Data types in C programming

2. Objectives
• Be able to explain to others what a data type is
• Be able to use basic data types in C programs
• Be able to see the inaccuracies and limitations introduced
by machine representations of numbers
CoxSimple Data Types2

3. What do you see?
CoxSimple Data Types3

4. What do you see?
CoxSimple Data Types4

5. Last one
CoxSimple Data Types5

6. CoxSimple Data Types6
Everything is Just a Bunch of Bits
• Bits can represent many different things
• Depends on interpretation
• You and your program must keep track of what kind of data
is at each location in the computer’s memory
• E.g., program data types

7. CoxSimple Data Types7
Big Picture
• Processor works with finite-sized data
• All data implemented as a sequence of bits
• Bit = 0 or 1
• Represents the level of an electrical charge
• Byte = 8 bits
• Word = largest data size handled by processor
• 32 bits on most older computers
• 64 bits on most new computers

8. CoxSimple Data Types8
Data types in C
•Only really four basic types:
• char
• int (short, long, long long, unsigned)
• float
• double
•Size of these types on
•CLEAR machines:
•Sizes of these types
•vary from one machine
•to another!
Type Size (bytes)
char 1
int 4
short 2
long 8
long long 8
float 4
double 8

9. CoxSimple Data Types9
Characters (char)
• Roman alphabet, punctuation, digits, and other symbols:
• Encoded within one byte (256 possible symbols)
• ASCII encoding (man ascii for details)
• In C:
char a_char = ’a’;
char newline_char = ’n’;
char tab_char = ’t’;
char backslash_char = ’’;

10. CoxSimple Data Types10
ASCII
From “man ascii”:
| 0 NUL| 1 SOH| 2 STX| 3 ETX| 4 EOT| 5 ENQ| 6 ACK| 7 BEL|
| 8 BS | 9 HT | 10 NL | 11 VT | 12 NP | 13 CR | 14 SO | 15 SI |
| 16 DLE| 17 DC1| 18 DC2| 19 DC3| 20 DC4| 21 NAK| 22 SYN| 23 ETB|
| 24 CAN| 25 EM | 26 SUB| 27 ESC| 28 FS | 29 GS | 30 RS | 31 US |
| 32 SP | 33 ! | 34 " | 35 # | 36 $ | 37 % | 38 & | 39 ' |
| 40 ( | 41 ) | 42 * | 43 + | 44 , | 45 - | 46 . | 47 / |
| 48 0 | 49 1 | 50 2 | 51 3 | 52 4 | 53 5 | 54 6 | 55 7 |
| 56 8 | 57 9 | 58 : | 59 ; | 60 < | 61 = | 62 > | 63 ? |
| 64 @ | 65 A | 66 B | 67 C | 68 D | 69 E | 70 F | 71 G |
| 72 H | 73 I | 74 J | 75 K | 76 L | 77 M | 78 N | 79 O |
| 80 P | 81 Q | 82 R | 83 S | 84 T | 85 U | 86 V | 87 W |
| 88 X | 89 Y | 90 Z | 91 [ | 92 | 93 ] | 94 ^ | 95 _ |
| 96 ` | 97 a | 98 b | 99 c |100 d |101 e |102 f |103 g |
|104 h |105 i |106 j |107 k |108 l |109 m |110 n |111 o |
|112 p |113 q |114 r |115 s |116 t |117 u |118 v |119 w |
|120 x |121 y |122 z |123 { |124 | |125 } |126 ~ |127 DEL|
Special
control
characters

11. CoxSimple Data Types11
Characters are just numbers
•What does this function do?
Char
fun(char c)
{
char new_c;
if ((c >= ’A’) && (c <= ’Z’))
new_c = c - ’A’ + ’a’;
else
new_c = c;
return (new_c);
}
return type
procedure
name
argument type
and name
local variable
type and name
Math on
characters!
comparisons
with characters!

12. CoxSimple Data Types12
FP vs. Integer Results
int i = 20 / 3;
float f = 20.0 / 3.0;
True mathematical answer: 20  3 = 6 2/3
i = ?
f = ?
6
6.666667
Integer division ignores remainder
FP arithmetic rounds result

13. CoxSimple Data Types13
Integer Representations
Base 2 Base 16 Unsigned 2’s Comp.
0000 0 0 0
0001 1 1 1
0010 2 2 2
0011 3 3 3
0100 4 4 4
0101 5 5 5
0110 6 6 6
0111 7 7 7
1000 8 8 -8
1001 9 9 -7
1010 A 10 -6
1011 B 11 -5
1100 C 12 -4
1101 D 13 -3
1110 E 14 -2
1111 F 15 -1
0000 0001
0010
0011
0100
0101
0110
011110001001
1011
1100
1101
1010
1110
1111
0
1
2
3
4
5
6
789
A
D
C
B
E
F
0 1
2
3
4
5
6
789
10
11
12
13
14
15
0 1
-8
3
4
5
6
7
2
-7
-6
-5
-4
-3
-2
-1
Why one more negative than positive?

14. CoxSimple Data Types14
Integer Representations
Math for n bits:
Define




1
0
2)(2
n
i
i
i
xxUB







2
0
1
1
22)(2
n
i
i
i
n
n
xxxTB

01 xxx n 


sign bit
0=non-negative
1=negative
Base 2 Base 16 Unsigned 2’s Comp.
0000 0 0 0
0001 1 1 1
0010 2 2 2
0011 3 3 3
0100 4 4 4
0101 5 5 5
0110 6 6 6
0111 7 7 7
1000 8 8 -8
1001 9 9 -7
1010 A 10 -6
1011 B 11 -5
1100 C 12 -4
1101 D 13 -3
1110 E 14 -2
1111 F 15 -1

15. 15
Memory Layout of Records
initials
(2 bytes)
married
(1 byte)
ss_number
(4 bytes)
• Fields are stored adjacently in memory.
• Memory is allocated for records based on
the order the fields are created.
• Variables are aligned for easy reference.
initials
(2 bytes)
ss_number
(4 bytes)
married
(1 byte)
Optimized for space Optimized for memory alignment
4 bytes / 32 bits 4 bytes / 32 bits