# How can I use numpy.genfromtxt to read the lower triagular matrix into a numpy array?

I have a bottom diagonal matrix like this

```
1
2 3
4 5 6
```

in a text file and I want to read it into a numpy array with zeros above the main diagonal. The simplest code I can think of

```
import io
import scipy
data = "1\n2 3\n4 5 6"
scipy.genfromtxt(io.BytesIO(data.encode()))
```

doesn't work with

```
ValueError: Some errors were detected !
Line #2 (got 2 columns instead of 1)
Line #3 (got 3 columns instead of 1)
```

which makes sense because there is nothing in the text file at the top diagonal of the matrix, so numpy doesn't know what to interpret as missing values.

Looking at the documentation , I want something like a parameter `invalid_raise = False`

, except that I don't want to skip the "invalid" lines,

With some changes below, the last code I am using is

```
import scipy
with open("data.txt", "r") as r:
data = r.read()
n = data.count("\n") + 1
mat = scipy.zeros((n, n))
mat[scipy.tril_indices_from(mat)] = data.split()
```

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1 answer

`np.tril_indices_from()`

makes it easy to populate the bottom triagular matrix of an array using fancy idexing:

```
data = "1\n2 3\n4 5 6"
n = len(data.split('\n'))
data = data.replace('\n', ' ').split()
a = np.zeros((n, n))
a[np.tril_indices_from(a)] = data
print(a)
#array([[ 1., 0., 0.],
# [ 2., 3., 0.],
# [ 4., 5., 6.]])
```

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