所属专题：[Python社区](README.md)
 
## 问题
**输入：** 一个最多1000行的文本文件。
**输出：** 一个文件，包含原始输入文件的所有偶数行内容。设定行数从1开始计算。
**样例数据：**
```
Bravely bold Sir Robin rode forth from Camelot
Yes, brave Sir Robin turned about
He was not afraid to die, O brave Sir Robin
And gallantly he chickened out
He was not at all afraid to be killed in nasty ways
Bravely talking to his feet
Brave, brave, brave, brave Sir Robin
He beat a very brave retreat
```
**样例输出：**
```
Yes, brave Sir Robin turned about
And gallantly he chickened out
Bravely talking to his feet
He beat a very brave retreat
```
 
## 背景知识
该问题涉及Python语言的文件读写。详情请查阅ROSALIND网站上[关于该问题的背景说明](http://rosalind.info/problems/ini5/)。
 
## 解答
```python
import linecache
def evenlines(fin, fout):
"""从输入文件中提取偶数序的行，输出到另一个文件（行序从1开始）"""
with open(fin, 'r') as f1:
nrow = len(f1.readlines())
with open(fout, 'w') as f2:
for i in range(1, nrow+1):
if i%2==0:
f2.write(linecache.getline(fin, i))
## --main--
evenlines("rosalind_ini5.txt", "rosalind_ini5_out.txt")
```
