CS-2303, System Programming Concepts

1
Digression on Loop Invariants

• CS-2303, System Programming Concepts
• (Slides include materials from The C
  Programming Language, 2nd edition, by Kernighan
  and Ritchie and from C How to Program, 5th and
  6th editions, by Deitel and Deitel)

2
Definition Loop Invariant

• Something that is true at a certain point of each
  iteration of the loop
• Often at start of iteration
• E.g., a relationship of the variables
• Expressed as a logical statement called an
  assertion
• Needs to be preserved from one iteration to the
  next
• Does not necessarily remain true within loop
  body, but only at the specific point each time
  through

3
Loop Invariant

• Allows you to reason about a program
• and to convince yourself (and other people)
  that it is correct
• For many ordinary programs, you do this
  subconsciously
• For seriously difficult programs, you must write
  out the loop invariants!

4
Thinking Through the Calendar Assignment

• int startingDay / init for your year/
• for (int month 0 month lt 12 month)
• // for month

5
Calendar Assignment (continued)

• int month, date
• int startingDay / init from your year/
• for (month 0 month lt 12 month)
• // for month

At beginning of each iteration, startingDay
indicates the day of the week on which that
particular month starts. It is the
responsibility of the loop to update startingDay
for the next month.
This is the beginning of a Loop Invariant!
6
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek 0
• printf() //month name
• printf() //days of week
• // for month

7
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)
• // for month

8
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)
• // for month

9
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)

• for (int date 1 date lt
• daysInMonth date)
• // for date
• // for month

10
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)

• for (int date 1 date lt
• daysInMonth date)
• // for date
• // for month

11
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)

• for (int date 1 date lt
• daysInMonth date)
• printf("", date)
• if (dayOfWeekgt6)
• printf("\n")
• dayOfWeek 0
• // for date
• // for month

12
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)

• for (int date 1 date lt
• daysInMonth date)
• printf("", date)
• if (dayOfWeekgt6)
• printf("\n")
• dayOfWeek 0
• // for date
• // for month

13
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)

• for (int date 1 date lt
• daysInMonth date)
• printf("", date)
• if (dayOfWeekgt6)
• printf("\n")
• dayOfWeek 0
• // for date
• if (dayOfWeek ! 0)
• printf("\n")
• // for month

14
Calendar Assignment (continued)

• int month, date
• int startingDay / init from user input/
• for (month 0 month lt 12 month)
• const int daysInMonth
• / set of days /
• int dayOfWeek
• printf() //month name
• printf() //days of week
• for (dayOfWeek 0
• dayOfWeekltstartingDay
• dayOfWeek)
• printf(/blanks/)

• for (int date 1 date lt
• daysInMonth date)
• printf("", date)
• if (dayOfWeekgt6)
• printf("\n")
• dayOfWeek 0
• // for date
• if (dayOfWeek !0)
• printf("\n")
• startingDay dayOfWeek
• // for month

15
Questions?