What is array.inject() in Ruby?

The inject method in Ruby

The inject method in Ruby is defined in enum.c, also known as enumerable. Enumerable is a collection of classes such as arrays, hashes, and structs that includes search and traversal methods. 

The inject method sequentially accesses the elements of the enumerator and operates on them. It is most commonly used for arrays and behaves similarly to Ruby's reduce method. 

The syntax for inject is given below: 

## Defining initial operand and symbol
inject(initial, sym) ⇒ Object  
## Defining initial value before the memo, obj, and operation
inject(initial) {|memo, obj| ... } ⇒ Object  
## Calling inject method without specifying initial value
inject {|memo, obj| ... } ⇒ Object

  • Line 2: We only define the initial operand and the symbol. This means that we can assign an initial value on which we wish to start applying the operation indicated by the symbol. 

  • Line 5: We define the initial value before and then the memo, obj and the operation to be applied which is denoted by .... The obj accesses each value of the enumerator, applies the operation, and stores the final value in memo.

  • Line 8: We do not specify an initial value while calling the inject method. 

Parameters

The array.inject method takes the mapping function as input to apply it to the elements of an array. Along with the operator that will be applied to the array. We also specify an accumulator value memo and an element obj.  The obj collects each element of the array and memo stores the value after the operation has been applied 

Return value

The array.inject method returns a value corresponding to the mapping function performed on the array.

Example

The following example will help explain how one can use the array.inject method. 

num = [1,2,3,4,5]
## Calling inject function with subtract operator
puts num.inject(:-)
## Calling inject function with modified operator
puts num.inject{|difference, n| difference-n}

In the code above, we started by defining an array called num. After this, we call the inject function using the - operator. This means that the function will take each element of the array and subtract it from the next. In this case, it performs (((1-2)-3)-4)-5 which results in -13.

We can redefine this function by taking each number n and subtracting it from the difference. The variable difference saves the result of each subtraction to facilitate the subtraction from the next element of the array.

Setting the initial operand

The syntax indicates that the inject method can be called in different ways. The main difference lies in setting the initial operand.

num = [1,2,3,4,5]
## Calling inject method with the operand and operator
puts num.inject(0, :-)
## Setting initial operator before defining memo, obj, and operator
puts num.inject(0){|difference, n| difference-n}

As shown in the code above:

  • Line 4: We set the initial operand with the symbol.

  • Line 7: We set the initial operand before defining the memo, obj, and operator. 

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