(7-2) Modular Programming II H&K Chapter 6. Instructor - Andrew S. O Fallon CptS 121 (February 22, 2019) Washington State University

Transcription

1 (7-2) Modular Programming II H&K Chapter 6 Instructor - Andrew S. O Fallon CptS 121 (February 22, 2019) Washington State University

2 Pitfalls of Pointers and Output Parameters Not assigning a pointer a valid memory address Improper use of the dereference or indirection operator *ptr = &variable // most likely incorrect ptr = number // intention is *ptr = number Overusing pointers Defining a function to compute too many results and breaking our notion of proper top-down design Overcomplicating the concept of a pointer Recall it s just a variable Others? 2

3 Functions with Output Params (1) Example problem: Write a function that prompts the reader for a date string in the form "mm/dd/yyyy." The function should return a valid month, day, and year as output parameters. Check date entered for validity, reprompting the user for a valid date string if necessary. 3

4 Functions with Output Params (2) Algorithm Pseudocode for get_date Set valid to true Do prompt user to enter date string scan date string into month, seperator1, day, separator2, year if (status of scan is not equal to 5) or (separator1 is not a '/') or (separator2 is not a '/') or (day <0) or (day > 31) or (month < 1) or (month > 12) or (year < 0) set valid to false Tell user that date string is invalid end if while not valid return day, month, year End get_date 4

5 Functions with Output Params (3) You Try It: Write get_date: void get_date(int *month, int *day, int *year) 5

6 Functions with Output Params (4) Possible solution void get_date(int *month, int *day, int *year) { int valid, status; char sep1, sep2; do { valid = 1; printf("please enter a date (mm/dd/yyyy): "); status = scanf("%d %c%d %c%d", month,&sep1,day,&sep2,year); if (status < 5 sep1!= '/' sep2!= '/' *day < 0 *day > 31 *month < 1 *month > 12 *year < 0 *year > 9999) { valid = 0; printf("the date string you entered is invalid.\n"); } } while (!valid); } 6

7 An Extended Example (1) Date arithmetic Write a program that asks the user to enter two dates, determines which date is before the other date, and finally determines the number of days that separate the two dates. Note: You may need to write helper functions 7

8 An Extended Example (2) Conceptualizing the date difference problem: Case 1: Dates are in different years days_to_end days_between_years days_from_beginning date1 date2 Case 2: Dates are in the same year 8 date1 date2

9 An Extended Example (3) Top-down design: Date arithmetic application day, month, year day1, day2, month1, month2, year1, year2 day1, day2, month1, month2, year1, year2 year day, month, year get_date Is_before day_diff is_leap _year print_ date day1,day2 month1,month2 year day, month year1, year2 day, month 9 day_diff_within_year days_to_end Days_between_years days_to_end

10 An Extended Example (4) main function int main(void) { char cont; int month1, month2, day1, day2, year1, year2; get_date(&month1, &day1, &year1); get_date(&month2, &day2, &year2); if (!is_before(month1,day1,year1, month2,day2,year2)) { if (!is_before(month2,day2,year2, month1,day1,year1)) /* dates are equal */ { print_date(month1,day1,year1); printf(" is the same date as "); print_date(month2,day2,year2); printf("!\n"); printf("there are 0 days between these dates.\n"); } 10

11 An Extended Example (5) main function (cont.): } else /* date2 is before date1 */ { print_date(month2,day2,year2); printf(" is before "); print_date(month1,day1,year1); printf("\n"); printf("there are %d days between these dates.\n", day_diff(month2,day2, year2,month1,day1, year1)); } 11

12 An Extended Example (6) main function (cont.): else /* date1 is before date2 */ { print_date(month1,day1,year1); printf(" is before "); print_date(month2,day2,year2); printf("\n"); printf("there are %d days between these dates.\n", day_diff(month1,day1,year1,month2,day2, year2)); } return(1); } /* end of main */ 12

13 An Extended Example (7) You Try It: Write the easy case: int day_diff_within_year(int day1, int month1, int day2, int month2, int year) Given these functions: /* days_in_month given an integer month code (1 = January, 2 = February,, 12 = December), this function returns the number of days in that month. */ int days_in_month(int month); /* is_leap_year returns 1 if year is a leap year, 0 otherwise. */ int is_leap_year(int year) 13

14 An Extended Example (8) You Try It: Write the other case: int day_diff(int month1, int day1, int year1, int month2, int day2, int year2): Given these functions: int day_diff_within_year(int day1, int month1, int day2, int month2, int year); int days_to_end(int month, int day, int year); int days_from_beginning(int month, int day, int year); int days_between_years(int year1, int year2); 14

15 An Extended Example (9) Notes Need for a specific function is not always clear in advance For example, the day_diff_within_year and days_in_month functions were not included in my preliminary top-down design Do not feel obligated to write out every function! A "stub" that displays a text message can be useful initially; refine gradually Do not be afraid to use the debugger 15

16 References J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (8 th Ed.), Addison- Wesley, 2016 P.J. Deitel & H.M. Deitel, C How to Program (7 th Ed.), Pearson Education, Inc.,

17 Collaborators Chris Hundhausen 17