Chapter 12 Variables and Operators

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Chapter 12 Variables and

Operators

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12-2

Basic C Elements

Variables

• named, typed data items

Operators

• predefined actions performed on data items

• combined with variables to form expressions, statements

Rules and usage

Implementation using LC-3

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12-3

Data Types

C has three basic data types

int integer (at least 16 bits)

double floating point (at least 32 bits) char character (at least 8 bits)

Exact size can vary, depending on processor

• int is supposed to be "natural" integer size;

for LC-3, that's 16 bits -- 32 bits for most modern processors

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Variable Names

Any combination of letters, numbers, and underscore (_)

Case matters

• "sum" is different than "Sum"

Cannot begin with a number

• usually, variables beginning with underscore are used only in special library routines

Only first 31 characters are used

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Examples

Legal

iwordsPerSecond words_per_second _green

aReally_longName_moreThan31chars

aReally_longName_moreThan31characters

Illegal

10sdigit ten'sdigit done?

double

reserved keyword

same identifier

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Literals

Integer

123 /* decimal */

-123

0x123 /* hexadecimal */

Floating point 6.023

6.023e23 /* 6.023 x 10²³ */

5E12 /* 5.0 x 10¹² */

Character 'c'

'\n' /* newline */

'\xA' /* ASCII 10 (0xA) */

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12-7

Scope: Global and Local

Where is the variable accessible?

Global: accessed anywhere in program

Local: only accessible in a particular region

Compiler infers scope from where variable is declared

• programmer doesn't have to explicitly state

Variable is local to the block in which it is declared

• block defined by open and closed braces { }

• can access variable declared in any "containing" block

Global variable is declared outside all blocks

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Example

#include <stdio.h>

int itsGlobal = 0;

main() {

int itsLocal = 1; /* local to main */

printf("Global %d Local %d\n", itsGlobal, itsLocal);

{

int itsLocal = 2; /* local to this block */

itsGlobal = 4; /* change global variable */

}

Output

Global 0 Local 1 Global 4 Local 2 Global 4 Local 1

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12-9

Operators

Programmers manipulate variables

using the operators provided by the high-level language.

Variables and operators combine to form expressions and statements

which denote the work to be done by the program.

Each operator may correspond to many machine instructions.

• Example: The multiply operator (*) typically requires multiple LC-3 ADD instructions.

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Expression

Any combination of variables, constants, operators, and function calls

• every expression has a type,

derived from the types of its components (according to C typing rules)

Examples:

counter >= STOP x + sqrt(y)

x & z + 3 || 9 - w-- % 6

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Statement

Expresses a complete unit of work

• executed in sequential order

Simple statement ends with semicolon

z = x * y; /* assign product to z */

y = y + 1; /* after multiplication */

; /* null statement */

Compound statement groups simple statements using braces.

• syntactically equivalent to a simple statement

{ z = x * y; y = y + 1; }

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12-12

Operators

Three things to know about each operator (1) Function

• what does it do?

(2) Precedence

• in which order are operators combined?

• Example:

"a * b + c * d" is the same as "(a * b) + (c * d)"

because multiply (*) has a higher precedence than addition (+)

(3) Associativity

• in which order are operators of the same precedence combined?

• Example:

"a - b - c" is the same as "(a - b) - c"

because add/sub associate left-to-right

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Assignment Operator

Changes the value of a variable.

x = x + 4;

1. Evaluate right-hand side.

2. Set value of left-hand side variable to result.

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Assignment Operator

All expressions evaluate to a value,

even ones with the assignment operator.

For assignment, the result is the value assigned.

• usually (but not always) the value of the right-hand side

type conversion might make assigned value different than computed value

Assignment associates right to left.

y = x = 3;

y gets the value 3, because (x = 3) evaluates to the value 3.

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Arithmetic Operators

Symbol Operation Usage Precedence Assoc

* multiply x * y ⁶ ^l-to-r

/ divide x / y ⁶ ^l-to-r

% modulo x % y ⁶ ^l-to-r

+ addition x + y ⁷ ^l-to-r

- subtraction x - y ⁷ ^l-to-r

All associate left to right.

* / % have higher precedence than + -.

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12-16

Arithmetic Expressions

If mixed types, smaller type is "promoted" to larger.

x + 4.3

if x is int, converted to double and result is double

Integer division -- fraction is dropped.

x / 3

if x is int and x=5, result is 1 (not 1.666666...)

Modulo -- result is remainder.

x % 3

if x is int and x=5, result is 2.

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Bitwise Operators

~ bitwise NOT ~x ⁴ ^r-to-l

<< left shift x << y ⁸ ^l-to-r

>> right shift x >> y ⁸ ^l-to-r

& bitwise AND x & y ¹¹ ^l-to-r

^ bitwise XOR x ^ y ¹² ^l-to-r

| bitwise OR x | y ¹³ ^l-to-r

Operate on variables bit-by-bit.

• Like LC-3 AND and NOT instructions.

Shift operations are logical (not arithmetic).

Operate on values -- neither operand is changed.

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Logical Operators

! logical NOT !x ⁴ ^r-to-l

&& logical AND x && y ¹⁴ ^l-to-r

|| logical OR x || y ¹⁵ ^l-to-r

Treats entire variable (or value)

as TRUE (non-zero) or FALSE (zero).

Result is 1 (TRUE) or 0 (FALSE).

