Logical Data in C
Data is called logical if it conveys a
value of true or false.
If a data value is zero, it is considered
false. If a value is non-zero, it is
considered true.
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Logical operators
The relational operators are the Logical AND, Logical
OR and and the NOT operator.
Value 1
T
T
F
F
Value2
T
F
T
F
&&
T
F
F
F
||
T
T
T
F
! (Value 1)
F
F
T
T
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Logical Operator Examples
#include <stdio.h>
int main(void)
{
int a = 7, b=0;
printf( "\n The value of a && b is %d\n", a && b );
// prints 0
printf( "\n The value of a || b is %d\n", a || b );
// prints 1
printf( "\n The value of !a && b is %d\n", !a && b ); // prints 0
printf( "\n The value of !a || !b is %d\n", !a || b );
// prints 0
printf( "\n The value of a && !b is %d\n", a && !b ); // prints 1
printf( "\n The value of !(a || b) is %d\n", !(a || b) );
// prints 0
printf( "\n The value of !(a && b) is %d\n", !(a && b) ); // prints 1
return 0;
} // main
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Relational operators
Six relational operators support logical relationships.
They are all binary operators that accept two
operands and compare them. The value of an
expression involving a relational operator is either
true or false.
The operators are:
<
<=
>
>=
==
!=
less than
less than or equal
greater than
greater than or equal
equal
not equal
higher priority than
the other two
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Examples
x = 1;
y = 4;
z = 14;
x <= 1 && y == 3
x <= 1 || y == 3
!( x > 1 )
!x > 1
!( x <= 1 || y == 3 )
x >= 1 && y == 3 || z < 14
0
1
1
0
0
0
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Two-way Selection
The basic decision statement in the computer is the
two-way selection. The decision is described to the
computer as a conditional statement that can be
answered either true or false. If the answer is true,
one or more action statements is executed. If the
answer is false, then a different action or set of
actions is executed. Regardless of which set of
actions is executed, the program continues with the
next statement, after the selection.
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if statement
if is a mechanism for executing a statement/s
conditionally.
if ( expression )
action 1
else
action 2
In this case only action 1 or action 2 is executed,
depending on whether the expression is true or false.7
if statement cont.
Usually the expression in an if
statement is a relational, equality or
logical expression.
if ( j < k )
min = j;
if ( j < k )
printf( “ j is smaller than k\n”);
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if statement cont.
if ( j < k )
{
min = j;
printf( “ j is smaller than k\n”);
}
else
printf(“ j is larger than k\n”);
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Nested if statements
if ( expression 1 )
{
action 1
if ( expression 2 )
action 2
else
action 3
}
else
action 4
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Dangling else problem
Once you start nesting if…else statements, you can
encounter the ‘dangling else’ problem. This problem
is created when there is no matching else for every if.
C has a simple rule:
an else is always paired with the most recent unpaired if
This might result in your program not matching your
actual intent.
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Dangling else problem
Write the code for the following pseudocode:
if x is greater to 5
if x is less than 10
print that the value is between 5 and 10
else
print that the value is less than 5
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The conditional operator
The conditional operator ?: takes in3 expressions and
is in the form
conditional_expression ::= expr1 ? expr2 : expr3
eg:
x = y<z ? y : z;
// if y is < z then x=y otherwise x=z
x = flag ? Y : Y * Y; // if flag is true then x=y otherwise x = y*y
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The switch statement
The switch is a multi-conditional statement
generalizing the if-else statement.
switch ( expression )
{
case case-label-1 :
statement-list
case case-label-2 :
statement-list
...
default :
statement-list
}
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The switch statement cont.



Switch works by evaluating expression, passing
control to the case labeled with its value (or the
default) and executing the case’s statement-list.
The case-labels have to have expressions that the
compiler can evaluate to an integer constant, and all
the case-labels have to be unique.
A break statement is placed at the end of each
statement list.
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The switch statement cont.
Switch ( operator )
{
case '+' : // when the value of operator is +
result = op1 + op 2;
break;
case '-' : // when the value of operator is result = op1 - op 2;
break;
case '*' : // when the value of operator is *
result = op1 * op 2;
break;
case '/' : // when the value of operator is /
if ( op2 != 0.0 )
result = op1 / op 2;
break;
default : // when the value of operator is not any of the above
printf("Unknown operator\n");
result = 0.0;
break;
}
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The else-if statement
The switch statement only works when the case
values are integral. If it is necessary to make a
multiway decision on the basis of a value that is not
integral, the else if statement could be used.
The else-if is pretty much the same as the if..else
statement with a style change. The last statement in
the series concludes with an else. This is the default
condition; that is, the condition that is to be executed
if all other statements are false.
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Example
#include <stdio.h>
main()
{
int j =4;
int k, min;
k = j * 4 % 5;
if ( j < k )
{
// this is for the case of j is less than k
min = j;
printf( “ j is smaller than k\n”);
}
else if ( j == k )
{
min = j = k;
printf(“j and k are equal\n”);
}
else
which if
printf(“ j is larger than k\n”);
printf(“min = %d\n”, min );
}
does this else correspond to?
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More Standard Library
Functions
Another standard library that comes with C is the
<ctype.h> library. This contains different functionality
dealing with characters.
int iscntrl ( int testChar );
int isalnum ( int testChar );
int isdigit ( int testChar );
int toupper ( int convChar );
int tolower ( int convChar );
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