Return values in C - Step-by-Step Execution

Concept Flow - Return values

Function called

↓

Execute function body

↓

Return statement reached

↓

Return value sent back

↓

Function call replaced by return value

When a function is called, it runs its code and sends back a value using return, which replaces the function call.

Execution Sample

C

int add(int a, int b) {
    return a + b;
}

int main() {
    int result = add(2, 3);
    return 0;
}

This code defines a function that adds two numbers and returns the sum, which is stored in 'result'.

Execution Table

Step	Action	Variables	Return Value	Notes
1	Call add(2, 3)	a=2, b=3	N/A	Function starts with parameters a=2, b=3
2	Calculate a + b	a=2, b=3	N/A	Sum is 5
3	Return 5	a=2, b=3	5	Return value 5 sent back to caller
4	Assign return to result	result=5	N/A	Variable 'result' now holds 5
5	End main	result=5	N/A	Program ends

💡 Function returns value 5, which is assigned to 'result', then program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
a	N/A	2	2	2	N/A
b	N/A	3	3	3	N/A
result	undefined	undefined	undefined	undefined	5

Key Moments - 2 Insights

Why does the function stop running after the return statement?

Because return immediately sends the value back and exits the function, no code after return runs (see Step 3 in execution_table).

What happens to the return value after the function finishes?

The return value replaces the function call in the calling code and can be stored in a variable like 'result' (see Step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the return value at Step 3?

A 5

B 2

C 3

D Undefined

💡 Hint

Check the 'Return Value' column at Step 3 in the execution_table.

At which step is the variable 'result' assigned a value?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Variables' and 'Notes' columns in the execution_table for when 'result' changes.

If the return statement was missing, what would happen to the return value?

A It would be 0 by default

B The function would return 5 anyway

C The function would return garbage or undefined

D The program would crash

💡 Hint

In C, missing return in non-void functions leads to undefined return value.

Concept Snapshot

Return values in C:
- Use 'return' to send a value back from a function.
- The function stops running after return.
- The returned value replaces the function call.
- Store it in a variable to use later.
- Missing return in non-void functions causes undefined behavior.

Full Transcript

This example shows how a function in C returns a value. When 'add' is called with 2 and 3, it calculates the sum and returns 5. The return statement ends the function immediately. The returned 5 replaces the function call in 'main' and is stored in 'result'. Variables 'a' and 'b' hold the input values during the function execution. Beginners often wonder why code after return doesn't run; it doesn't because return exits the function. Also, the return value is what the caller receives and can use. If a return is missing in a function that should return a value, the result is unpredictable.