Introduction to Computers and Programming

1
Introduction to Computers and Programming

• Class 9
• Introduction to C
• Professor Avi Rosenfeld

2
Loop Structures

• Event Controlled
• While Loops
• Do While Loops
• Counter Controlled
• Counter while loops
• For loops (next time)

3
Parts of a while Loop

• Every while loop will always contain three main
  elements
• Priming initialize your variables
• Testing test against some known condition
• Updating update the variable that is tested

4
Infinite Loop

• Infinite Loop A loop that never ends
• Generally, you want to avoid these!
• There are special cases, however, when you do
  want to create infinite loops on purpose
• Common Exam Questions
• Given a piece of code, identify the bug in the
  code
• You may need to identify infinite loops

5
Infinite Loop Example 1

• include ltstdio.hgt
•  
• main ()
• int index 1
•   while (index lt 10)
• printf ("Index d\n", index)

Index 1 Index 1 Index 1 Index 1 Index
1 forever
Here, I have deleted part 3 the index index
1 statement.
6

• include ltstdio.hgt
• void main()
• int number,EvenNumbers 0, OddNumbers 0
• printf("\nEnter a number between 0 9 inclusive
  (999 to end program) ")
• scanf("d", number) / why is this scanf
  needed? /
• while (number ! 999)
• switch (number)
• case 0 case 2 case 4 case 6 case 8
• EvenNumbers /what is the ? /
• printf("You entered a valid EVEN ")
• break
• case 1 case 3 case 5 case 7 case 9
• OddNumbers
• printf("You entered a valid ODD ")
• break
• default

7

• include ltstdio.hgt
• void main()
• int number,EvenNumbers 0, OddNumbers 0
• do
• printf("\nEnter a number between 0 9
  inclusive (999 to end program) ")
• scanf("d", number)
• switch (number)
• case 0 case 2 case 4 case 6 case 8
• EvenNumbers
• printf("You entered a valid EVEN ")
• break
• case 1 case 3 case 5 case 7 case 9
• OddNumbers
• printf("You entered a valid ODD ")
• break
• default

8
Shortcuts

• C provides abbreviations for some common
  operations
• Assignment operators
• Increment/Decrement operators

9
Assignment Operators

• Abbreviations are provided for the basic binary
  operations
• Addition
• Subtraction
• Multiplication
• Division
• Modulus ()

10
Shortcut Heaven (or Hell?)

• include ltstdio.hgt
• void main()
• int x 2, y 3
• x 3 / same as x x 3 x is 6 /
• printf("X is now d\n", x)
• y / 2 / same as y y / 2 y is 1 /
• printf("Y is now d\n", y)
• y 2 / same as y y 2 y is 3 /
• printf("Y is now d\n", y)
• x - 2 / same as x x - 2 x is 4 /
• printf("X is now d\n", x)
• x y / same as x x y x is 1 /
• printf("X is now d\n", x)

11
Samples

• Assume variable int c 10

Operator Initial Value Sample Meaning Assigns
10 c 7 c c 7 17 to c
- 10 c - 3 c c 3 7 to c
10 c 6 c c 6 60 to c
/ 10 c / 5 c c / 5 2 to c
10 c 6 c c 7 3 to c
12
Increment and Decrement Operators

• C provides unary increment operators and
  decrement operators - (no spaces between them)
• Increment operators add 1
• Decrement operators subtract 1
• Not for other assignment operators (, /, )
• Guess where C comes from???

13
Postincrement

• The operator is after the variable
• Causes the initial value of the variable to be
  used in the expression where it appears AND THEN
  adds the 1 to the variable
• For example,
• int iCount 5
• printf( d\n, iCount )
• Would print 5 but iCount is worth 6 after the
  statement

14
Preincrement

• The operator is before the variable
• Adds 1 to the initial value of the variable
  BEFORE it is used in the expression where it
  appears
• For example,
• int iCount 5
• printf( d\n, iCount)
• Would print 6 and iCount is worth 6 after the
  statement

15
More examples postincrement vs. preincrement

• int iTotal 0
• int iCount 5
• iTotal iCount 2
• printf( d\n, iTotal)
• printf( d\n, iCount)
• Would print 10 for iTotal and then 6 for iCount
• iTotal iCount 2
• printf( d\n, iTotal)
• printf( d\n, iCount)
• Would print 12 for iTotal and then 6 for iCount

16
Decrement Operator --

• Similar to increment in syntax/operation
• Instead of writing
• iCount iCount - 1 or
• iCount - 1
• You can write
• iCount-- or --iCount
• Subtle difference between the two new options
  call postdecrement and predecrement

17
Postdecrement

• The -- operator is after the variable
• Causes the initial value of the variable to be
  used in the expression where it appears AND THEN
  subtracts the 1 from the variable
• For example,
• int iCount 5
• printf( d\n, iCount-- )
• Would print 5 but iCount is worth 4 after the
  statement

18
Predecrement

• The -- operator is before the variable
• Subtracts 1 from the initial value of the
  variable BEFORE it is used in the expression
  where it appears
• For example,
• int iCount 5
• printf( d\n, --iCount)
• Would print 4 and iCount is worth 4 after the
  statement

19
For our last trick

• include ltstdio.hgt
• void main()
• int x 2, y 3
• printf("What is this number d\n", x y)
• printf("But x is still d and y is d\n", x, y)
• printf(What d ???\n", y / x-- x y)
• / 0 /
• printf("But x is still d and y is d\n", x, y)

20
Summary Table
Operator Sample Explanation
c Use value of c in expression THEN add 1 to c
c Add 1 to c THEN use the new value of c in expression
-- c-- Use value of c in expression THEN subtract 1 from c
-- --c Subtract 1 from c THEN use new value of c in expression
21
Introducing for loops

• include ltstdio.hgt
• void main()
• int counter 100
• while (counter--gt 0)
• printf("The counter is at d ", counter)