For loop (rarely used in Ruby) - Step-by-Step Execution

Concept Flow - For loop (rarely used in Ruby)

Initialize loop variable

↓

Check condition: variable in range?

No→EXIT

Yes↓

Execute loop body

↓

Update loop variable to next value

↩Back to Check

The for loop starts by setting a variable, checks if it is within a range, runs the loop body if yes, then moves to the next value until the condition fails.

Execution Sample

Ruby

for i in 1..3
  puts i
end

This code prints numbers 1 to 3 using a for loop.

Execution Table

Iteration	i value	Condition (i in 1..3)	Action	Output
1	1	true	Print i	1
2	2	true	Print i	2
3	3	true	Print i	3
4	4	false	Exit loop

💡 Next value 4 is outside 1..3, so condition is false and loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	undefined	1	2	3	4

Key Moments - 2 Insights

Why does the loop stop after printing 3?

Because the condition checks if i is in 1..3. After i=3, next would be 4 (see execution_table row 4), it is outside the range, so the loop ends.

Is the loop variable i accessible after the loop ends?

Yes, in Ruby the loop variable i keeps its last value after the loop (which is 4 here, see variable_tracker final).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i during iteration 2?

A 1

B 2

C 3

D 4

💡 Hint

Check the 'i value' column in execution_table row for iteration 2.

At which iteration does the loop condition become false?

A Iteration 4

B Iteration 1

C Iteration 3

D Iteration 2

💡 Hint

Look at the 'Condition' column in execution_table where it changes to false.

If the range changed to 1..2, how many times would the loop run?

A 3 times

B 1 time

C 2 times

D 4 times

💡 Hint

Refer to how the range controls iterations in execution_table and variable_tracker.

Concept Snapshot

for loop syntax:
for variable in range
  # code
end

Runs code for each value in range.
Stops when values end.
Loop variable keeps last value after loop.

Full Transcript

This visual trace shows how a for loop works in Ruby. The loop variable i starts at 1 and goes up to 3. Each iteration prints i. After printing 3, next would be 4 but condition fails and the loop stops. The variable i keeps its last value after the loop ends. This example helps beginners see each step clearly.

Practice

1. What does the following Ruby for loop do?

for i in 1..3
puts i
end

easy

A. Prints numbers 1, 2, and 3 each on a new line

B. Prints numbers 1 to 2 only

C. Prints numbers 0 to 3

D. Causes a syntax error

Solution

1. Step 1: Understand the range 1..3

   The range 1..3 includes the numbers 1, 2, and 3.

2. Step 2: Loop through each number and print

   The loop prints each number on its own line using puts.

3. Final Answer:

   Prints numbers 1, 2, and 3 each on a new line -> Option A

4. Quick Check:

   for loop with 1..3 prints 1, 2, 3 [OK]

Hint: Remember 1..3 includes 3, so loop prints 1, 2, 3 [OK]

Common Mistakes:

• Thinking 1..3 excludes 3
• Confusing puts with print
• Expecting syntax error

2. Which of the following is the correct syntax for a for loop in Ruby to iterate over an array ["a", "b", "c"]?

easy

A. for x to ["a", "b", "c"] puts x end

B. for (x : ["a", "b", "c"]) puts x end

C. for x in ["a", "b", "c"] do puts x end

D. for x in ["a", "b", "c"] puts x

Solution

1. Step 1: Check the correct Ruby for loop syntax

   Ruby uses for variable in collection do ... end or without do.

2. Step 2: Validate each option

   for x in ["a", "b", "c"] do puts x end uses correct syntax with for x in array do ... end. for (x : ["a", "b", "c"]) puts x end and C use invalid syntax. for x in ["a", "b", "c"] puts x misses end to close the loop.

3. Final Answer:

   for x in ["a", "b", "c"] do puts x end -> Option C

4. Quick Check:

   Correct Ruby for loop uses 'for x in collection do ... end' [OK]

Hint: Remember: for x in collection do ... end is correct Ruby syntax [OK]

Common Mistakes:

• Using colon or 'to' instead of 'in'
• Omitting the 'end' keyword
• Adding parentheses incorrectly

3. What is the output of this Ruby code?

sum = 0
for i in 1..4
sum += i
end
puts sum

medium

A. Error

B. 9

C. 0

D. 10

Solution

1. Step 1: Understand the loop range and sum initialization

   The loop runs from 1 to 4, and sum starts at 0.

2. Step 2: Calculate sum after each iteration

   sum = 0+1=1, then 1+2=3, then 3+3=6, then 6+4=10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Sum of 1 to 4 is 10 [OK]

Hint: Add numbers 1 through 4 to get 10 quickly [OK]

Common Mistakes:

• Forgetting to include last number 4
• Starting sum at 1 instead of 0
• Syntax errors in loop

4. Identify the error in this Ruby for loop:

for i in 1..3
puts i


medium

A. Wrong range syntax

B. Missing end to close the loop

C. Variable i not defined

D. Using puts inside loop is invalid

Solution

1. Step 1: Check loop syntax completeness

   The loop starts with for but does not have a matching end.

2. Step 2: Confirm other parts are correct

   The range 1..3 and variable i are valid, and puts is allowed inside loops.

3. Final Answer:

   Missing end to close the loop -> Option B

4. Quick Check:

   Every for loop needs an 'end' [OK]

Hint: Always close for loops with 'end' [OK]

Common Mistakes:

• Forgetting 'end' keyword
• Thinking variable must be declared first
• Believing puts is invalid inside loops

5. You want to create a Ruby for loop that prints only even numbers from 2 to 10 inclusive. Which code snippet achieves this correctly?

hard

A. for i in 2..10 do
puts i if i % 2 == 0
end

B. for i in 2..10 do
puts i if i % 2 == 1
end

C. for i in 1..10 do
puts i if i % 2 == 1
end

D. for i in 2..10
puts i
end

Solution

1. Step 1: Identify the range and condition for even numbers

   The range 2..10 includes numbers 2 through 10. Even numbers satisfy i % 2 == 0.

2. Step 2: Check each option's condition

   for i in 2..10 do puts i if i % 2 == 0 end prints numbers in 2..10 only if even. for i in 2..10 do puts i if i % 2 == 1 end prints odd numbers. for i in 1..10 do puts i if i % 2 == 1 end prints odd numbers 1,3,5,7,9. for i in 2..10 puts i end prints all numbers without filtering.

3. Final Answer:

   for i in 2..10 do puts i if i % 2 == 0 end -> Option A

4. Quick Check:

   Filter with 'if i % 2 == 0' inside for loop [OK]

Hint: Use 'if i % 2 == 0' inside for loop to print evens [OK]

Common Mistakes:

• Using wrong modulo condition for even numbers
• Not filtering numbers inside loop
• Starting range at 1 instead of 2