Discover data structures and algorithms using Python. Gain insights into solving real-world problems and typical interview questions with detailed reviews, explanations, and hands-on coding exercises.

Data structures and algorithms are among the most fundamental concepts of Computer Science. Whether it’s real-world problems you’re trying to solve or the typical coding question asked in an interview, almost every problem requires you to demonstrate a deep understanding of data structures and algorithms. 

This course is a detailed review of some of the most common data structures and algorithms that you’ll see in interviews and your everyday work. With implementation details, thorough explanations, and hands-on coding exercises, you’ll quickly gain the confidence you need to solve any problem, no matter the situation.

Data Structures and Algorithms in Python

In this lesson, we will review the solution to the challenge in the last lesson. The problem is as follows:

**Given some numeric string as input, convert the string to an integer.**

You already know that we are not allowed to use the built-in `int()` function. Now to convert a string into an integer, we can make use of the following arithmetic:

As you can see, by multiplying each digit with the appropriate base power, we can get the original integer back. Let's implement this in Python. 

# Implementation

def str_to_int(input_str):

    output_int = 0

    if input_str[0] == '-':
        start_idx = 1
        is_negative = True
    else:
        start_idx = 0
        is_negative = False

    for i in range(start_idx, len(input_str)):
        place = 10**(len(input_str) - (i+1))
        digit = ord(input_str[i]) - ord('0')
        output_int += place * digit

    if is_negative:
        return -1 * output_int
    else:
        return output_int



s = "554"
x = str_to_int(s)
print(type(x))

s = "123"
print(str_to_int(s))

s = "-123"
print(str_to_int(s))

# Explanation

On **line 3**, `output_int` is initialized to `0`. The code on **lines 5-10** deal with the polarity of the number represented by `input_str`:
```
    if input_str[0] == '-':
        start_idx = 1
        is_negative = True
    else:
        start_idx = 0
        is_negative = False
```
If the first character of `input_str` is `-`, `start_idx` is set to `1` and `is_negative` is set to `True` (**lines 6-7**). `start_idx` is to indicate that we'll start processing `input_str` from the first index as `-` is on the zeroth index. However, if `input_str[0]` is not equal to `-`, `start_idx` and `is_negative` are set to `0` and `False` respectively on **lines 9-10**. 

Next, let's discuss the `for` loop on **line 12**:

```python
    for i in range(start_idx, len(input_str)):
        place = 10**(len(input_str) - (i+1))
        digit = ord(input_str[i]) - ord('0')
        output_int += place * digit
```

This `for` loop will run from `start_idx` to `len(input_str) - 1`. On **line 13**, `place` is set to `10` raised to the power of `len(input_str - (i+1))`. For example, if `input_str` is `123`, `place`  will have the following values:
```python
input_str = 123
len(input_str) = 3

i = 0 
place = 10** (3 - (0+1)) = 10** (2) = 100 
i = 1
place = 10** (3 - (1+1)) = 10** (1) = 10
i = 2
place = 10** (3 - (2+1)) = 10** (0) = 1
```

You may notice that `place` is the base power calculated according to the place of the digit in the number. For the unit place, `place` will equal `1`. Once we have calculated the base power, we need to extract the digit as a number. Therefore, **on line 13**, we get the digit as an integer type by subtracting the Unicode code point of `0` from the Unicode code point of `input_str[i]`. The  Unicode code points are calculated using the `ord` function.

Finally, all that is left for us is to multiply `digit` with `place` and add it to `output_int` which is done on **line 15**. 

**Lines 17-20** return `output_int` with its corresponding polarity. 
```
    if is_negative:
        return -1 * output_int
    else:
        return output_int
```

If `is_negative` is `True`, `output_int` is returned after multiplication with `-1` on **line 18**; otherwise, it is returned as it is on **line 20**. 

That's all, folks! Now we have come to an end to the course. I hope you had an amazing learning experience!

This lesson contains the solution review to convert a string into an integer in Python.  

Introduction

Stack

Singly Linked Lists

Circular Linked Lists

Doubly Linked Lists

Arrays

Binary Trees

Binary Search Trees

Binary Search

Recursion

String Processing

Solution Review: String to Integer