所屬專題：[Python社區](README.md)
 
## 問題
**輸入：** 一個最多1000行的文本文件。
**輸出：** 一個文件，包含原始輸入文件的所有偶數行內容。設定行數從1開始計算。
**樣例數據：**
```
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
```
**樣例輸出：**
```
Yes, brave Sir Robin turned about
And gallantly he chickened out
Bravely talking to his feet
He beat a very brave retreat
```
 
## 背景知識
該問題涉及Python語言的文件讀寫。詳情請查閱ROSALIND網站上[關于該問題的背景說明](http://rosalind.info/problems/ini5/)。
 
## 解答
```python
import linecache
def evenlines(fin, fout):
"""從輸入文件中提取偶數序的行，輸出到另一個文件（行序從1開始）"""
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")
```
