Convert a String to an Int (without Int(“1”) nor radix)

REQUIREMENTS — YOUR SOLUTION SHALL NOT USE ANY OF THE FOLLOWING APPROACHES:

• Int(“123”)
• Int(“123”, radix: 10)
• Character.wholeNumberValue
• NumberFormatter()

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ASSUMPTIONS:

The input string that will be passed to the function you create:

• will only contain whole numbers
• will not contain floating-point numbers
• will not be an empty string

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THE SOLUTIONS BELOW:

• O(N) time
• O(1) auxilliary space
• Swift 5

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SOLUTION #1/3

• L5: ‘.reversed()’ == O(1) time
• L6: ‘base’ type == Decimal.
• L7: We subtract 48 to get a # between 0–9

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SOLUTION #2/3

This solution is risky & will not work for every scenario. I advise against using it. I initially liked it because it appear to perform ~1 less operation per loop iteration compared to the solution above.

This solution will not return accurate results for every input string that is passed to the function; due to rounding issues that arise from the use of exponents, which are used to represent the number.

IEEE-754 can help you understand when this solution will & will not be accurate, but after all is said & done, any extra operations needed to turn this into a dependable solution will likely make this solution slower than solution #1.

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SOLUTION #3/3

The requirements say not to use this solution, but in case you want to see how it can be used:

#wholeNumberValue

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RESOURCES:

• https://developer.apple.com/documentation/swift/int/integer_operators
• https://www.thoughtco.com/definition-of-base-10-2312365
• https://github.com/apple/swift/blob/main/stdlib/public/core/IntegerParsing.swift
• https://developer.apple.com/documentation/swift/int
• https://sarunw.com/posts/how-to-convert-string-to-int-in-swift/
• https://en.wikipedia.org/wiki/IEEE_754