YouTipPython Exercise Example38
# Python2.x Python Exercise 38
[ Python 100 Examples](#)
**Question:** Calculate the sum of the main diagonal elements of a 3*3 matrix.
**Program Analysis:** Use a double for loop to control the input of the two-dimensional array, then accumulate a and output the result.
Program source code:
## Example
```python
#!/usr/bin/python
# -*- coding: UTF-8 -*-
if __name__ == '__main__':
a = []
sum = 0.0
for i in range(3):
a.append([])
for j in range(3):
a.append(float(input("input num:n")))
for i in range(3):
sum += a
print(sum)
The output result of the above example is:
input num:
78
input num:
34
input num:
23
input num:
34
input num:
56
input num:
33
input num:
12
input num: