Output formatting basics in Go

Introduction

Output formatting helps you show information clearly and nicely when your program runs.

You want to print a message with numbers aligned.

You need to show a date or time in a specific style.

You want to display a list of items in columns.

You want to control how many decimal places a number shows.

Syntax

Go

fmt.Printf("format string", values...)

Use fmt.Printf to print formatted output.

The format string uses special codes like %d for integers, %s for strings, and %f for floats.

Examples

Prints a string inside a message.

Go

fmt.Printf("Hello, %s!\n", "world")

Prints an integer number.

Go

fmt.Printf("You have %d apples.\n", 5)

Prints a float with 2 decimal places.

Go

fmt.Printf("Price: $%.2f\n", 3.456)

Prints a left-aligned string and right-aligned number in columns.

Go

fmt.Printf("%-10s %5d\n", "Item", 42)

Sample Program

This program shows how to print strings, integers, and floats with formatting. It also shows alignment in columns.

Go

package main

import "fmt"

func main() {
    name := "Alice"
    age := 30
    score := 95.6789

    fmt.Printf("Name: %s\n", name)
    fmt.Printf("Age: %d years\n", age)
    fmt.Printf("Score: %.1f%%\n", score)
    fmt.Printf("%-10s | %5d | %8.2f\n", "Test", 7, 123.456)
}

Important Notes

Always end your format string with \n to move to the next line.

Use %% to print a percent sign.

Try different format codes to see how output changes.

Summary

Use fmt.Printf with format codes to control output.

Format codes like %s, %d, and %f help print different data types.

You can align text and numbers for neat columns.

Practice

1. What does the %d format specifier do in fmt.Printf in Go?

easy

A. Prints an integer value

B. Prints a floating-point number

C. Prints a string

D. Prints a boolean value

Solution

1. Step 1: Understand format specifiers in Go

   The %d specifier is used to print decimal integers in Go's fmt.Printf.

2. Step 2: Match specifier to data type

   Since %d prints integers, it matches integer values, not floats, strings, or booleans.

3. Final Answer:

   Prints an integer value -> Option A

4. Quick Check:

   %d = integer [OK]

Hint: Remember %d is for decimal integers in printf [OK]

Common Mistakes:

• Confusing %d with %f for floats
• Using %d for strings
• Thinking %d prints booleans

2. Which of the following is the correct syntax to print a string variable name using fmt.Printf in Go?

easy

A. fmt.Printf("%d", name)

B. fmt.Printf("%f", name)

C. fmt.Printf("%s", name)

D. fmt.Printf(name)

Solution

1. Step 1: Identify the correct format specifier for strings

   The format specifier for strings in Go's fmt.Printf is %s.

2. Step 2: Check the syntax correctness

   fmt.Printf("%s", name) uses fmt.Printf("%s", name), which correctly prints the string variable name.

3. Final Answer:

   fmt.Printf("%s", name) -> Option C

4. Quick Check:

   %s prints strings [OK]

Hint: Use %s for strings in fmt.Printf [OK]

Common Mistakes:

• Using %d or %f for strings
• Omitting quotes around format string
• Passing variable without format specifier

3. What is the output of the following Go code?

package main
import "fmt"
func main() {
    x := 7
    y := 3.14
    fmt.Printf("x = %d, y = %.1f\n", x, y)
}

medium

A. x = 7, y = 3.1

B. x = 7, y = 3.14

C. x = 7, y = 3.0

D. x = 7, y = 3

Solution

1. Step 1: Understand the format specifiers used

   %d prints integer x as 7. %.1f prints floating-point y rounded to 1 decimal place.

2. Step 2: Apply rounding to y

   3.14 rounded to 1 decimal place is 3.1, so output is "x = 7, y = 3.1".

3. Final Answer:

   x = 7, y = 3.1 -> Option A

4. Quick Check:

   %.1f rounds float to 1 decimal [OK]

Hint: %.1f rounds float to 1 decimal place [OK]

Common Mistakes:

• Expecting full float without rounding
• Confusing %d with %f
• Ignoring newline character

4. Identify the error in this Go code snippet:

package main
import "fmt"
func main() {
    name := "Alice"
    age := 30
    fmt.Printf("Name: %s, Age: %f\n", name, age)
}

medium

A. Missing newline character in format string

B. Missing import statement

C. Incorrect variable declaration

D. Using %f to print an integer variable

Solution

1. Step 1: Check format specifiers against variable types

   name is a string, printed with %s correctly. age is an integer but printed with %f, which expects a float.

2. Step 2: Identify the mismatch causing error

   Using %f for an integer causes a runtime error or incorrect output.

3. Final Answer:

   Using %f to print an integer variable -> Option D

4. Quick Check:

   Match %f with float, not int [OK]

Hint: Match %f only with float variables [OK]

Common Mistakes:

• Using %f for integers
• Ignoring type mismatch warnings
• Assuming %f works for all numbers

5. You want to print a table of product names and prices aligned in columns using fmt.Printf. Which format string correctly aligns the name left in 10 spaces and price right in 6 spaces with 2 decimals?

fmt.Printf("...", name, price)

hard

A. "%10s %-6.2f\n"

B. "%-10s %6.2f\n"

C. "%10s %6.2f\n"

D. "%-10s %-6.2f\n"

Solution

1. Step 1: Understand alignment flags in format strings

   In Go, %-10s means left-align string in 10 spaces; %6.2f means right-align float in 6 spaces with 2 decimals.

2. Step 2: Match alignment to requirements

   We want product name left-aligned in 10 spaces and price right-aligned in 6 spaces with 2 decimals, so "%-10s %6.2f\n" fits.

3. Final Answer:

   "%-10s %6.2f\n" -> Option B

4. Quick Check:

   Left-align string with %-10s, right-align float with %6.2f [OK]

Hint: Use - for left-align strings, no - for right-align numbers [OK]

Common Mistakes:

• Swapping alignment flags
• Using wrong width or precision
• Forgetting to specify decimal places for floats