3 Operators and Expressions

Chapter 3Operators and Expressions

PROGRAMMING IN ANSI C

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Operators

Arithmetic + - * / % ++ --

Relational < <= > >= == !=

Logical ！ && ||Bitwise << >> ~ | ^ &

Assignment

= (Shorthand Assignment: += -= etc.)

Comma , Explicit conversion

(type)

Conditional ?: Arrow ->

Pointer * & Array element [ ]

Number of bytes

sizeof others ( ) - .

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Expressions

An expression is a sequence of operands and

operators that produces a single value.

This value may be any data type except void.

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How to learn operators and expressions? When we study operators and expressions, we

must pay attention to such 5 points:

1. The function of operators.

2. The relation of operators and operands:

① How many operands does the operator need?

② Which types does the operator require?

3. The precedence of the operator.

4. The associativity of the operator.

5. The type of the calculated result.

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Arithmetic Operators & Expressions

+ ： Addition or unary plus e.g. 3+2 ， +3.5

- ： Subtraction or unary minus e.g. 3-2 ， -3.5

* ： Multiplication e.g. 3*2 ， 3.5*2

/ ： Division e.g. 3/2 ， 3.5/2 If the 2 operands both are integers, the result is an integer a

nd its fractional part is truncatedtruncated. e.g. 8/5=1

% ： Modulo division e.g. 5%2 = 1 Both of the 2 operands must be integers integers .

The sign of the result is the same as the dividend.

5 % 2 =

-5 % 2 =

5 % -2 =

-5 % -2 =

5 % 1 =

5 % 1.0 =

5 / - 2 =

5 / - 2.0 =

- 2

- 2.5

1

- 1

1

- 1

0

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+ ， - ， * ， / ， %

Precedence: + - (Unary) higher than * / % higher than + -

Associativity ： + ， - (Unary): Right to left others ： Left to right

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Notice ：

If the operands are all integers, the result must be a

n integer.

If one of the operands is a real number, the result

must be a real number.

The modulo division operator cannot used on real

numbers but only integers, and the sign of result is

the same as the sign of dividend.

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An arithmetic expression is a combination of

variables, constants, and operators arranged as

per the syntax of the language.

e.g. a * b / c + 1.5 – (3.28 + 'f') * (5 % 3)

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You must pay attention to :

1. The expression “a multiplied by b” must be written to “a*b” but not “ab”;

2. C doesn’t supply exponential operator, so you can write some multiplication, or you can use mathematic function pow(x,y) in the C function library to express xy;

3. Notice the precedence of those arithmetic operators, and you should reasonably use parentheses.

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4. When expressions include real values, then it

is important to take necessary precautions to

guard against certain computational errors.

e. g. a = 1/3.0;

b = a * 3.0;

5. Don’t make any expression divide zero.

6. Avoid data overflow.

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Relational Operators & Expressions

< <= > >= == !=

Value of Relational Expression: 1(True) & 0(False)

Precedence ： < <= > >= higher than == != Arithmetic operators higher than relational operators.

Associativity ： Left to right

Relational operators are used in the test condition of

decision or loop statements to decide the course of ac

tion of running program.

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int a=3, b=2, c=1, d, f;

1. a > b

2. c == a > b

3. a < b + c

4. d = a > b

5. f = a > b > c

3>2 ， the value is 1

1==1 ， the value is 1

b+c=3 ， the value is 0

d=1

a>b is 1 ， 1>c is 0 ， so: f = 0

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Notice

Avoid carrying out “==” and “!=” between real nu

mbers.

e.g. 1.0 / 3.0 * 3.0 == 1.0 /*not always 1 */

fabs ( 1.0 / 3.0 * 3.0 - 1.0 ) < 1e-6

Pay attention to distinguish “=” and “==”.

e.g. int a = 3, b = 2;

a = b == a; /* a = 0 */

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Logical Operators & Expressions

! && ||

Precedence ： ! higher than relational operators higher than && higher than ||

Associativity ： ! ： Right to left

&& , ||: Left to right

Value of logical Expression: 1(True) & 0(False)

Operands: 0 denotes false, others denote true.

e.g. 3.0 && 0

a b !a a&&b a||b

0 0 1 0 0

0 1 1 0 1

1 0 0 0 1

1 1 0 1 1

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int a = 4, b = 5;

1. !a

2. a&&b

3. a||b

4. !a||b

5. 4&&0||2

6. 5>3&&0||8<4-!0

7. 'c'&&'d'

!4 => 0

4 && 5 => 1

4 || 5 => 1

0 || 5 => 1

0 || 2 => 1

((5>3)&&0)||(8<(4-(!0))) = 0||0

=> 0

, 99 || 100 => 1

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Question ： Write the expression of leap year.If the “year” is a leap year, it must satisfy one of the 2 conditions:

1) It can be divided exactly by 4, and it can’t be divided exactly by 100.

