How to Convert an Array of Digits to an Integer in Ruby?

In Ruby, you can convert an array of digits to an integer in the following ways:

• Looping, Shifting and Adding Digits to the Right;
• Convert Array to String and Then to Integer.

Looping, Shifting and Adding Digits to the Right

To convert an array of digits to an integer, you can use the Enumerable#inject method (or its alias Enumerable#reduce), in the following way:

array.inject(0) { | memo, digit | memo * 10 + digit }

Passing 0 as the initial operand (or base case) is important as nil would be returned for empty arrays otherwise.

This works in the following way:

• An accumulator/memo value is passed to the block (as the first argument), which contains the result of the calculation of previous iteration;
• By multiplying the "accumulated" (memo) value by 10, each value in the array is widened to the right, to which then the current digit is added. This results in each digit in the array being added to the resulting integer one by one;
• At the end of the iteration, the final (single/reduced) value of accumulator/memo is returned.

For example:

num = [1, 2, 3, 4, 5].inject(0) {
    | memo, digit | memo * 10 + digit
}

puts num #=> 12345
puts num.class #=> Integer

This can be visualized like so:

# Iteration#1 (memo = 0, digit = 1) => 0 * 10 + 1 = 1
# Iteration#2 (memo = 1, digit = 2) => 1 * 10 + 2 = 12
# Iteration#3 (memo = 12, digit = 3) => 12 * 10 + 3 = 123
# Iteration#4 (memo = 123, digit = 4) => 123 * 10 + 4 = 1234
# Iteration#5 (memo = 1234, digit = 5) => 1234 * 10 + 5 = 12345
# Iteration#6 (memo = 12345, digit = nil) => 12345

Convert Array to String and Then to Integer

You can use the Array#join method to convert an array to a string, and then call String#to_i to convert the resulting string to an integer:

array.join.to_i

For example:

num = [1, 2, 3, 4, 5].join.to_i

puts num #=> 12345
puts num.class #=> Integer