Given an array (or list), and a target element, find all the indices that the element appears in it. For example,

Array: [1, 2, 3, 4, 5, 5, 6, 7, 8], and find the element 5, which appears in index 4 and 5, thus return [4, 5].

This is a trivial question and most programmers know how to do this using a for loop. For example,

In Python:

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def getIndices(data, element):
    x = []
    for i in range(len(data)):
        if data[i] == element:
            x.append(i)
    return x

def getIndices(data, element):
    x = []
    for i in range(len(data)):
        if data[i] == element:
            x.append(i)
    return x

Or similarly in Javascript:

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function getIndices(data, element) {
    let x = [];
    for (let i = 0; i < data.length; i += 1) {
        if (data[i] === element) {
            x.push(i);
        }
    }
    return x;
}

function getIndices(data, element) {
    let x = [];
    for (let i = 0; i < data.length; i += 1) {
        if (data[i] === element) {
            x.push(i);
        }
    }
    return x;
}

However, as a spirit of avoiding loops as much as you can, we can simplify the implementation. In Python, we can use the enumerate and the list comprehension which gives us the following short and concise solution to return the list of the indices for a target element in the array.

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def getIndices(data, element):
    return [x for x, y in enumerate(data) if y == element]

def getIndices(data, element):
    return [x for x, y in enumerate(data) if y == element]

In Javascript, we could similarly implement a enumerate function, alternatively, we can chain map and filter function.

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function getIndices(data, element) {
    let i = -1;
    return data.map(x => {
        i ++; // incrementing the index
        // return index for target element
        return x == element ? i : false;
    }).filter(x => typeof x !== 'boolean');  
}

function getIndices(data, element) {
    let i = -1;
    return data.map(x => {
        i ++; // incrementing the index
        // return index for target element
        return x == element ? i : false;
    }).filter(x => typeof x !== 'boolean');  
}

The map function maps those target elements with their indices, and others similar set to false – which will be filtered out via filter.

–EOF (The Ultimate Computing & Technology Blog) —