python 3.x - Why doesn't variable increment in list comprehension -


given; 

a = [[1,2,3,4],      [5,6,7,8],      [9,0,1,2],      [3,4,5,6]]

i want list of diagonal - using single list comprehension statement. 

[1,6,1,6]

i expected with

i = -1 # (cheating didn not know how else it) dia_1 = [r[i] (r,i) in [(r, i+1) r in a]]

but result;

[1, 5, 9, 3]

so, must not incrementing:

q1: why doesn't 'i' increment?

q2: how fix statement achieve desirede result?

why not like

dia_1 = [a[n][n] n in range(len(a))]

or

dia_1 = [r[n] n, r in enumerate(a)]

while second approach more aesthetically pleasing me personally, yield identical results. both assume nth row has nth element, regardless of actual shape of list.

to answer question incrementing, there no assignment i anywhere in comprehension. i + 1 has constant value 0, fetching first column, expected.


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