2) It can be divided exactly by 400.

year%4==0 year%100!=0 year%400==0( && ) ||

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Short-circuit

When a logical expression is being evaluated,

it is not each part linked by && or || all is

evaluated. But only when it has to be

evaluated, it is done. a&&b /* Only when a is true, b is evaluated. If a is false, b is not evaluated.*/

a||b /* Only when a is false, b is evaluated. If a is true, b is not evaluated.*/

a=1; b=2; c=3; d=4; m=1; n=2;( m = a >b ) && ( n = c > d ); /* m=0,n=2*/

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Assignment Operators & Expressions

=

Format ： variable = expression

Precedence ： = lower than ||

Associativity ： Right to left

a = b = 5;

int a = b = 5; Wrong!

int a = 5, b = 5; Right!

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int a, b, c;

1. a = b = c = 5;

2. a = 5 + (c=6);

3. a = (b=4) + (c=6);

c = 5, b = c, a = b /*a=5, b=5, c=5*/

c = 6, a = 5+c /*a=11, c=6*/

b = 4, c = 6, a = b+c /*a=10, b=4, c=6*/

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“shorthand ” assignment operators: +=, -=, *=, /=, %=

variable op = expression;is equivalent to:

variable = variable op expression e.g.

x * = y + 1; is equivalent to:

x = x * (y + 1);

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int a = 2;

a += a -= a*a ; a += a –= 4

a += (a = a – 4)

a += (a = -2)

a += a a = a + a

a = (-2) + (-2) a = -4

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Increment and Decrement Operators

++ --

m ++; or ++ m; is equivalent to: m = m + 1; m --; or -- m; is equivalent to: m = m - 1;

Precedence ： same as unary +, unary -, !


(10 / m) ++ Wrong!

10 / m ++ Right!

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int m=5, n;

n = 10 / m++;

is equivalent to: n = 10/m;

m++;

n = 10 / ++m;

is equivalent to: ++m;

n = 10/m;

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1. j=3; k=++j;

2. j=3; k=j++;

3. j=3; printf("%d",++j);

4. j=3; printf("%d",j++);

5. a=3;b=5;c=(++a)*b;

6. a=3;b=5;c=(a++)*b;

j=j+1; k=j; result ： k=4, j=4

k=j; j=j+1; result ： k=3, j=4

j=j+1; printf(); output:4

printf(); j=j+1; output:3

a=a+1; c=a*b; result ： a=4 ， c=20

c=a*b; a=a+1; result ： a=4 ， c=15

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Notice

The operand of ++ or -- must be a variable and

not any expression or any constant.

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Comma Operator & Expressions

expression 1, expression 2, ……, expression n

Value of Relational Expression:

the value of expression n.

Precedence ： the lowest

Associativity ： Left to right

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Comma Operator & Expressions

1. a = 3*4, 5*2 ;

2. b = (a = 3*4, 5*2) ;

3. a=1; b=2; c=3;

printf("%d,%d,%d", a, b, c);

printf("%d,%d,%d", (a, b, c), b, c);

a = 12

a = 12, b = 10

output: 1, 2, 3

output:

3, 2, 3

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Implicit Type Conversion

The rules of conversion:

P67

In all expressions except

assignments, any implicit

type conversions are made

from a lower size type to a

higher size type as shown

here:

short & char

int

unsigned int

long

unsigned long

float

double

long double

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During assignment:

1. If expression is real type and the variable is integer

type, the value of expression will be truncated its

fractional part.

2. If expression is double type and the variable is float

type, the value of expression will be round its digits.

3. If expression is long type and the variable is int type,

the value of expression will be drop its higher byte.

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int i, x; float f; double d; long L;

x = L / i + i * f -

d; long float

float

float

double

double

doubleint

Notice: in the whole process of type

conversion, only the type of the interim value

used in evaluation is converted, but all the

types of variables are not converted.

Except the variable “x” assigned a value, all

the values of other variables are not changed.

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Explicit Type Conversion

Form ： (type-name) expression

Precedence ： same as unary +, unary -, !, ++, --


e.g. (int) (x+y) (int) x + y

main(){ float x, y ;

x = 3.6 ; y = (int) x * 2 ;

printf("x=%.2f, y=%.2f", x, y);}

x=3.60, y=6.00

Notice: like the implicit type conversion, only the type of the interim value used in evaluation is converted, but all the types of variables are not converted. Of course, the values of these variables are not changed.

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Homework

Review Questions P76

3.1~3.8 & 3.10 write down in your exercise book

Programming Exercises

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3 Operators and Expressions