DAY 2 — Core Go Building Blocks (Functions, Errors, Collections)

Goal (End of Day 2)

• Write non-trivial functions outside main
• Use slices and maps correctly
• Handle errors idiomatically
• Structure small programs with clear data flow
• Read and reason about basic Go code confidently

1. Functions (Beyond main)

Concepts

• Function declaration syntax
• Parameters vs return values
• Multiple return values
• Named vs unnamed returns (know both, prefer unnamed)
• Early returns
• Functions as units of behavior

Key Rules

• Go uses explicit returns
• Errors are returned, not thrown

Exercises

• Implement: add(a int, b int) int
• Implement: divide(a int, b int) (int, error)
• Implement: isEven(n int) bool

2. Error Handling (Go Style)

Concepts

• error is an interface
• nil means “no error”
• if err != nil {} is mandatory
• errors.New vs fmt.Errorf
• Fail fast, return early

Key Rules

• Errors are values, not control flow
• Never ignore errors unless absolutely certain

Exercises

• Update divide to return an error on division by zero
• Handle and print user-friendly error messages in main

3. Slices (Primary Data Structure)

Concepts

• Slice literals vs make
• len vs cap
• append
• for range iteration
• Zero value of a slice is nil

Key Rules

• Slices are descriptors over arrays
• append may reallocate underlying array

Exercises

• Write functions to compute sum, min, and max from a slice of ints

4. Maps (Key–Value Storage)

Concepts

• Map literals vs make
• Zero value of a map is nil
• Insert, read, delete
• Comma-ok idiom

Key Rules

• Missing key returns zero value
• Use comma-ok to distinguish missing vs zero

Exercises

• Build a word-frequency counter for a string
• Print word counts (sorted optional)

5. Mini Program (Mandatory)

Program: CLI Stats Tool

Requirements

• Hardcode a slice of integers
• Compute count, sum, and average using functions
• Handle empty slice gracefully

Suggested Structure

package main

func main() { // orchestration only }

func computeStats(nums []int) (sum int, avg float64, err error) { }

6. Tooling Rules (Non-Negotiable)

• Use gofmt or editor auto-formatting
• No unused variables
• No commented-out dead code
• Keep main thin; logic goes into functions

Exit Criteria (Must All Be True)

• At least 3 functions outside main
• Used both slices and maps
• Returned and handled real errors
• Can explain why Go avoids exceptions
• Code formats cleanly with gofmt

Day 3 Preview

• Structs
• Methods
• Value vs pointer receivers
• Basic program modeling