1. ECE 103 Engineering Programming Chapter 17 Relational and Logical Operators
Herbert G. Mayer, PSU CS
Status 7/19/2015
Initial content copied verbatim from
ECE 103 material developed by
Professor Phillip PSU ECE

2. Syllabus
Expressions
Relational Operators
Logical Operators
Examples

3. Expressions An expression is any combination of:
Constants and variables
parentheses
function calls
operators
that, when evaluated, produces a value
An expression can be tested to make one selection out of several possible choices
expr
choice 1
choice 2

4. Table 1: C Operator Precedence Table in Decreasing Order
Description
Operator
Associativity 1
Function call
( )
left-to-right 5
Bitwise left shift
<<
Array subscript
[ ]
Bitwise right shift
>>
Struct member . 6
Less than
<
Struct dereference
->
Greater than
>
Increment (post)
expr++
LT or equal to
<=
Decrement (post)
expr--
GT or equal to
>= 2
Indirection *
right-to-left 7
Equal to ==
Reference (addr)
&
Not equal to !=
Unary plus + 8
Bitwise AND
Unary minus - 9
Bitwise XOR ^
Logical negation ! 10
Bitwise OR |
Bitwise NOT ~ 11
Logical AND
&&
Increment (pre)
++expr 12
Logical OR ||
Decrement (pre)
--expr 13
Conditional
? :
Cast
( type ) 14
Assignment
= += -= *= /= %= >>= <<=
&= ^= |=
Size in bytes
sizeof 3
Multiplication 15
Comma ,
Division /
Highest precedence is Level 1.
Lowest precedence is Level 15.
Modulo % 4
Addition
Subtraction 3

5. Relational Operators
Relational operators compare one value against another value, and yield boolean result
Operator
Description
>
Greater than
>=
Greater than or equal to
<
Less than
<=
Less than or equal to ==
Equal to !=
Not equal to 4

6. the expression value is
Important: == (equal to) is not the same as = (assignment)
The value of a relational expression is true or false
Be careful when comparing:
Numeric values of differing data types
Calculated floating point numbers
if expr evaluates to
the expression value is
true 1
false 5

7.  Expr Initial values Evaluates to Value 5 >= 1 - 'A' > 'B'
m < 3
m ← 2
m ← 5
sin(x) > w+1
x ← 0, w ← 0
x ← 0, w ← -2
k == 100
k ← 75
k ← 100
k != 100
1 < u <= 3
u ← 5
true 1
false
true 1
false
false
true 1
false
true 1
true 1
false
true 1

8. Logical operators are used to perform Boolean logic evaluations
A value of zero is interpreted as false
A non-zero value is interpreted as true
Operator
Description
&&
Logical AND ||
Logical OR !
Unary negation 7

9. Suppose A and B are general expressions:
C uses short-circuit evaluation; not all languages do!
Short-circuit evaluation  Evaluation stops as soon as the true or false status is determined
That means, some subexpressions may not get evaluated during run time A !A
false 1
true A B
A && B
A || B
false
true 1 8

10. Example: Expr Initial values Evaluates to Value (5 >= 1) && 0 -
'A'<'B' && 'B'<'C'
(k<1&&w==2)||n!=3
k← -2, w←2, n←3
m >= 0 && m <= 5
m ← -2
m ← 2
m ← 6
m < 0 || m > 5
false
true 1
true 1
false
true 1
false
true 1
false
true 1 9

11. Right: -1 <= x && x <= 7 Better: ( -1 <= x ) && ( x <= 7 )
Example:
Write an expression to check if a variable x is between -1 and 7, inclusive.
Wrong: -1 <= x <= 7
Right: -1 <= x && x <= 7
Better: ( -1 <= x ) && ( x <= 7 )
-- the “Golden Rule”: When in
-- doubt: Parenthesize!! 10

12. Example: Expr Initial values Evaluates to Value sum sum ← 0 sum ← 1
false
true 1
true
645
true 1
false
false 11

13. Relational and logical expressions can be assigned Example:
int x = -3;
bool state = ( x>5 ) || ( x<0 ); // true
int a, b=0, c=3;
a = (b==(c==(c++)));//( c==(c++) ) 1
// ( b==1 )  0
// a=0  0 12

14. C does not directly support set membership:
Inclusion or exclusion tests are done individually, or, if the set is ordered, by using a range test
Example:
Check if c is one of these characters: 'A', 'B', 'C', or 'D'
c=='A' || c=='B' || c=='C' || c=='D'
Alternative: c >= 'A' && c <= 'D’
Better: ( c >= 'A' ) && ( c <= 'D’ )
Check if x is specifically one of these values: 5, 10, 120
x == 5 || x == 10 || x == 120
Why would ( x >= 5 ) && ( x <= 120 ) not be correct? 13