The Simple Assignment Operator

The assignment operator = stores a value in a variable. It is used in assignment expressions and assignment statements. The rule is simple: the left operand must be a modifiable variable (something with a memory location), while the right operand can be a variable, a constant or any expression. The right side is evaluated first, then its value is stored in the left side.

Assignment	Meaning
level = 8	Constant 8 is stored in level
total = a + b - 2	Value of the expression is stored in total
copy = original	Value of original is copied into copy
8 = level	Compile error: a constant cannot be on the left

Expression Versus Statement

In C, x = 8 is an expression whose value is 8. Put a semicolon after it and it becomes an assignment statement: x = 8;. Because assignment is an expression, it can sit inside a bigger expression, for example r = (x = 8) + 2.

Multiple Assignment (Chaining)

Assignment evaluates right to left. In x = y = z = 20, first z = 20 runs and yields 20, which is assigned to y, which yields 20, assigned to x. All three hold 20, and the whole expression has the value 20.

Compound Assignment Operators in C

When the variable on the left also appears on the right, you can avoid writing it twice with a compound assignment operator. For example x = x + 5 becomes x += 5.

Compound form	Equivalent to
x += 5	x = x + 5
x -= 5	x = x - 5
x *= 5	x = x * 5
x /= 5	x = x / 5
x %= 5	x = x % 5

Besides saving typing, compound operators evaluate the left operand only once, which matters when the left side is a complicated expression such as an array element with a function call as index. In embedded code you will later meet |=, &=, ^=, <<= and >>= for setting and clearing hardware register bits.

Hands-On Assignment Operator Examples

Compile with gcc -Wall -Wextra -std=c11 file.c -o out.

Example 1: Assignment and Swapping Two Values

/* cp4_4_ex1.c */
#include <stdio.h>

int main(void)
{
    int a = 3, b = 9, temp;

    printf("before: a=%d b=%d\n", a, b);
    temp = a;
    a = b;
    b = temp;
    printf("after : a=%d b=%d\n", a, b);
    return 0;
}

before: a=3 b=9
after : a=9 b=3

Assignment overwrites the old value. Without temp the original a would be lost, so swapping needs a third variable.

Example 2: Chained Assignment and Expression Value

/* cp4_4_ex2.c */
#include <stdio.h>

int main(void)
{
    int x, y, z, r;

    x = y = z = 20;
    printf("x=%d y=%d z=%d\n", x, y, z);

    r = (x = 8) + 2;
    printf("x=%d r=%d\n", x, r);
    return 0;
}

x=20 y=20 z=20
x=8 r=10

Example 3: Every Compound Operator in Sequence

/* cp4_4_ex3.c */
#include <stdio.h>

int main(void)
{
    int score = 10;

    score += 5;  printf("after += 5 : %d\n", score);
    score -= 3;  printf("after -= 3 : %d\n", score);
    score *= 2;  printf("after *= 2 : %d\n", score);
    score /= 5;  printf("after /= 5 : %d\n", score);
    score %= 3;  printf("after %%= 3 : %d\n", score);
    return 0;
}

after += 5 : 15
after -= 3 : 12
after *= 2 : 24
after /= 5 : 4
after %= 3 : 1

Example 4: Precedence and Narrowing Traps

/* cp4_4_ex4.c */
#include <stdio.h>

int main(void)
{
    int x = 10;
    x *= 2 + 3;
    printf("x = %d\n", x);

    int n = 7.9;
    printf("n = %d\n", n);

    int total = 10;
    total += 2.7;
    printf("total = %d\n", total);
    return 0;
}

x = 50
n = 7
total = 12

Line one: 2 + 3 is 5, so x = 10 * 5 = 50. Line two: assigning a double to an int truncates, it does not round. Line three: total + 2.7 is 12.7 in floating-point, which is truncated to 12 when stored back into the int.

The = Versus == Bug

The most famous assignment mistake in C is writing = where you meant the equality test ==:

int mode = 0;
if (mode = 1) {      /* assigns 1, condition is always true */
    printf("enabled\n");
}

$ gcc -Wall -Wextra bug.c
bug.c:4:13: warning: suggest parentheses around assignment used as truth value [-Wparentheses]

This compiles, overwrites mode, and the branch always runs. Always build with -Wall so the compiler warns you.

Real-World Applications

• Embedded: accumulating sensor readings with sum += reading, decrementing retry counters, updating hardware registers.
• Linux programs: accumulating byte counts, advancing offsets with offset += n.
• General: running totals, scores, balances.

Common Mistakes and Troubleshooting

Mistake	Symptom	Fix
= instead of == in a condition	Condition always true or false	Use ==; build with -Wall
Assuming x *= a + b multiplies only by a	Wrong totals	Remember the right side is grouped
Assigning a float to an int	Fraction silently lost	Cast or round deliberately
Chaining with different types	Surprising truncation along the chain	Assign separately
Using an uninitialised variable on the right	Random values	Initialise at declaration

Best Practices

• Initialise variables when you declare them.
• Use compound operators when updating a variable from its own value; it reads clearly.
• Avoid cramming assignments inside conditions unless you add extra parentheses to show intent.
• Keep chained assignment to variables of the same type.
• Use const for values that must never be reassigned; the compiler then rejects accidental assignment.

Conclusion

Assignment looks trivial yet drives every program you will write. You can now read and write simple, chained and compound assignments and avoid the classic traps. Increment and decrement operators come next in this free C programming course, completing the family of operators that modify variables.

Quick Quiz

1. What is the value of the expression x = 8?
   Show answer

   8.
2. What does k %= 5 mean?
   Show answer

   k = k % 5.
3. If x = 4, what is x after x *= 3 + 1?
   Show answer

   16.
4. Is 10 = n legal?
   Show answer

   No. The left side must be a variable.
5. What does int n = 5.8; store?
   Show answer

   5.

Interview Questions

1. Explain the difference between an assignment expression and an assignment statement.
2. How is a = b = c = 0 evaluated?
3. What bug does if (x = 5) cause, and how can the compiler help?
4. Why does x *= a + b differ from x = x * a + b?
5. Which compound operators are common in embedded register code?

Frequently Asked Questions

Is this free C programming course in order?

Yes. Chapter 4 goes from arithmetic operators to assignment and then to increment, relational, logical and bitwise operators.

What is the difference between = and ==?

The single equals stores a value; the double equals compares two values and gives 1 or 0.

What is a compound assignment operator?

A shorthand such as += that applies an operation to a variable and stores the result back in it.

Can I assign one variable to several in one line?

Yes, a = b = c = 0; works because assignment evaluates right to left.

Does assigning a float to an int round the number?

No. It truncates toward zero.

Where can I find more free C tutorial lectures?

Browse the course index of this free C programming course for every chapter, quiz and interview question set.