print it out. CMSC 104, Lecture 19 2

Increment Programming Review (con’t) o o o Get that working (compile and run). Next, change the code so that you use the value in a calculation and print out the answer. Get that working (compile and run). Continue this process until you have the final version. Get the final version working. • Bottom line: Always have a working version of your program! CMSC 104, Lecture 19 3

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. CMSC 104, Lecture 19 4

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 CMSC 104, Lecture 19 5

Report Design Interest rate : 7. 0000 % Period : 20 years Principal at start of period : 1000. 00 Interest accrued : 2869. 68 Total amount at end of period : 3869. 68 CMSC 104, Lecture 19 6

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); return 0; } CMSC 104, Lecture 19 7

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. CMSC 104, Lecture 19 8

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 thatn”) ; printf (“compounds interest annually. You will need to enter the amountn”) ; printf (“of the principal, the interest rate and the number of years. nn”) ; /* 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 = %dn”, principal, rate, years ) ; return 0 ; } CMSC 104, Lecture 19 9

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 : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 principal = 1000. 000000, rate = 0. 070000, years = 20 CMSC 104, Lecture 19 10

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* 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); CMSC 104, Lecture 19 11

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 = %dn”, principal, rate, years ) ; printf (“amount = %f, interest = %fn”); return 0 ; } CMSC 104, Lecture 19 12

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 : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 principal = 1000. 000000, rate = 0. 070000, years = 20 amount = 1070. 000000, interest = 70. 000000 CMSC 104, Lecture 19 13

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* 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); CMSC 104, Lecture 19 14

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 = %dn”, principal, rate, years ) ; printf (“amount = %f, interest = %fn”); return 0 ; } CMSC 104, Lecture 19 15

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 : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 principal = 1000. 000000, rate = 0. 070000, years = 20 amount = 1144. 900000, interest = 144. 900000 CMSC 104, Lecture 19 16

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* 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); CMSC 104, Lecture 19 17

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 = %dn”, principal, rate, years ) ; printf (“amount = %f, interest = %fn”); return 0 ; } CMSC 104, Lecture 19 18

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 : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 principal = 1000. 000000, rate = 0. 070000, years = 20 amount = 3869. 680000, interest = 2869. 680000 CMSC 104, Lecture 19 19

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* 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); CMSC 104, Lecture 19 20

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 : %. 4 f %%n”, 100 * rate ) ; printf (“ Period : %d yearsnn”, years ) ; printf (“ Principal at start of period : %9. 2 f”, principal ); printf (“ Interest accrued : %9. 2 f”, interest ); printf (“Total amount at end of period : %9. 2 f”, amount); return 0 ; } CMSC 104, Lecture 19 21

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 : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 Interest rate : 7. 0000 % Period : 20 years Principal at start of period : 1000. 00 Interest accrued : 2869. 68 Total amount at end of period : 3869. 68 CMSC 104, Lecture 19 22

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! CMSC 104, Lecture 19 23

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. CMSC 104, Lecture 19 24

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. CMSC 104, Lecture 19 25

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* Get principal from user */ printf (“Enter the principal amount : “); scanf (“%f”, &principal); /* Validate principal amount */ while ( principal < 0. 0 || principal > 100000. 0 ) { printf (“The principal amount must be a value between 0 and 100000n”) ; printf (“Enter the principal amount : “); scanf (“%f”, &principal); } CMSC 104, Lecture 19 26

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 : %. 4 f %%n”, 100 * rate ) ; printf (“ Period : %d yearsnn”, years ) ; printf (“ Principal at start of period : %9. 2 f”, principal ); printf (“ Interest accrued : %9. 2 f”, interest ); printf (“Total amount at end of period : %9. 2 f”, amount); } CMSC 104, Lecture 19 27

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 : 100000. 01 The principal amount must be between 0 and 100000. Enter the principal amount : -. 01 The principal amount must be between 0 and 100000. Enter the principal amount : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 Interest rate : 7. 0000 % Period : 20 years Principal at start of period : 1000. 00 Interest accrued : 2869. 68 Total amount at end of period : 3869. 68 CMSC 104, Lecture 19 28

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* Get valid principal amount from user */ printf (“Enter the principal amount : “); scanf (“%f”, &principal); while ( principal < 0. 0 || principal > 100000. 0 ) { printf (“The principal amount must be a value between 0 and 100000n”) ; printf (“Enter the principal amount : “); scanf (“%f”, &principal); } CMSC 104, Lecture 19 29

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 1n”) ; 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 ; } CMSC 104, Lecture 19 30

Improved Interest #2 (con’t) /* Calculate accrued interest */ interest = amount - principal ; /* Print report */ printf (“Interest rate : %. 4 f %%n”, 100 * rate ) ; printf (“ Period : %d yearsnn”, years ) ; printf (“ Principal at start of period : %9. 2 f”, principal ); printf (“ Interest accrued : %9. 2 f”, interest ); printf (“Total amount at end of period : %9. 2 f”, amount); } CMSC 104, Lecture 19 31

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 : 1000. 00 Enter the interest rate as a decimal (for 7% enter. 07) : 1. 0001 The interest rate must be between 0 and 1. Enter the interest rate as a decimal (for 7% enter. 07) : -. 0001 The interest rate must be between 0 and 1. Enter the interest rate as a decimal (for 7% enter. 07) : . 07 Enter the number of years : 20 Interest rate : 7. 0000 % Period : 20 years Principal at start of period : 1000. 00 Interest accrued : 2869. 68 Total amount at end of period : 3869. 68 CMSC 104, Lecture 19 32

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 thatn”); printf (“compounds interest annually. You will need to enter the amountn”); printf (“of the principal, the interest rate and the number of years. nn”); /* Get valid principal amount from user */ printf (“Enter the principal amount : “); scanf (“%f”, &principal); while ( principal < 0. 0 || principal > 100000. 0 ) { printf (“The principal amount must be a value between 0 and 100000n”) ; printf (“Enter the principal amount : “); scanf (“%f”, &principal); } CMSC 104, Lecture 19 33

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 1n”) ; printf (“Enter the interest rate as a decimal (for 7%% enter. 07) : “) ; scanf (“%f”, &rate); } /* 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, inclusiven”) ; printf (“Enter the number of years : “); scanf (“%d”, &years); } CMSC 104, Lecture 19 34

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 : %. 4 f %%n”, 100 * rate ) ; printf (“ Period : %d yearsnn”, years ) ; printf (“ Principal at start of period : %9. 2 f”, principal ); printf (“ Interest accrued : %9. 2 f”, interest ); printf (“Total amount at end of period : %9. 2 f”, amount); } CMSC 104, Lecture 19 35

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 : 1000. 00 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 The number of years must be between 1 and 100, inclusive. Enter the number of years : 20 Interest rate : 7. 0000 % Period : 20 years Principal at start of period : 1000. 00 Interest accrued : 2869. 68 Total amount at end of period : 3869. 68 CMSC 104, Lecture 19 36

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? CMSC 104, Lecture 19 37

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 (9 + 9) 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. CMSC 104, Lecture 19 38

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 ? 1000 * 3 • There must be a quicker and easier way. CMSC 104, Lecture 19 39

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 0, 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. CMSC 104, Lecture 19 40

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%dn”, i, j, k ) ; counter++ ; } } How long does this take ? 1000 ( 10 * 10 ) CMSC 104, Lecture 19 41

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 o 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. o Increment the counter. • Print the count • • CMSC 104, Lecture 19 42

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 CMSC 104, Lecture 19 43