Two elements of array sum - AndrewMZ6/python_cheat_sheet GitHub Wiki

Create algorithm that finds the indexes of elements whos pair sum is equals to 9.
The algo have to be smarter than just looping through every possible pair for each element

Green colored boxes are number pairs that were already checked at previous iteration

nums = [1, 3, 5, 6, 11, 23]

L = []

for i in range(len(nums) -1):
	for k in range(len(nums[i + 1:])):
		if nums[i] + nums[i+k+1] == 9:
			L.append((i, i+k+1))

print(L)

At the first iteration i = 0 and (since len(nums) = 6, thus i = range(6 - 1)) it's range is i=0, 1, 2, 3, 4
The range for k is defined by the slice len(nums[i + 1:]) and at the first iteration equals to len(nums[1:]).
This means that we count every element that has higher index than that of the current. Sequently the range of k is range(5) = 0, 1, 2, 3, 4. Notice that we don't subtract 1 in the second for loop.
nums[i] + nums[i+k+1] at the first iterations gives nums[0] + nums[1] and as k grows to it's biggest value the sum gives nums[0] + nums[5]
And thus the first iteration of nested for loop os over. i changes it's value to 2 and the nested for loop starts again

MATLAB code

indexes = cell(0, 0);
k = 1;

a = [1, 3, 5, 6, 11, 23];
for i = 1:length(a)
    for j = i + 1:length(a)
        if a(i) + a(j) == 9
            indexes{k} = [i, j];
            k = k + 1;
        end
    end
end