Accessing the index or a row in a matrix/ list of lists

Question

Without using numpy, my question is related to the for loop in Python, as it doesn't have for i=0, i++ ...

only for i in list

I need

for i in list 
  # print(i.index)
  # i.index being 0 at the first element of the list and
  # ordered to len(list)-1 at the last element

There is something I am missing :

mat=[[1,0,1],
     [1,1,1],
     [1,0,1]]
 
dict1= []
dict2= []
n=len(mat[0])
for row in mat :
    for j in range(n):           
        if row[j]==0: 
            dict1.append(mat.index(row))
            dict2.append(j)
print(dict1)
print(dict2)

output should be [0,2] and [1,1] but instead I am getting

[0,0] 
[0,1]

How do we explain this?

Does this answer your question? [Accessing the index in 'for' loops](https://stackoverflow.com/questions/522563/accessing-the-index-in-for-loops) — ssp, May 04 '22 at 14:47

score 1 · Answer 1 · answered May 04 '22 at 14:46

1

Another possibility is for i,v in enumerate(my_list): print(i, v). The i value goes from 0 to len(my_list) - 1. The functionenumerate has keyword argument start that allow to specify where i starts (as above this defaults to 0).

answered May 04 '22 at 14:46

thebadgateway

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score 0 · Answer 2 · answered May 04 '22 at 14:22

0

Use .index()

l=[1,2,3]
for i in l:
    print(i,l.index(i))

1 0
2 1
3 2

answered May 04 '22 at 14:22

Sruthi

2,908
1
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1

.. What if `l = [1, 2, 1]` – ssp May 04 '22 at 14:49
is this behavior standard in python? never at all the indices get mixed, say after altering the list? – NotaChoice May 04 '22 at 15:30
1

@ssp , index() return just the first occurrence, so it is basically useless here – NotaChoice May 04 '22 at 16:14
This turns the simple loop into a nested for loop O(n^2) – Abhijit Sarkar Aug 15 '22 at 15:47

score 0 · Answer 3 · answered May 04 '22 at 14:38

Using .index() is okay, as long as the list does not have double entries, because index returns the index of the first occurrence of an item.

The for loop can iterate over an index i in some range as follows:

l=[1,2,3,5,6,7,3,1,89,2,77]
for i in range(0,len(l)):
    print(l[i], i)

Accessing the index or a row in a matrix/ list of lists

3 Answers3