Returning errors in Go - Step-by-Step Execution

Concept Flow - Returning errors

Start function

↓

Perform operation

↓

Error occurred?

No→Return result, nil

Yes↓

Return zero value, error

↓

Function ends

The function starts, does some work, checks if an error happened, and returns either the result with no error or a zero value with an error.

Execution Sample

Go

func divide(a, b int) (int, error) {
    if b == 0 {
        return 0, fmt.Errorf("cannot divide by zero")
    }
    return a / b, nil
}

This function divides two numbers and returns an error if dividing by zero.

Execution Table

Step	Input (a,b)	Condition (b==0)	Action	Return values
1	a=10, b=2	false	Calculate 10/2	5, nil
2	a=10, b=0	true	Return error	0, error("cannot divide by zero")

💡 Function returns after either successful division or error detection.

Variable Tracker

Variable	Start	After Step 1	After Step 2
a	10	10	10
b	2 or 0	2	0
error	nil	nil	error("cannot divide by zero")
result	undefined	5	0

Key Moments - 2 Insights

Why do we return 0 along with the error instead of just the error?

In Go, functions must return all declared return values. When an error occurs, we return the zero value for the result (here 0) and the error. See execution_table row 2.

What does 'nil' mean when returned as the error?

'nil' means no error happened. In execution_table row 1, error is nil, so the division was successful.

Visual Quiz - 3 Questions

Look at the execution_table, what is returned when b=0?

A 5, nil

B 0, error("cannot divide by zero")

C 10, nil

D 0, nil

💡 Hint

Check execution_table row 2 for b=0 case.

At which step is the condition b==0 false?

A Both steps

B Step 2

C Step 1

D Neither step

💡 Hint

Look at the Condition column in execution_table.

If we change the return to 'return a / b, error' without checking b==0, what happens?

A Program panics at runtime if b=0

B Returns zero and nil error

C Always returns error

D Compiles but never runs

💡 Hint

Think about division by zero in Go and error handling in execution_table.

Concept Snapshot

func functionName(...) (resultType, error) {
  if errorCondition {
    return zeroValue, error
  }
  return result, nil
}

- Return zero value with error on failure
- Return result with nil error on success
- Always return both values

Full Transcript

This example shows how a Go function returns errors. The function divide takes two integers and returns an integer result and an error. It checks if the divisor is zero. If yes, it returns zero and an error message. If no, it returns the division result and nil error. The execution table shows two cases: dividing 10 by 2 returns 5 and no error; dividing 10 by 0 returns zero and an error. Variables a and b keep their input values. The error variable is nil when no error occurs and holds an error message otherwise. Returning zero with an error is required because Go functions must return all declared values. Nil means no error. If we skip the error check and divide by zero, the program will crash at runtime.