For loop as while in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a 'for' loop without initialization and post statements act like in Go?

It acts like a 'while' loop, running as long as the condition is true.

beginner

How do you write a 'while' loop in Go using a 'for' loop?

Use 'for' with only a condition, like:

for condition {
    // code
}

intermediate

Why does Go not have a separate 'while' keyword?

Because the 'for' loop is flexible enough to act as a 'while' loop by omitting init and post statements.

beginner

Example: What will this Go code print?

i := 0
for i < 3 {
    fmt.Println(i)
    i++
}

It will print:
0
1
2
Because the loop runs while i is less than 3, increasing i each time.

intermediate

Can you use 'break' and 'continue' inside a 'for' loop acting as a 'while' loop in Go?

Yes, 'break' stops the loop early, and 'continue' skips to the next iteration, just like in other loops.

In Go, how do you write a loop that behaves like a 'while' loop?

A repeat { /* code */ } until condition

B for condition { /* code */ }

C loop condition { /* code */ }

D while condition { /* code */ }

Go uses 'for' with only a condition to act like a 'while' loop. There is no 'while' keyword.

What happens if you write 'for {}' in Go?

A Loop runs once

B Syntax error

C Infinite loop

D Loop runs zero times

'for {}' is an infinite loop in Go because there is no condition to stop it.

Which part is missing in a 'for' loop acting as a 'while' loop?

A Initialization and post statements

B Condition

C Loop body

D Semicolon

A 'for' loop as 'while' omits initialization and post statements, keeping only the condition.

Can you use 'continue' inside a 'for' loop used as a 'while' loop in Go?

A Yes

B No

C Only with a label

D Only in Go 1.18+

'continue' works normally inside any 'for' loop in Go, including those acting as 'while' loops.

What will this code print?
i := 5
for i > 0 {
fmt.Println(i)
i--
}

A 0 1 2 3 4

B 1 2 3 4 5

C No output

D 5 4 3 2 1

The loop prints i while i is greater than 0, decreasing i each time from 5 down to 1.

Explain how to use a 'for' loop as a 'while' loop in Go and why Go uses this approach.

Think about what parts of a for loop you keep or remove to make it behave like while.

Write a Go code snippet that uses a 'for' loop as a 'while' loop to print numbers from 1 to 5.

Start with i=1, loop while i <= 5, print i, then increase i.

Practice

1. Which of the following is the correct way to write a while loop in Go using a for loop?

easy

A. for i < 5 { i++ }

B. while i < 5 { i++ }

C. for (i < 5) { i++ }

D. loop i < 5 { i++ }

Solution

1. Step 1: Understand Go's while loop syntax

   Go uses for condition { } to act like a while loop.

2. Step 2: Identify the correct syntax

   for i < 5 { i++ } uses for i < 5 { i++ }, which is valid Go syntax for a while loop.

3. Final Answer:

   for i < 5 { i++ } -> Option A

4. Quick Check:

   Go while loop = for condition { } [OK]

Hint: Remember: Go uses 'for' with condition as while loop [OK]

Common Mistakes:

• Using 'while' keyword which Go does not support
• Adding parentheses around the condition unnecessarily
• Using 'loop' keyword which is invalid in Go

2. Which of the following Go code snippets correctly uses a for loop as a while loop to print numbers from 1 to 3?

easy

A. i := 1
for i <= 3 {
fmt.Println(i)
i++
}

B. i := 1
for (i <= 3) {
fmt.Println(i)
i++
}

C. i := 1
for i < 3 {
fmt.Println(i)
i++
}

D. i := 1
while i <= 3 {
fmt.Println(i)
i++
}

Solution

1. Step 1: Check syntax for Go while loop

   Go uses for condition { } without parentheses around condition.

2. Step 2: Verify loop condition and output

   i := 1 for i <= 3 { fmt.Println(i) i++ } uses i <= 3 and increments i, printing 1, 2, 3 correctly.

