For loop basics in Go

Introduction

A for loop helps you repeat actions many times without writing the same code again and again.

When you want to count from 1 to 10 and do something each time.

When you have a list of names and want to greet each person.

When you want to add numbers from 1 to 100.

When you want to repeat a task until a condition changes.

Syntax

Go

for initialization; condition; post {
    // code to repeat
}

The loop runs while the condition is true.

Initialization runs once at the start, post runs after each loop.

Examples

Prints numbers 0 to 4, increasing i by 1 each time.

Go

for i := 0; i < 5; i++ {
    fmt.Println(i)
}

Counts down from 10 to 1.

Go

for i := 10; i > 0; i-- {
    fmt.Println(i)
}

Prints "Hello" three times.

Go

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

Sample Program

This program prints the word "Number" followed by numbers 1 to 5, each on a new line.

Go

package main

import "fmt"

func main() {
    for i := 1; i <= 5; i++ {
        fmt.Println("Number", i)
    }
}

Important Notes

You can leave out parts of the for loop to create different loop styles.

Be careful to change the loop variable, or the loop might run forever.

Go only has one loop type: the for loop, but it can do many jobs.

Summary

A for loop repeats code while a condition is true.

It has three parts: start, check, and change.

Use it to do tasks many times without writing code again.

Practice

1. What does a for loop do in Go programming?

easy

A. Stops the program immediately

B. Imports a package

C. Defines a new variable

D. Repeats a block of code while a condition is true

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is used to repeat code multiple times based on a condition.

2. Step 2: Identify the correct description

   Only Repeats a block of code while a condition is true correctly describes repeating code while a condition is true.

3. Final Answer:

   Repeats a block of code while a condition is true -> Option D

4. Quick Check:

   For loop = Repeat code [OK]

Hint: For loops repeat code while condition is true [OK]

Common Mistakes:

• Thinking for loop stops program
• Confusing for loop with variable declaration
• Mixing for loop with import statements

2. Which of the following is the correct syntax for a basic for loop in Go?

easy

A. for i := 0; i < 5; i++ { }

B. for (i = 0; i < 5; i++) { }

C. for i in range(5) { }

D. loop i from 0 to 5 { }

Solution

1. Step 1: Recall Go for loop syntax

   Go uses for init; condition; post { } syntax without parentheses.

2. Step 2: Compare options

   for i := 0; i < 5; i++ { } matches Go syntax exactly. Options B, C, and D use other languages or invalid syntax.

3. Final Answer:

   for i := 0; i < 5; i++ { } -> Option A

4. Quick Check:

   Go for loop = for init; condition; post [OK]

Hint: Go for loops use semicolons, no parentheses [OK]

Common Mistakes:

• Using parentheses like in C or Java
• Using Python-style for-in loops
• Writing 'loop' instead of 'for'

3. What is the output of this Go code?

for i := 1; i <= 3; i++ {
    fmt.Print(i, " ")
}

medium

A. 1 2 3

B. 0 1 2 3

C. 1 2 3 4

D. Syntax error

Solution

1. Step 1: Analyze loop range and output

   The loop starts at i=1 and runs while i <= 3, so i takes values 1, 2, 3.

2. Step 2: Understand print output

   Each iteration prints i followed by a space, so output is "1 2 3 "

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Loop prints 1 to 3 with spaces [OK]

Hint: Check loop start, end, and print inside loop [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Including extra number beyond 3
• Confusing print with println

4. Identify the error in this Go for loop:

for i := 0; i < 5 i++ {
    fmt.Println(i)
}

medium

A. Using := instead of = for i

B. Missing semicolon between condition and post statement

C. Using parentheses around for loop

D. Missing curly braces

Solution

1. Step 1: Check for loop syntax parts

   Go requires semicolons between init, condition, and post parts in for loops.

2. Step 2: Locate missing semicolon

   The code misses a semicolon between i < 5 and i++.

3. Final Answer:

   Missing semicolon between condition and post statement -> Option B

4. Quick Check:

   For loop parts separated by semicolons [OK]

Hint: Check semicolons between for loop parts [OK]

Common Mistakes:

• Omitting semicolons in for loop header
• Confusing := with = in loop variable
• Forgetting curly braces

5. How can you use a for loop in Go to sum all even numbers from 2 to 10 inclusive?

hard

A. for i := 2; i < 10; i += 2 { sum += i }

B. for i := 0; i <= 10; i += 2 { sum += i }

C. for i := 1; i <= 10; i++ { if i%2 == 0 { sum += i } }

D. for i := 2; i < 10; i++ { sum += i*2 }

Solution

1. Step 1: Understand the goal

   We want to add even numbers from 2 to 10 inclusive.

2. Step 2: Analyze each option

   for i := 2; i < 10; i += 2 { sum += i } increments by 2 starting at 2, but i < 10 so sums only 2+4+6+8 (misses 10).
   for i := 1; i <= 10; i++ { if i%2 == 0 { sum += i } } loops from 1 to 10, checks if number is even, then adds it. This is correct and clear.
   for i := 2; i < 10; i++ { sum += i*2 } incorrectly multiplies i by 2 and sums, which is wrong.
   for i := 0; i <= 10; i += 2 { sum += i } starts at 0, which includes 0 in sum, not desired.

3. Final Answer:

   for i := 1; i <= 10; i++ { if i%2 == 0 { sum += i } } -> Option C

4. Quick Check:

   Check even with modulo inside loop [OK]

Hint: Use modulo to check even numbers inside loop [OK]

Common Mistakes:

• Starting loop at 0 including unwanted numbers
• Multiplying instead of checking evenness
• Forgetting to check condition inside loop