C Programming #12: Operators - Assignment

Following article will discuss the assignment operators in C. Previously when discussing logical operators i mentioned that == is equality operator while = is assignment operator. Assignment operator is used to change the value of a variable. Simplest usage of assignment operator is


a = b + 1;

In above C statement we are adding 1 to value of b and storing it in a.

In C there are two kinds of value

1. R-value - Read Value e.g. Value stored in variable, constant value.
2. L-value - Location Value (Address) e.g Address of variable.

Hence R-value from Right hand side of = are evaluated and stored in the L-value left hand side. Sometimes people say R-value as right value and L-value as left value. Right hand side part of = can contain variables and constants while the left hand side should be always variable.


There are other short hand assignment operators.

1. Add assignment (+=) e.g. a += b; which is equivalent to  a = a + b
2. Subtract assignment (-=) e.g. a -= b; which is equivalent to a = a - b
3. Multiply assignment (*=)
4. Division assignment (/=)
5. Remainder assignment (%=)
6. Bit-wise and assignment (&=)
7. Bit-wise or assignment (|=)
8. Bit-wise xor assignment(^=)
9. Bit-wise left shift assignment (<<=)
10. Bit-wise right shift assignment (>>=)

NOTE : Logical operators do not have this shortcut assignments.

Following example shows the usage of the above assignment operators.

#include <stdio.h>

int main() 
{
   int a;
   int b = 1;

   a = b + 5;
   b += 2;

   printf("Value of a is %d\n", a);
   printf("Value of b is %d\n", b);

   return 0;
}

output of above program is

Value of a is 6
Value of b is 3

Can we mix data types on right hand side ? Yes, and it is already discussed when discussing operators.

Can we mix data types in = operator ? There are two possibility which is shown in below program first and then explained.

#include <stdio.h>
int main()
{
   int a = 1;
   float b = 2.3f;

   a = b + 1;
   b = a - 10;

   printf("Value of a is %d\n", a);
   printf("Value of b is %f\n", b);

   return 0;
}

output of above program is

Value of a is 3
Value of b is -7.000000

In following expression -

• a = b + 1; 
• first b + 1; is evaluated, since b is float and 1 is int, 1 is promoted to float and addition is carried which results in 3.3.
• then 3.3 is assigned to a which is integer, then approximation is done and only 3 is stored.
• b = a - 10;
• first a - 10 is evaluated, since a and 10 are integer hence no data type upgradation. 
• During assignment -7 is upgraded to float and stored in b.

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