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In this article, we are going to discuss another operator provided by C++, known as decrement operator. Decrement operator is denoted by -- and there are two kinds of Decrement operations in C -
• Pre-decrement operation
• Post-decrement operation

Pre-decrement operation

This operation is named pre-decrement because the decrement operator(--) comes before the operand.

Sequence of operation in pre-decrement operation is -

• First, the original value is decremented and then,
• Secondly, this value is read or used.

//Decrement Operator Example in C++

#include<iostream>

using namespace std; 

int main()
{
char ch= 'b';;
short sh= 99;
int i=9;
long l=9999;
float f=19.5f;
double d= 10.5;


cout<< "Original char : " << ch <<"\n";
cout<< "Decremented char : " << --ch << "\n";


cout<< "Original short : " << sh << "\n";
cout<< "Decremented short : " << --sh <<"\n";


cout<< "Original int : " << i << "\n";
cout<< "Decremented int : " << --i <<"\n";

cout<< "Original float : " << f << "\n";
cout<< "Decremented float : " << --f <<"\n";

cout<< "Original double : " << d << "\n";
cout<< "Decremented double : " << --d << "\n";

return 0;
}

Output is

Original char : b
Decremented char : a
Original short : 99
Decremented short : 98
Original int : 9
Decremented int : 8
Original float : 19.5
Decremented float : 18.5
Original double : 10.5
Decremented double : 9.5

Post-Decrement operator

Post-decrement operation is named so because the decrement operator(--) comes after the operand.

In a post-decrement operation, the sequence of operation is -
• Firstly, the original value of operand is read or used and then,
• Secondly, this value is decremented.

// Post-decrement operator example in C++

#include<iostream>

using namespace std; 

int main()
{
char ch= 'd';;
short sh= 101;
int i=1;
long l=999999;
float f=2.5f;
double d= 19.555;


cout<< "Original char : " << ch <<"\n";
cout<< "Decremented char : " << ch-- << "\n";


cout<< "Original short : " << sh << "\n";
cout<< "Decremented short : " << sh-- <<"\n";


cout<< "Original int : " << i << "\n";
cout<< "Decremented int : " << i-- <<"\n";

cout<< "Original float : " << f << "\n";
cout<< "Decremented float : " << f-- <<"\n";

cout<< "Original double : " << d << "\n";
cout<< "Decremented double : " << d-- << "\n";

return 0;
}

Output -

Original char : d
Decremented char : d
Original short : 101
Decremented short : 101
Original int : 1
Decremented int : 1
Original float : 2.5
Decremented float : 2.5
Original double : 19.555
Decremented double : 19.555

Program Analysis



In the output, you don't see the decrement of any original value because -
• We are applying an post-decrement operator on a primitive variable within cout.
• Hence, each original value of a variable is first read and printed right away using cout.
• Next, this original value is decremented, but it is not read or used to display in the output.





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Displaying decremented value after a post-decrement operation




// Post-decrement taking effect in C++

#include<ostream>

using namespace std; 

int main()
{
char ch= 'd';;
short sh= 101;
int i=1;
long l=999999;
float f=2.5f;
double d= 19.555;


cout<<"Original char : " <<ch <<"\n";
ch++;
cout<<"Decremented char : " <<ch <<"\n";


cout<<"Original short : " <<sh <<"\n";
sh++;
cout<<"Decremented short : " <<sh <<"\n";


cout<<"Original int : " <<i <<"\n";
i++;
cout<<"Decremented int : " <<i <<"\n";

cout<<"Original float : " <<f <<"\n";
f++;
cout<<"Decremented float : " <<f <<"\n";

cout<<"Original double : " <<d <<"\n";
d++;
cout<<"Decremented double : " <<d <<"\n";

return 0;
}





Output-




Original char : d
Decremented char : c
Original short : 101
Decremented short : 100
Original int : 1
Decremented int : 0
Original float : 2.5
Decremented float : 1.5
Original double : 19.555
Decremented double : 18.555





Program Analysis



Now in the output, you see the Decrementation of every original value because -
• Each original value of a variable is not read or used by any expression or a method.
• Hence, this original value is decremented and its decremented value is printed in the output using cout in the next line.







Post and pre-decrement operators in arithmatic expressions.




//Post-decrement and pre-decrement operator in arithmetic expressions in C++ 

#include<iostream>
 
using namespace std;

int main()
{
int a = 10, b=10, c=10, d=10;

int result = a-- - 10 ;  	//first expression
cout << "result1 = "<< result <<"\n";
cout << "a = "<< a << "\n";


result = --b - 10; 		 //second expression
cout << "result2 = " << result <<"\n";
cout << "b = " << b << "\n";


result = --c + c--; 	 //third expression
cout<< "result3 = " << result <<"\n";
cout<< "c = " << c << "\n";


result = d-- + 10 --d;		 //fourth expression
cout<< "result4 = " << result << "\n";
cout<< "d = " << d << "\n";

return 0;
}





Output-




Result1 = 0
a = 9
Result2 = -1
b = 9
Result3 = 17
c = 8
Result4 = 28
d = 8








Explaining the sequence of operations in our last code.





a-- - 10

1. Due to post-decrement operator with a, its value 10 is first read to be subtracted by 10.
2. Next, the original value of a is decremented to 9.
3. Finally, result of 10-10 = 0, is passed to the result.





--b - 10

1. Due to pre-decrement operator with b, its value 10 is first decremented to 9
2. Next, this decremented value of b, 9 is subtracted by 10
3. Finally, result of 9-10 = -1 , is passed to the result.






--c + c--

1. Due to pre-decrement operator with c, first its value 10 is decremented to 9
2. In the next expression, due to post-decrement operator with c, first the current value of c i.e. 9 is read and added to 9 from the previous expression and then c is decremented to 8
3. Finally, the result of 9+9 = 18, is passed to the result.






d-- + 10 + --d

1. Due to post-decrement operator with d, first its value is read to be added to 10, resulting 20 and then d is decremented to 9
2. In the next expression, due to pre-decrement operator with d, first the current value of d 9 is first decremented to 8 and then it is added to 20 from the previous expression.
3. Finally, the result of 20+8 = 28, is passed to the result.






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