3. Final Answer:

   i := 1 for i <= 3 { fmt.Println(i) i++ } -> Option A

4. Quick Check:

   Correct Go while loop syntax = i := 1 for i <= 3 { fmt.Println(i) i++ } [OK]

Hint: No parentheses needed around condition in Go for loop [OK]

Common Mistakes:

• Using 'while' keyword which is invalid in Go
• Using '<' instead of '<=' causing missing last number
• Adding parentheses around condition causing syntax error

3. What will be the output of the following Go code?

i := 0
for i < 3 {
    fmt.Print(i, " ")
    i++
}

medium

A. 1 2 3

B. 0 1 2

C. 0 1 2 3

D. 1 2

Solution

1. Step 1: Analyze loop condition and initial value

   Variable i starts at 0 and loop runs while i < 3.

2. Step 2: Trace loop iterations and output

   i prints 0, then increments to 1; prints 1, increments to 2; prints 2, increments to 3; loop stops.

3. Final Answer:

   0 1 2 -> Option B

4. Quick Check:

   Loop prints 0 to 2 with spaces [OK]

Hint: Loop runs while condition true, stops when false [OK]

Common Mistakes:

• Assuming loop includes i=3 output
• Starting i from 1 instead of 0
• Forgetting to increment i causing infinite loop

4. Identify the error in the following Go code that tries to use a for loop as a while loop:

i := 0
for i < 3 {
    fmt.Println(i)
}

medium

A. Variable i should be declared inside the loop

B. Incorrect loop condition syntax

C. Using fmt.Println instead of fmt.Print

D. Missing increment of i inside the loop causing infinite loop

Solution

1. Step 1: Check loop condition and body

   The loop condition is valid, but i is never changed inside the loop.

2. Step 2: Understand consequences of missing increment

   Without incrementing i, the condition i < 3 remains true forever, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the loop causing infinite loop -> Option D

4. Quick Check:

   Change loop variable inside loop to avoid infinite loop [OK]

Hint: Always update loop variable inside for-as-while loop [OK]

Common Mistakes:

• Forgetting to increment or change loop variable
• Thinking fmt.Println causes error
• Declaring variable inside loop unnecessarily

5. You want to use a for loop as a while loop in Go to read user input until the user types "exit". Which code snippet correctly implements this?

hard

A. var input string
for input != "exit" {
fmt.Print("Enter text: ")
fmt.Scanln(&input)
}

B. var input string
for input == "exit" {
fmt.Print("Enter text: ")
fmt.Scanln(&input)
}

C. var input string
for {
fmt.Print("Enter text: ")
fmt.Scanln(&input)
if input == "exit" { break }
}

D. var input string
while input != "exit" {
fmt.Print("Enter text: ")
fmt.Scanln(&input)
}

Solution

1. Step 1: Understand the loop condition for reading input

   We want to keep reading until input equals "exit".

2. Step 2: Analyze options for correct loop behavior

   var input string for { fmt.Print("Enter text: ") fmt.Scanln(&input) if input == "exit" { break } } uses an infinite loop with a break when input is "exit", which is safe and common in Go.

3. Step 3: Why other options fail

   var input string for input != "exit" { fmt.Print("Enter text: ") fmt.Scanln(&input) } uses input before reading it, so loop condition is false initially (empty string != "exit" is true, but input is empty initially). var input string for input == "exit" { fmt.Print("Enter text: ") fmt.Scanln(&input) } loops only if input == "exit" which is opposite. var input string while input != "exit" { fmt.Print("Enter text: ") fmt.Scanln(&input) } uses invalid 'while' keyword.

4. Final Answer:

   Use infinite for loop with break on "exit" input -> Option C

5. Quick Check:

   Use for { ... if condition { break } } for while loops with input [OK]

Hint: Use infinite loop with break for input-based while loops [OK]

Common Mistakes:

• Using 'while' keyword which is invalid in Go
• Checking condition before first input read
• Using wrong loop condition causing no input read