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Data Structures and Algorithms in Python

Solution Review: String to Integer

Explore how to convert a numeric string to an integer in Python without the int() function. Understand handling number polarity, calculating place value, and extracting digits using Unicode code points. This lesson reviews a step-by-step implementation that solidifies your grasp of string processing and arithmetic conversion techniques.

We'll cover the following...

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

Python 3.5
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 ...