String concatenation and << in Ruby - Step-by-Step Execution

Concept Flow - String concatenation and <<

Start with string s = "Hello"

↓

Use << to add " World"

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s now "Hello World"

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Use + to add "!"

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s + "!" returns new string "Hello World!" but s unchanged

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Print s and s + "!"

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END

Start with a string, add more text using << which changes the original string, and + which creates a new string without changing the original.

Execution Sample

Ruby

s = "Hello"
s << " World"
puts s
puts s + "!"

This code adds " World" to s using <<, then prints s and s with "!" added using +.

Execution Table

Step	Code executed	s value	Result/Output
1	s = "Hello"	"Hello"	No output
2	s << " World"	"Hello World"	No output
3	puts s	"Hello World"	Hello World
4	puts s + "!"	"Hello World"	Hello World!

💡 All code executed; s was mutated by << but not by +; program ends.

Variable Tracker

Variable	Start	After Step 2	After Step 4 (final)
s	"Hello"	"Hello World"	"Hello World"

Key Moments - 2 Insights

Why does s change after using << but not after using +?

Because << modifies the original string s (see Step 2 in execution_table), while + creates a new string without changing s (see Step 4).

What does puts s + "!" print if s is not changed?

It prints the new string "Hello World!" created by +, but s itself remains "Hello World" (see Step 4).

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the value of s after s << " World"?

A " World"

B "Hello"

C "Hello World"

D "Hello World!"

💡 Hint

Check the 's value' column at Step 2 in execution_table.

At which step does the program output "Hello World!"?

A Step 2

B Step 4

C Step 3

D No step outputs "Hello World!"

💡 Hint

Look at the 'Result/Output' column in execution_table for each step.

If we replaced s << " World" with s + " World", what would be the value of s after Step 2?

A "Hello"

B "Hello World"

C " World"

D "Hello World!"

💡 Hint

Refer to variable_tracker and remember + does not change s, it returns a new string.

Concept Snapshot

String concatenation in Ruby:
- << adds text to the original string (mutates it)
- + creates a new string without changing the original
- Use << to change the string in place
- Use + to combine strings without mutation

Full Transcript

This example starts with a string s set to "Hello". Using << adds " World" to s, changing s to "Hello World". When printing s, it shows "Hello World". Using + to add "!" creates a new string "Hello World!" but does not change s. Printing s + "!" shows "Hello World!" but s remains "Hello World". This shows << changes the original string, while + returns a new string without changing the original.