我的目标是逐行读取文件,检查该行是否包含某个数字,如果是,则重写该行.然后继续阅读该文件.
我已经成功地为一行做了这个,但我无法弄清楚如何继续阅读文件的其余部分.
这是我如何替换一行(每行是已知的固定大小):
while(getline(fs, line)){
if(condition){
pos = fs.tellg(); //gets current read position (end of the line I want to change)
pos -= line.length()+1; //position of the beginning of the line
fs.clear(); //switch to write mode
fs.seekp(pos); //seek to beginning of line
fs << new_data; //overwrite old data with new data (also fixed size)
fs.close(); //Done.
continue;
}
}
如何切换回读取并继续getline循环?
最佳答案 我有同样的问题,TB级文件,我想修改文件开头的一些标题信息.
显然,当最初为任何新内容创建文件时,必须留出足够的空间,因为没有办法增加文件大小(除了附加到它)并且新行必须具有与原始文件完全相同的行长度.
这是我的代码的简化:
#include <iostream>
#include <fstream>
using namespace std;
bool CreateDummy()
{
ofstream out;
out.open("Dummy.txt");
// skip: test if open
out<<"Some Header"<<endl;
out<<"REPLACE1 12345678901234567890"<<endl;
out<<"REPLACE2 12345678901234567890"<<endl;
out<<"Now ~1 TB of data follows..."<<endl;
out.close();
return true;
}
int main()
{
CreateDummy(); // skip: test if successful
fstream inout;
inout.open("Dummy.txt", ios::in | ios::out);
// skip test if open
bool FoundFirst = false;
string FirstText = "REPLACE1";
string FirstReplacement = "Replaced first!!!";
bool FoundSecond = false;
string SecondText = "REPLACE2";
string SecondReplacement = "Replaced second!!!";
string Line;
size_t LastPos = inout.tellg();
while (getline(inout, Line)) {
if (FoundFirst == false && Line.compare(0, FirstText.size(), FirstText) == 0) {
// skip: check if Line.size() >= FirstReplacement.size()
while (FirstReplacement.size() < Line.size()) FirstReplacement += " ";
FirstReplacement += '\n';
inout.seekp(LastPos);
inout.write(FirstReplacement.c_str(), FirstReplacement.size());
FoundFirst = true;
} else if (FoundSecond == false && Line.compare(0, SecondText.size(), SecondText) == 0) {
// skip: check if Line.size() >= SecondReplacement.size()
while (SecondReplacement.size() < Line.size()) SecondReplacement += " ";
SecondReplacement += '\n';
inout.seekp(LastPos);
inout.write(SecondReplacement.c_str(), SecondReplacement.size());
FoundSecond = true;
}
if (FoundFirst == true && FoundSecond == true) break;
LastPos = inout.tellg();
}
inout.close();
return 0;
}
输入是
Some Header
REPLACE1 12345678901234567890
REPLACE2 12345678901234567890
Now ~1 TB of data follows...
输出是:
Some Header
Replaced first!!!
Replaced second!!!
Now ~1 TB of data follows...