Iterating over tables

It is pretty common after we have a table filled with some data, we may want to do something with all these "elements" of a table. For tables with sequential numeric indices it seems easy with for loop. Here is an example where we want to sum all elements:

numbers = {3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5}

sum = 0

for i = 1,#numbers do
    sum = sum + numbers[i]
end

print('Total is:', sum)

However, we now know that table may have non-numeric indices! How to deal with them? We shall learn in a minute. Moreover we shall learn about slightly more convenient approach even to iterating over tables with simple sequential indexing.

Magical "pairs" function

Regard the table containing salaries of employees. Employee names are used as keys:

salaries = {
    ['John M Carpenter'] = 1800,
    ['Jane L Puppetmaster'] = 2700,
    ['Jess W Beerman'] = 2150,
}

sum = 0

for name, money in pairs(salaries) do
    print(name, 'earns', money, 'dollars')
    sum = sum + money
end

print('In total they earn ' .. sum .. ' per month')

What new do we see here? The pairs(...) function which takes some table as argument and then on every call returns new pair of key and value from it. We assign such "key-value" pairs to name and money variables on every iteration. Note slightly differing for syntax with the word in word in place of some equals sign.

Inside the function body we do two things: print the current name-salary pair - and then add the salary to the sum variable. Try running it and the idea hopefully is obvious.

The same approach could be used with numeric indices!

However if your table has sequential indexing (i.e. from 1, 2, 3 up to size of the table) - it is advisable to use ipairs function. It has slight differences in behavior, which could be overlooked at the first glance:

• it stops immediately if it doesn't find next numeric index (i.e. if there is some gap like 1, 2, 4, 5)
• it ignores non-numeric, negative and fractional indices, if they exist in the table
• it guarantees the proper order (which is not true with pairs).

So we can rewrite our first example with modified loop:

for _, n in ipairs(numbers) do
    sum = sum + n
end

Note that we don't need indices returned by ipairs - only values are of interest. Hence we assign them to _ variable - it is usual way to mark that we don't care about whatever is assigned there.

For exercise please try solving Array Counters task. You'll see it mentions another problem, Vowel Count - which is simpler in that it doesn't require array but will make to recollect something about string operations - so it would be good to solve it too.