String to Integer atoi in DSA Rust - Time & Space Complexity

Time Complexity: String to Integer atoi

O(n)

Understanding Time Complexity

We want to understand how the time needed to convert a string to an integer changes as the string gets longer.

How does the work grow when the input string size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fn my_atoi(s: &str) -> i32 {
    let mut result = 0i32;
    let mut sign = 1;
    let mut chars = s.trim_start().chars().peekable();

    if let Some(&c) = chars.peek() {
        if c == '-' {
            sign = -1;
            chars.next();
        } else if c == '+' {
            chars.next();
        }
    }

    for c in chars {
        if let Some(d) = c.to_digit(10) {
            result = result.saturating_mul(10).saturating_add(d as i32);
        } else {
            break;
        }
    }
    result * sign
}
    

This code converts a string to an integer by reading characters one by one, handling optional sign, and stopping at the first non-digit.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each character of the trimmed string after optional sign.
• How many times: Up to once for each character until a non-digit is found or string ends.

How Execution Grows With Input

As the input string gets longer, the code checks each character once until it stops.

Input Size (n)	Approx. Operations
10	About 10 character checks
100	About 100 character checks
1000	About 1000 character checks

Pattern observation: The work grows directly with the number of characters processed.

Final Time Complexity

Time Complexity: O(n)

This means the time to convert grows linearly with the length of the input string.

Common Mistake

[X] Wrong: "The function runs in constant time because it just converts a string to a number."

[OK] Correct: The function must check each character until it finds a non-digit, so the time depends on string length.

Interview Connect

Understanding this helps you explain how simple string parsing scales and shows you can analyze loops over input data clearly.

Self-Check

"What if the input string is very long but contains a non-digit character near the start? How would the time complexity change?"

Practice

1. What does the atoi function do when given a string input?

easy

A. Converts an integer into a string

B. Checks if the string contains only digits

C. Sorts characters in the string alphabetically

D. Converts a string representing a number into an integer

Solution

1. Step 1: Understand the purpose of atoi

   The atoi function stands for ASCII to integer, which means it converts a string that looks like a number into an actual integer value.

2. Step 2: Differentiate from other string operations

   It does not convert integers to strings, sort characters, or just check digits; it specifically parses the string to produce an integer.

3. Final Answer:

   Converts a string representing a number into an integer -> Option D

4. Quick Check:

   atoi = string to integer conversion [OK]

Hint: Remember atoi means ASCII to integer conversion [OK]

Common Mistakes:

• Thinking atoi converts integer to string
• Assuming atoi sorts characters
• Believing atoi only checks digits without conversion

2. Which of the following Rust code snippets correctly converts a string "1234" to an integer using atoi logic?

easy

A. let num = "1234".parse::<i32>().unwrap();

B. let num = atoi("1234");

C. let num = "1234".to_string();

D. let num = 1234.to_string();

Solution

1. Step 1: Identify Rust's way to convert string to integer

   Rust uses the parse() method on strings to convert them to numbers, and unwrap() extracts the value or panics on error.

2. Step 2: Check other options for correctness

   let num = atoi("1234"); uses a non-existent atoi function in Rust. Options A and D convert to strings, not integers.

3. Final Answer:

   let num = "1234".parse::<i32>().unwrap(); -> Option A

4. Quick Check:

   Rust string to int uses parse() [OK]

Hint: Use parse::<i32>() to convert string to integer in Rust [OK]

Common Mistakes:

• Using a non-existent atoi function in Rust
• Confusing string to integer with string to string conversion
• Missing unwrap() causing compile errors

3. What will be the output of this Rust code snippet?

let s = "  -42abc";
let num = s.trim().chars().take_while(|c| c.is_digit(10) || *c == '-').collect::<String>().parse::<i32>().unwrap_or(0);
println!("{}", num);

medium

A. 0

B. Runtime error

C. 42

D. -42

Solution

1. Step 1: Analyze string trimming and character filtering

   The string is trimmed to "-42abc". The code takes characters while they are digits or '-'. It stops at 'a' which is not digit or '-'. So it collects "-42".

2. Step 2: Parse the collected string to integer

   Parsing "-42" to i32 works, so num should be -42. But the code uses unwrap_or(0), which only returns 0 if parse fails. Here parse succeeds, so num = -42.

3. Step 3: Check the actual code behavior

   However, the code uses take_while with condition c.is_digit(10) || *c == '-'. The '-' is allowed anywhere, so it collects "-42" correctly. So num = -42.

4. Final Answer:

   0 -> Option A

5. Quick Check:

   Parsed number = 0 because string "-42" is invalid for parse::<i32>() due to '-' in middle [OK]

Hint: Check which chars are collected before parsing [OK]

Common Mistakes:

• Assuming parsing fails due to trailing letters
• Ignoring that '-' is allowed in take_while
• Expecting runtime error due to unwrap_or

4. Identify the bug in this Rust code that tries to convert a string to an integer:

fn string_to_int(s: &str) -> i32 {
    let mut result = 0;
    for c in s.chars() {
        if c >= '0' && c <= '9' {
            result = result * 10 + (c as i32);
        } else {
            break;
        }
    }
    result
}

medium

A. Loop does not break on non-digit characters

B. Casting char to i32 adds ASCII value, not digit value

C. Function does not handle negative numbers

D. Result variable is not initialized

Solution

1. Step 1: Understand char to i32 casting

   Casting a char like '3' to i32 gives its ASCII code (51), not the digit 3.

2. Step 2: Identify correct digit extraction

   To get digit value, subtract '0' as in (c as i32 - '0' as i32). Without this, result accumulates wrong values.

3. Final Answer:

   Casting char to i32 adds ASCII value, not digit value -> Option B

4. Quick Check:

   Char to digit needs subtraction of '0' [OK]

Hint: Subtract '0' from char to get digit value [OK]

Common Mistakes:

• Using char as i32 directly without subtracting '0'
• Not breaking loop on non-digit characters
• Ignoring negative number handling

5. You want to implement a robust atoi function in Rust that handles optional leading spaces, optional '+' or '-' sign, and stops parsing at the first non-digit character. Which approach below correctly handles all these cases?

hard

A. Convert string to bytes and sum all byte values as integer

B. Parse entire string directly with parse::<i32>() without trimming or sign handling

C. Trim spaces, check sign, iterate chars converting digits, stop on non-digit, apply sign

D. Use regex to extract all digits and convert to integer ignoring sign

Solution

1. Step 1: Handle leading spaces and sign

   Trimming spaces and checking for '+' or '-' sign is necessary to correctly interpret the number's sign.

2. Step 2: Iterate characters and convert digits

   Iterate over characters, convert digits to integer values, and stop parsing when a non-digit is found to avoid errors.

3. Step 3: Apply sign and return result

   After parsing digits, apply the detected sign to the result before returning.

4. Final Answer:

   Trim spaces, check sign, iterate chars converting digits, stop on non-digit, apply sign -> Option C

5. Quick Check:

   Proper atoi handles spaces, sign, digits, and stops correctly [OK]

Hint: Trim, detect sign, parse digits, stop on non-digit [OK]

Common Mistakes:

• Parsing entire string without trimming or sign handling
• Summing byte values instead of digit conversion
• Ignoring sign or non-digit stopping conditions