For loop in Rust - Step-by-Step Execution

Concept Flow - For loop

Start

↓

Get iterable

↓

Take next item?

No→Exit loop

Yes↓

Execute loop body

↓

Repeat: Take next item?

The for loop gets an iterable, takes each item one by one, runs the loop body, and stops when no items remain.

Execution Sample

Rust

fn main() {
    for i in 1..4 {
        println!("{}", i);
    }
}

This code prints numbers 1, 2, and 3 using a for loop over a range.

Execution Table

Iteration	i value	Condition (next item?)	Action	Output
1	1	Yes	Print 1	1
2	2	Yes	Print 2	2
3	3	Yes	Print 3	3
4	-	No	Exit loop	-

💡 No more items in range 1..4, loop ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	-	1	2	3	-

Key Moments - 2 Insights

Why does the loop stop after printing 3 and not include 4?

The range 1..4 in Rust includes 1, 2, and 3 but excludes 4, so the loop ends after 3 as shown in execution_table row 4.

What happens if the range is empty, like 5..5?

The loop never runs because there are no items to take, so the condition 'next item?' is No immediately, similar to execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'i' at iteration 2?

A 1

B 3

C 2

D -

💡 Hint

Check the 'i value' column in execution_table row 2

At which iteration does the loop condition become false?

A Iteration 3

B Iteration 4

C Iteration 1

D Never

💡 Hint

Look at the 'Condition (next item?)' column in execution_table row 4

If the range changed to 1..3, how many times would the loop run?

A 2 times

B 1 time

C 3 times

D 0 times

💡 Hint

Ranges exclude the upper bound, so 1..3 includes 1 and 2 only, check variable_tracker for similar pattern

Concept Snapshot

Rust for loop syntax:
for variable in iterable {
    // code
}

Runs loop body once per item in iterable.
Ranges like 1..4 include start but exclude end.
Loop stops when no more items.

Full Transcript

This visual trace shows how a Rust for loop works by taking each item from an iterable one by one. The example uses a range 1..4 which includes 1, 2, and 3 but not 4. Each iteration assigns the current number to variable 'i' and prints it. The loop stops when there are no more items. The variable tracker shows how 'i' changes each iteration. Key moments clarify why 4 is excluded and what happens with empty ranges. The quiz tests understanding of iteration values and loop stopping conditions.

Practice

1. What does a for loop do in Rust?

easy

A. Declares a variable

B. Defines a new function

C. Repeats actions for each item in a collection

D. Stops the program immediately

Solution

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

   A for loop runs code repeatedly for each item in a list or range.

2. Step 2: Match the description to the options

   Only Repeats actions for each item in a collection describes repeating actions over items in a collection.

3. Final Answer:

   Repeats actions for each item in a collection -> Option C

4. Quick Check:

   For loop = repeat over items [OK]

Hint: For loops repeat actions over collections [OK]

Common Mistakes:

• Confusing for loop with function definition
• Thinking for loop declares variables
• Believing for loop stops program

2. Which of these is the correct syntax for a for loop iterating over numbers 1 to 3 in Rust?

easy

A. for i = 1 to 3 { println!("{}", i); }

B. for i in 1..=3 { println!("{}", i); }

C. for (i in 1..3) println!("{}", i);

D. for i in range(1,3) { println!("{}", i); }

Solution

1. Step 1: Recall Rust for loop syntax

   Rust uses for variable in range { } with ranges like 1..=3 to include 3.

2. Step 2: Check each option

   for i in 1..=3 { println!("{}", i); } uses correct Rust syntax with inclusive range. Others use invalid syntax or functions not in Rust.

3. Final Answer:

   for i in 1..=3 { println!("{}", i); } -> Option B

4. Quick Check:

   Rust for loop uses in and ranges [OK]

Hint: Use 'for var in start..=end' for inclusive ranges [OK]

Common Mistakes:

• Using 'to' instead of 'in' keyword
• Using parentheses around loop variable
• Using Python or other language range syntax

3. What is the output of this Rust code?

let mut sum = 0;
for i in 1..4 {
    sum += i;
}
println!("{}", sum);

medium

A. 6

B. 10

C. 9

D. 7

Solution

1. Step 1: Identify the loop range and iterations

   The loop runs for i = 1, 2, 3 because 1..4 excludes 4.

2. Step 2: Calculate the sum of iterations

   sum = 0 + 1 + 2 + 3 = 6

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Remember range excludes the end number [OK]

Common Mistakes:

• Including 4 in the sum
• Starting sum at 1 instead of 0
• Confusing inclusive and exclusive ranges

4. Find the error in this Rust for loop code:

for i in 0..5
    println!("{}", i);
}

medium

A. Variable i is not declared

B. Range syntax is incorrect

C. println! macro is used incorrectly

D. Missing curly braces around loop body

Solution

1. Step 1: Check loop syntax

   Rust requires curly braces { } around the loop body if it has more than one statement or to be explicit.

2. Step 2: Identify the missing brace

   The code has a closing brace '}' without an opening brace '{' before println!().

3. Final Answer:

   Missing curly braces around loop body -> Option D

4. Quick Check:

   For loop body needs braces [OK]

Hint: Always use braces {} for loop bodies [OK]

Common Mistakes:

• Forgetting opening brace
• Assuming range syntax is wrong
• Thinking variable needs declaration outside loop

5. How can you use a for loop in Rust to print only even numbers from 2 to 10 inclusive?

hard

A. for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } }

B. for i in 2..10 { if i % 2 = 0 { println!("{}", i); } }

C. for i in 2..=10 { if i / 2 == 0 { println!("{}", i); } }

D. for i in 2..=10 { if i % 2 != 0 { println!("{}", i); } }

Solution

1. Step 1: Understand the range and condition

   We want numbers from 2 to 10 inclusive, so use 2..=10. To print even numbers, check if remainder when divided by 2 is zero.

2. Step 2: Check each option's condition

   for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } } correctly uses i % 2 == 0. for i in 2..10 { if i % 2 = 0 { println!("{}", i); } } uses assignment (=) instead of comparison (==). for i in 2..=10 { if i / 2 == 0 { println!("{}", i); } } uses division instead of modulo. for i in 2..=10 { if i % 2 != 0 { println!("{}", i); } } prints odd numbers.

3. Final Answer:

   for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } } -> Option A

4. Quick Check:

   Use modulo == 0 for even check [OK]

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

Common Mistakes:

• Using '=' instead of '==' in condition
• Using division instead of modulo
• Printing odd numbers by mistake