Agenda

1
Agenda

• Variables (Review)
• Example
• Input / Output
• Arithmetic Operations
• Casting
• Char as a Number (if time allow)

2
Why do We Need Variables?

• Computer programs manipulate data
• Data is given as input or calculated throughout
  the program
• To be later accessed, variables must be
  remembered
• Thus, variables are stored in the memory
• Variable name ? memory address

3
What is a Variable?

• A memory chunk that is linked to a name
• Stores data (of a predefined type)
• Read/write
• Definition type name
• Assignment name value
• Definition assignment type name value

4
Why do we Need Different Types?

• Saving memory
• Execution speed
• Makes compiler life easier

5
Data representation

• Bit a binary digit 0 or 1
• Bits are always grouped!
• Byte a group of 8 bits
• Therefore, a byte can represent up to 28256
  values
• The values can range from 0 to 255 or from -128
  to 127
• The fundamental data unit of a computer
• Word a group of (usually) 4 (or 8) bytes
• 4 bytes 32 bits
• Value range 0 to 232 -1 (4,294,967,295 )
• Or, more often -231 to 231-1 (-2,147,483,648 to
  2,147,483,647)

6
Data Types

• char a single byte character
• Integer
• short int (or short) usually 2 bytes (rarely
  used)
• int - usually 4 bytes
• long int (or long) 4 or 8 bytes (rarely used)
• Real
• float a single precision real number usually
  4 bytes
• double a double precision real number usually
  8 bytes
• long double - a double precision real number
  usually 8 bytes (rarely used)
• Signed vs. unsigned

7
Variable Naming Rules

• Letters, digits, underscores
• Legal i, CSE_5a, a_very_long_name_that_isnt_very_
  useful, fahrenheit
• Illegal
• 5a_CSE (first character cannot be a digit)
• my-var
• Case sensitive
• CSE_5a is different from cse_5a
• Variables names should have a meaning

8
Agenda

• Variables (Review)
• Example
• Input / Output
• Arithmetic Operations
• Casting
• Char as a Number (if time allow)

9
Example Swap Two Variables
/ Swap1 swaps two variables a,b using a
temporary variable / include ltstdio.hgt int
main() int a 2, b 5, temp temp a a
b b temp printf("a d, b
d\n",a,b) return 0
10
Example Swap Two Variables
How to swap two variables without using a
temporary variable?
11
Example Swap Two Variables
int a, b // User is requested to enter a and
b printf("Enter a ") scanf("d",
a) printf("Enter b ") scanf("d", b) a a
b // this way b is still "alive" b a - b //
a b - b a a a - b // a b - a
b printf("a d, b d\n", a, b)
12
Agenda

• Variables (Review)
• Example
• Input / Output
• Arithmetic Operations
• Casting
• Char as a Number (if time allow)

13
printf and scanf

• printf prints to the screen.
• Can also accept variables and print their values
• scanf gets values from the standard input and
  assigns to corresponding variables

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printf can Print Variable Values

• printf ("zd\n", z )
• The sequence d is a special sequence and is not
  printed
• It indicates to printf to print the value of an
  integer variable written after the printed string

15
scanf Gets Input from the User

• scanf("lf", var)
• Wait for the user to type in a double value, and
  store it in the variable var
• var address of var (we shall deal with it
  later on the course)
• To get 2 doubles from the user, use scanf("lf
  lf", var1, var2)

16
prinft/scanf Conversion Codes

• A ltconversion codegt in the printf/scanf string
  is replaced by a respective variable
• c a character
• d an integer, u an unsigned integer
• f a float
• lf a double
• g a nicer way to show a double (in printf)
• - the character (in printf)

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Example

• int a 2, b 3, c, e
• double h
• c a b
• printf(d\n,c) // 6
• printf(d d d\n,a,b,c) // 2 3 6
• printf(d d d\n,a,b,ab) // 2 3 6
• printf(d\n,23) // 6
• scanf(d lf, e, h)

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What does the means?

• scanf("d", var)
• var value of var
• var address of var
• Our intension is to insert the input to the vars
  cell in memory
• What happens if we discard the ?

