Output formatting basics in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What package is commonly used in Go for formatted output?

The fmt package is used for formatted output in Go. It provides functions like Println, Printf, and Sprintf.

beginner

What does fmt.Printf("Hello, %s!", name) do?

It prints the string Hello, followed by the value of name, replacing %s with that string value.

beginner

Name two common format verbs used in Go's fmt.Printf.

Two common verbs are:
- %d for integers
- %f for floating-point numbers

intermediate

How do you print a floating-point number with 2 decimal places in Go?

Use %.2f in fmt.Printf. For example: fmt.Printf("%.2f", 3.14159) prints 3.14.

beginner

What is the difference between fmt.Println and fmt.Printf?

fmt.Println prints values with spaces and adds a newline automatically.
fmt.Printf prints formatted output according to a format string you provide.

Which fmt function allows you to format output with verbs like %d and %s?

A fmt.Scan

B fmt.Println

C fmt.Sprint

D fmt.Printf

fmt.Printf formats output using verbs like %d for integers and %s for strings.

What does the verb %s represent in Go's formatting?

A String

B Integer

C Floating-point number

D Boolean

%s is used to format string values.

How do you print a float with exactly 3 decimal places?

A %3f

B %.f3

C %.3f

D %f3

Use %.3f to format a float with 3 decimal places.

Which function automatically adds a newline after printing?

A fmt.Println

B fmt.Printf

C fmt.Print

D fmt.Sprintf

fmt.Println adds a newline after printing.

What will fmt.Printf("%d", 42) print?

A %d

B 42

C 42.0

D Error

%d formats the integer 42 as '42'.

Explain how to use fmt.Printf to format a string and an integer in one line.

Think about how to combine text and variables using format verbs.

Describe the difference between fmt.Println and fmt.Printf in Go.

One adds a newline automatically, the other needs a format string.

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