Assignment operators in C - Step-by-Step Execution

Concept Flow - Assignment operators

Start

↓

Evaluate Right Side

↓

Apply Operator (=, +=, -=, etc.)

↓

Update Left Variable

↓

End

The program evaluates the right side expression, applies the assignment operator to update the left variable, then continues.

Execution Sample

C

int a = 5;
a += 3;
a *= 2;

This code assigns 5 to a, then adds 3 to a, then multiplies a by 2.

Execution Table

Step	Code Line	Variable 'a' Before	Operation	Variable 'a' After
1	int a = 5;	undefined	Assign 5	5
2	a += 3;	5	Add 3 (a = a + 3)	8
3	a *= 2;	8	Multiply by 2 (a = a * 2)	16
4	End	16	No operation	16

💡 All assignment operations completed, program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
a	undefined	5	8	16	16

Key Moments - 3 Insights

Why does 'a += 3;' change 'a' from 5 to 8?

Because 'a += 3;' means 'a = a + 3', so it adds 3 to the current value of 'a' which was 5, resulting in 8 (see execution_table step 2).

What happens if we use '=' instead of '+='?

Using '=' assigns the right side value directly, so 'a = 3;' would set 'a' to 3, ignoring its previous value (not shown in this trace but similar to step 1).

Does 'a *= 2;' multiply the old or new value of 'a'?

'a *= 2;' multiplies the current value of 'a' after previous operations, which is 8, so it becomes 16 (see execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'a' after step 2?

A 16

B 5

C 8

D 3

💡 Hint

Check the 'Variable a After' column in execution_table row for step 2.

At which step does 'a' become 16?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Variable a After' column and find when it changes to 16.

If we replace 'a += 3;' with 'a = 3;', what would be the value of 'a' after step 2?

A 3

B 8

C 5

D 16

💡 Hint

Remember '=' assigns the right side value directly, ignoring previous 'a' value.

Concept Snapshot

Assignment operators in C:
- '=' assigns value: a = 5;
- '+=' adds and assigns: a += 3; means a = a + 3;
- '*=' multiplies and assigns: a *= 2; means a = a * 2;
- They update the variable on the left with the result of the operation.
- Useful for short, clear updates to variables.

Full Transcript

This visual trace shows how assignment operators work in C. First, variable 'a' is assigned 5. Then 'a += 3;' adds 3 to 'a', making it 8. Next, 'a *= 2;' multiplies 'a' by 2, making it 16. Each step updates 'a' based on the operator used. The trace helps understand how these operators change variable values step-by-step.

Practice

1. Which of the following assignment operators adds the right operand to the left operand and then assigns the result to the left operand?

easy

A. -=

B. +=

C. *=

D. /=

Solution

1. Step 1: Understand the meaning of += operator

   The operator += adds the value on the right to the variable on the left and stores the result back in the left variable.

2. Step 2: Compare with other operators

   Other operators like -= subtract, *= multiply, and /= divide, so they do not add.

3. Final Answer:

   += -> Option B

4. Quick Check:

   += means add and assign [OK]

Hint: Remember '+=' means add then assign to left variable [OK]

Common Mistakes:

• Confusing '+=' with '=' which just assigns
• Mixing '+=' with '-=' or '*='
• Thinking '+=' only adds without assignment

2. Which of the following is the correct syntax using the compound assignment operator to multiply variable x by 5 and assign the result back to x?

easy

A. x = x *5;

B. x =* 5;

C. x = *5;

D. x *= 5;

Solution

1. Step 1: Identify correct compound assignment syntax

   The correct syntax for multiplying and assigning is 'x *= 5;'. It combines multiplication and assignment.

2. Step 2: Check other options for syntax errors

   x =* 5; 'x =* 5;' is invalid syntax. x = *5; 'x = *5;' is invalid because '*5' is not a valid expression. x = x *5; 'x = x *5;' is correct but not a compound assignment operator.

3. Final Answer:

   x *= 5; -> Option D

4. Quick Check:

   '*=' is multiply and assign operator [OK]

Hint: Use '*=' to multiply and assign in one step [OK]

Common Mistakes:

• Writing '=*' instead of '*='
• Forgetting space or semicolon
• Using 'x = *5;' which is invalid

3. What is the output of the following code?

int a = 10;
a += 5;
a -= 3;
printf("%d", a);

medium

A. 8

B. 15

C. 12

D. 10

Solution

1. Step 1: Calculate after first operation 'a += 5;'

   Starting with a = 10, adding 5 gives a = 15.

2. Step 2: Calculate after second operation 'a -= 3;'

   Subtracting 3 from 15 gives a = 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   10 + 5 - 3 = 12 [OK]

Hint: Apply each operator step-by-step to track value [OK]

Common Mistakes:

• Adding and subtracting in wrong order
• Confusing '+=' and '-=' effects
• Ignoring initial value of variable

4. Identify the error in the following code snippet:

int x = 7;
x +== 3;
printf("%d", x);

medium

A. Syntax error due to '+==' operator

B. Runtime error due to invalid operation

C. No error, output is 10

D. Logical error, output is wrong

Solution

1. Step 1: Check the operator '+==' in the code

   The operator '+==' does not exist in C. The correct operator for addition assignment is '+='.

2. Step 2: Understand the effect of invalid operator

   Using '+==' causes a syntax error and the code will not compile.

3. Final Answer:

   Syntax error due to '+==' operator -> Option A

4. Quick Check:

   Invalid operator '+==' causes syntax error [OK]

Hint: Use '+=' not '+==' for addition assignment [OK]

Common Mistakes:

• Typing '+==' instead of '+='
• Assuming '+==' works like '=='
• Ignoring compiler error messages

5. Given the code below, what is the final value of result?

int result = 20;
result /= 4;
result *= 3;
result %= 5;
result += 2;
printf("%d", result);

hard

A. 2

B. 3

C. 4

D. 1

Solution

1. Step 1: Perform division assignment 'result /= 4;'

   Starting with result = 20, dividing by 4 gives result = 5.

2. Step 2: Perform multiplication assignment 'result *= 3;'

   Multiplying 5 by 3 gives result = 15.

3. Step 3: Perform modulus assignment 'result %= 5;'

   15 modulo 5 is 0, so result = 0.

4. Step 4: Perform addition assignment 'result += 2;'

   Adding 2 to 0 gives result = 2.

5. Final Answer:

   2 -> Option A

6. Quick Check:

   20/4=5, 5*3=15, 15%5=0, 0+2=2 [OK]

Hint: Apply each operator in order carefully [OK]

Common Mistakes:

• Skipping modulus step or calculating it wrong
• Mixing up order of operations
• Forgetting that %= gives remainder