19
Agenda

• Variables (Review)
• Input / Output
• Arithmetic Operations
• Casting
• Char as a Number (if time allow)

20
Arithmetic operators

• An operator is a mathematical action performed on
  constants and/or variables (operands)
• Some operators
• Braces ()
• Assignment
• Addition
• Subtraction -
• Multiplication
• Division /
• Modulo

21
Example Reverse a 3-digit Number
int number, newNumber int ones, tens,
hundreds printf("Please enter a 3-digit number
") scanf("d", number) // divide number to
ones, tens, hundreds ones number 10 tens
(number100)/10 hundreds number/100 //
calculate the new number newNumber ones 100
tens 10 hundreds printf("The reversed number
is d\n", newNumber)
22
Operations on Different Types

• Operation on two different types (e.g., 5 6.5)
• The result is of the generalized operand (e.g.,
  int is a float is a double)
• 5/2.0 ? 2.5
• When the operands are of the same type, the
  result is of that type as well
• 5/2 ? 2

23
Examples

• 3 4 7
• 3.0 4 7.0
• 3 / 4 0
• 3.0 / 4 0.75

24
Agenda

• Variables (Review)
• Input / Output
• Arithmetic Operations
• Casting
• Char as a Number (if time allow)

25
Casting (????)

• Sometimes it is desirable for a variable of one
  type to be considered as belonging to another in
  an operation
• We say the variable is cast to the new type
• Syntax (type)operation/variable

26
Example find whats wrong

• include ltstdio.hgt
• int main()
• int a 10
• int b 20
• printf("The average of d and d is d\n", a, b,
  (a b) (1 / 2))
• return 0

27
Alternatives

• include ltstdio.hgt
• int main()
• int a 10
• int b 20
• printf(Avg of d and d is g\n", a, b, (a b)
  ((double)1 / 2))
• printf(Avg of d and d is g\n", a, b, (a b)
  (1.0 / 2))
• // will the following work?
• printf(Avg of d and d is g\n", a, b, (a b)
  (double)(1 / 2))
• return 0

28
More Examples
int a 11 int b 20 double x 4.5 int y
x // implicit cast printf("y g\n",y) //
ERROR - do not relay on implicit cast in
printf printf("y d\n",y) printf ("The average
of d and d is g\n", a, b, ((ab)
(1.0/2))) printf ("The average of d and d is
d\n", a, b, (int)((ab) (1.0/2))) printf
("The average of d and d is d\n", a,b, (a
b)(1.0 / 2)) // ERROR printf ("The average of
d and d is d\n", a,b, (int)((a b)(1.0 /
2)))
29
Will this Work?

• include ltstdio.hgt
• int main()
• int a 10
• int b 20
• printf ("The average of d and d is d\n", a,b,
  (a b)(1.0 / 2))
• return 0

30
Summary on Implicit Cast (QA)

• int a 4.5 // implicit cast from double (4.5)
  to int (4), it is legal but might cause problem,
  thus usually results with a compilation warning
  (possible lose of data)
• double x 4 // implicit cast from int to
  double, no compilation warning since "int is a
  double" relation hold
• printf("g\n", a) // printf/scanf is "dumb", in
  this case there is no compilation warnings, but
  the output will be wrong
• printf("d \n", 2.5) // same here no warnings,
  wrong output

31
Agenda

• Variables (Review)
• Input / Output
• Arithmetic Operations
• Casting
• Char as a Number (if time allow)

32
Char is also a number!

• A char variable is used to store a text
  character
• Letters.
• Digits.
• Keyboard signs.
• Non-printable characters.
• But also small numbers (0 to 255 or -128 to 127).

33
Text as numbers

• Every character is assigned a numeric code
• There are different sets of codes
• ASCII (American Standard Code for Information
  Interchange) most common
• EBCDIC ancient, hardly used today
• Maybe others
• We will use ASCII
• The ASCII table.

34
More about character encoding

• most of the time, you don't care what the
  particular numbers are
• The table above shows only 128 characters (7
  bits). Some are non-printable
• Extended ASCII code contains 256 characters

35
More about character encoding

• ASCII code 0 (NULL character) is important we
  will see it again
• Note contiguous sets of numbers, upper case and
  lower case characters

36
Example of char as both a character and a small
number

• include ltstdio.hgt
• int main(void)
• char i 'b'
• printf("i as a character is c\n", i)
• printf("i as an integer is d\n", i)
• printf("The character after c is c\n", i, i
  1)
• return 0

37
Another example

• / Get the position of a letter in the abc /
• include ltstdio.hgt
• int main(void)
• char letter
• printf("Please enter a lowercase letter\n")
• scanf("c", letter)
• printf("The position of this letter in the abc
  is d\n", letter-'a'1)
• return 0

38
Exercise

• Write a program that accepts as input
• A lowercase letter
• and outputs
• The same letter in uppercase
• (e.g., if the input is g, the output should be
  G)

39
Solution

• / Convert a letter to uppercase /
• include ltstdio.hgt
• int main(void)
• char letter
• printf("Please enter a lowercase letter\n")
• scanf("c", letter)
• printf("This letter in uppercase is c\n",
  letter-'a'A)
• return 0