File.readlines for line-by-line in Ruby - Step-by-Step Execution

Concept Flow - File.readlines for line-by-line

Open file

↓

Read all lines into array

↓

Iterate over array

↓

Process each line

↓

Close file

The program opens a file, reads all lines into an array, then processes each line one by one.

Execution Sample

Ruby

lines = File.readlines('example.txt')
lines.each do |line|
  puts line.chomp
end

Reads all lines from 'example.txt' into an array and prints each line without extra newline.

Execution Table

Step	Action	Evaluation	Result
1	Call File.readlines('example.txt')	Reads entire file	Array of lines: ["Hello\n", "World\n", "Ruby\n"]
2	Start iteration over array	First element: "Hello\n"	line = "Hello\n"
3	Print line.chomp	Remove trailing newline	Output: Hello
4	Next iteration	Second element: "World\n"	line = "World\n"
5	Print line.chomp	Remove trailing newline	Output: World
6	Next iteration	Third element: "Ruby\n"	line = "Ruby\n"
7	Print line.chomp	Remove trailing newline	Output: Ruby
8	End iteration	No more elements	Iteration stops

💡 All lines processed, iteration ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
lines	nil	["Hello\n", "World\n", "Ruby\n"]	["Hello\n", "World\n", "Ruby\n"]	["Hello\n", "World\n", "Ruby\n"]	["Hello\n", "World\n", "Ruby\n"]
line	nil	"Hello\n"	"World\n"	"Ruby\n"	nil

Key Moments - 3 Insights

Why does each line include a newline character at first?

File.readlines reads each line including the newline character '\n', as shown in execution_table step 1 where lines contain '\n'.

Why do we use line.chomp when printing?

line.chomp removes the trailing newline '\n' so the output lines don't have extra blank lines, as seen in steps 3, 5, and 7.

Does File.readlines keep the file open during iteration?

No, File.readlines reads all lines at once and closes the file immediately, so iteration works on the array in memory (step 1).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'line' at step 4?

A "Hello\n"

B "World\n"

C "Ruby\n"

D nil

💡 Hint

Check the 'Action' and 'Evaluation' columns at step 4 in execution_table.

At which step does the iteration stop because there are no more lines?

A Step 6

B Step 7

C Step 8

D Step 3

💡 Hint

Look for the step with 'End iteration' and 'No more elements' in execution_table.

If the file had 5 lines instead of 3, how would the variable 'lines' change in variable_tracker?

A It would have 5 elements in the array instead of 3

B It would still have 3 elements

C It would be nil

D It would be a string, not an array

💡 Hint

Variable 'lines' holds all lines read; more lines means a longer array (see variable_tracker).

Concept Snapshot

File.readlines('filename') reads all lines into an array.
Each element includes the newline character '\n'.
Use .each to loop line-by-line.
Use line.chomp to remove trailing newline when printing.
File is closed after reading all lines.

Full Transcript

This example shows how Ruby's File.readlines method reads all lines from a file into an array. Each line includes a newline character at the end. We then loop over this array using each, processing one line at a time. Using line.chomp removes the newline for clean output. The file is opened and closed automatically during the read. The variable 'lines' holds the array of all lines, and 'line' holds the current line during iteration. The iteration stops when all lines are processed.