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Relational Operators

> greater than x > y ⁹ ^l-to-r

>= greater than or equal x >= y ⁹ ^l-to-r

< less than x < y ⁹ ^l-to-r

<= less than or equal x <= y ⁹ ^l-to-r

== equal x == y ¹⁰ ^l-to-r

!= not equal x != y ¹⁰ ^l-to-r

Result is 1 (TRUE) or 0 (FALSE).

Note: Don't confuse equality (==) with assignment (=).

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Special Operators: ++ and --

Changes value of variable before (or after) its value is used in an expression.

Symbol Operation Usage Precedence Assoc

++ postincrement x++ ² ^r-to-l

-- postdecrement x-- ² ^r-to-l

++ preincrement ++x ³ ^r-to-l

<= predecrement --x ³ ^r-to-l

Pre: Increment/decrement variable before using its value.

Post: Increment/decrement variable after using its value.

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Using ++ and --

x = 4;

y = x++;

Results: x = 5, y = 4

(because x is incremented after assignment)

x = 4;

y = ++x;

Results: x = 5, y = 5

(because x is incremented before assignment)

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Practice with Precedence

Assume a=1, b=2, c=3, d=4.

x = a * b + c * d / 2; /* x = 8 */

same as:

x = (a * b) + ((c * d) / 2);

For long or confusing expressions,

use parentheses, because reader might not have memorized precedence table.

Note: Assignment operator has lowest precedence, so all the arithmetic operations on the right-hand side are evaluated first.

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Symbol Table

Like assembler, compiler needs to know information associated with identifiers

• in assembler, all identifiers were labels and information is address

Compiler keeps more information Name (identifier)

Type

Location in memory Scope

Name Type Offset Scope

amount hours minutes rate seconds time

int int int int int int

0 -3 -4 -1 -5 -2

main main main main main main

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Local Variable Storage

Local variables are stored in an

activation record, also known as a stack frame.

Symbol table “offset” gives the

distance from the base of the frame.

• R5 is the frame pointer – holds address of the base of the current frame.

• A new frame is pushed on the

run-time stack each time a block is entered.

• Because stack grows downward,

base is the highest address of the frame, and variable offsets are <= 0.

secon ds minut

es hours

time rate amoun

t R5

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Allocating Space for Variables

Global data section

• All global variables stored here (actually all static variables)

• R4 points to beginning

Run-time stack

• Used for local variables

• R6 points to top of stack

• R5 points to top frame on stack

• New frame for each block

(goes away when block exited)

Offset = distance from beginning of storage area

• Global: LDR R1, R4, #4

• Local: LDR R2, R5, #-3

instructions global data

run-time stack

0x0000

0xFFFF

PC R4

R6 R5

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Variables and Memory Locations

In our examples,

a variable is always stored in memory.

When assigning to a variable, must store to memory location.

A real compiler would perform code optimizations that try to keep variables allocated in registers.

Why?

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Example: Compiling to LC-3

#include <stdio.h>

int inGlobal;

main() {

int inLocal; /* local to main */

int outLocalA;

int outLocalB;

/* initialize */

inLocal = 5;

inGlobal = 3;

/* perform calculations */

outLocalA = inLocal++ & ~inGlobal;

outLocalB = (inLocal + inGlobal) - (inLocal - inGlobal);

/* print results */

printf("The results are: outLocalA = %d, outLocalB = %d\n", outLocalA, outLocalB);

}

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Example: Symbol Table

Name Type Offset Scope

inGlobal int 0 global

inLocal int 0 main

outLocalA int -1 main

outLocalB int -2 main

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Example: Code Generation

; main

; initialize variables AND R0, R0, #0

ADD R0, R0, #5 ; inLocal = 5 STR R0, R5, #0 ; (offset = 0) AND R0, R0, #0

ADD R0, R0, #3 ; inGlobal = 3 STR R0, R4, #0 ; (offset = 0)

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Example (continued)

; first statement:

; outLocalA = inLocal++ & ~inGlobal;

LDR R0, R5, #0 ; get inLocal ADD R1, R0, #1 ; increment STR R1, R5, #0 ; store

LDR R1, R4, #0 ; get inGlobal NOT R1, R1 ; ~inGlobal

AND R2, R0, R1 ; inLocal & ~inGlobal STR R2, R5, #-1 ; store in outLocalA ; (offset = -1)

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Example (continued)

; next statement:

; outLocalB = (inLocal + inGlobal)

; - (inLocal - inGlobal);

LDR R0, R5, #0 ; inLocal LDR R1, R4, #0 ; inGlobal ADD R0, R0, R1 ; R0 is sum LDR R2, R5, #0 ; inLocal LDR R3, R5, #0 ; inGlobal NOT R3, R3

ADD R3, R3, #1

ADD R2, R2, R3 ; R2 is difference NOT R2, R2 ; negate

ADD R2, R2, #1

ADD R0, R0, R2 ; R0 = R0 - R2

STR R0, R5, #-2 ; outLocalB (offset = -2)

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**Special Operators: +=, *=, etc.**

Arithmetic and bitwise operators can be combined with assignment operator.

Statement Equivalent assignment

x += y; x = x + y;

x -= y; x = x - y;

x *= y; x = x * y;

x /= y; x = x / y;

x %= y; x = x % y;

x &= y; x = x & y;

x |= y; x = x | y;

x ^= y; x = x ^ y;

x <<= y; x = x << y;

x >>= y; x = x >> y;

All have same precedence and associativity as =

and associate right-to-left.

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Special Operator: Conditional

?: conditional x?y:z ¹⁶ ^l-to-r

If x is TRUE (non-zero), result is y;

else, result is z.

Like a MUX, with x as the select signal.

x

y z

1 0