写入文件下一行

Question

我想从文件中取数字，给它们分配一个变量名，对变量做一些数学运算，然后写入文件中的下一行。例如，如果我有一个文件：1 2 3 4 5，那么这里是我到目前为止的缩写代码（Python 3.3）。我遇到的唯一问题是将计算结果写入下一行。预先感谢您的帮助。

with open('test.txt', 'r') as f: 
    read_data = f.read() 
    a1 = (read_data[0])`a1 = (read_data[0]) 
print(a1) # this is just a test to see whats happening 
f.close() 
with open('test.txt','w') as f: 
    f.write(a1) #how do I get a1 to write on the next line of the file 
exit() 

Answer 1

with open('test.txt', 'r') as f: 
    data = f.read() 
with open('test.txt', 'w') as f: 
    for line in data.split('\n'): 
     a1 = line # You didn't specify what modifications you did with the data so... 
     f.write(line + '\n') 
     f.write(a1 + '\n') 

另外请注意，我离开了f.close()，因为你不需要它。
忘了它叫什么（旁边的上下文管理器旁边的另一个花哨词）但with是一个自动关闭语句，每当你离开块f将有.close()由Python自动调用。

既然你没有写你想要做什么样的calulcations的，这里有一个由例如：

with open('test.txt', 'r') as f: 
    data = f.read() 
with open('test.txt', 'w') as f: 
    for line in data.split('\n'): 
     a1 = int(line)*100 
     f.write(line + '\n') 
     f.write(str(a1) + '\n') 

而且从你的下一行提到的例子读书，但没有指定如果该数据已经由线分离，所以服用的答案从djas你可以合并到这些：

with open('test.txt', 'r') as f: 
    data = f.read() 
with open('test.txt', 'w') as f: 
    for line in data.split(): 
     a1 = int(line)*100 
     f.write(line + '\n') 
     f.write(str(a1) + '\n') 

Answer 2

假设输入文件看起来像

04 80 52 67 

，下面的代码工作：

with open('input.txt', 'r') as f: 
    line = f.read() 

with open('output.txt','w') as f: 
    for element in line.split(): 
     f.write("%s\n" % ''.join(element)) 

编辑：你也可以更换f.write("%s\n" % ''.join(element))与f.write(element+'\n')和去除f.close（）由@Torxed的建议。

Answer 3

with open会自动关闭该文件，因此您可以在with open声明中执行程序的主逻辑。例如，该程序会做你的需要：

with open('test.txt', 'r') as f: 
    lines = f.readlines() 
newlines = [] 
for line in lines: 
    newlines.append(line) 
    newline = #do some logic here 
    newlines.append(newline) 
with open('test.txt','w') as f: 
    for line in newlines: 
    f.write(line + '\n') 

如果只有一条线，可以消除for line in lines:循环，直接修改该文件来代替。另一方面，如果您希望这是一个迭代过程，如其中一个问题的评论所示 - 也就是说，您希望新的第二行成为新的第一行，以此类推，您会把上面的一些功能如下：

def func(first_line_index): 
    """ 
    Takes the index of the current first line as first_line_index and modifies 
    the next line as per your assignment, then writes it back to the file 
    """ 
    with open('test.txt', 'r') as f: 
    lines = f.readlines() 
    line1 = lines[first_line_index] 
    newlines = [] 
    for line in lines[:first_line_index+1]: 
    newlines.append(line) 
    newlines.append(lines[first_line_index]) 
    newline = #do some logic here based on the line above 
    newlines.append(newline) 
    for line in lines[first_line_index+2:]: 
    newlines.append(line) 
    with open('test.txt','w') as f: 
    for line in newlines: 
     f.write(line + '\n') 

def __main__(): 
    counter = 0 
    while #some condition- depends on your assignment: 
    func(counter) 
    counter += 1