c++ - fread 正在读取文件中的垃圾
问题描述
我正在做我的作业,并在 C++ 中遇到了 fread() 的问题。当我修改文件中的名称时,它会根据需要完美地修改它,但是当我尝试读取文件之后会出现问题,它会读取整个文件,但之后它并没有停止,它总共运行了 146 次,而只有3个名字。
我的代码:-
#include <bits/stdc++.h>
using namespace std;
struct person{
int id;
string fname;
}s;
void write(){
FILE *outfile;
struct person input;
int num,ident;
string sname[] = {"a","b","c"};
outfile = fopen ("C:\\Users\\Amritesh\\Desktop\\students.txt","wb");
if (outfile == NULL)
{
fprintf(stderr, "\nError opend file\n");
exit (1);
}
scanf("%d",&num);
for(int i=0;i<num;i++){
s.fname = sname[i];
cin >> s.id;
fwrite (&s, sizeof(s), 1, outfile);
}
fclose(outfile);
}
void read(){
FILE *file1;
int i=0;
file1 = fopen ("C:\\Users\\Amritesh\\Desktop\\students.txt","r");
while(fread(&s, sizeof(s), 1, file1) == 1) {
cout << "ID " << s.id << " Name " <<s.fname << endl;
}
fclose (file1);
}
void modify(){
FILE *file;
file = fopen ("C:\\Users\\Amritesh\\Desktop\\students.txt","r+");
while(fread(&s, sizeof(s), 1, file)) {
if(s.fname == "a"){
s.fname = "d";
fseek(file,-sizeof(s),SEEK_CUR);
fwrite (&s, sizeof(s), 1,file);
}
}
fclose (file);
}
int main(){
write();
modify();
read();
}
编辑代码:-
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
struct person
{
int id;
string fname;
}s,temp;
void read()
{
int num;
ifstream fin;
fin.open("C:\\Users\\Amritesh\\Desktop\\student.txt",ios::in);
fin.seekg(0,ios::beg);
//scanf("%d",&num);
while(fin){
cout << s.fname << s.id << endl;
}
fin.close();
}
void write(){
int i=0;
ofstream fout;
fout.open("C:\\Users\\Amritesh\\Desktop\\student.txt");
while(i!=2) {
cin >> s.id >> s.fname;
fout << "ID " << s.id << " Name " <<s.fname << endl;
i++;
}
fout.close();
}
void modify(){
fstream mod;
mod.open ("C:\\Users\\Amritesh\\Desktop\\student.txt");
while(mod) {
if(s.fname == "a"){
s.fname = "d";
mod.seekg(-sizeof(s),ios::cur);
mod << s.fname;
}
}
mod.close();
}
int main(){
write();
read();
modify();
}
感谢您的任何回答!
解决方案
以下是基于我们讨论的三个想法。我将从用于读取和写入person
对象的免费函数开始,因为看起来您现在正处于那个阶段。为了方便起见,我将继续在您的person
类中添加成员函数,并以添加流运算符结束。
免费(非会员)read
和write
功能的示例:
#include <iostream>
#include <string>
#include <fstream>
struct person {
int id;
std::string fname;
};
std::ostream& write(std::ostream& os, const person& p) {
os << p.id << ',' << p.fname << '\n'; // stream out the properties of a person
return os; // look at the next example for an alternative doing the same thing
}
std::istream& read(std::istream& is, person& p) {
// extract "id" and if it succeeds, check if the next char is a , char
if(is >> p.id && is.peek() == ',') {
is.ignore(); // step over the , char
std::getline(is, p.fname); // read the rest of the line into p.fname
} else {
// we didn't get id or the , char, so set the stream in a failed state
is.setstate(is.failbit);
}
return is;
}
int main() {
// write to file
{
std::ofstream file("C:\\Users\\Amritesh\\Desktop\\student.txt");
person test1{10, "Foo Bar"};
person test2{20, "Apa Bepa"};
write(file, test1);
write(file, test2);
}
// read from file
{
std::ifstream file("C:\\Users\\Amritesh\\Desktop\\student.txt");
person test;
while(read(file, test)) {
std::cout << test.fname << '\n';
}
}
}
read
制作和write
成员函数的示例person
:
#include <iostream>
#include <string>
#include <fstream>
struct person {
int id;
std::string fname;
std::ostream& write(std::ostream& os) const {
// this does the same thing as in the first example
return os << id << ',' << fname << '\n';
}
std::istream& read(std::istream& is) {
if(is >> id && is.peek() == ',') {
is.ignore(); // step over the , char
std::getline(is, fname);
} else {
is.setstate(is.failbit); // we didn't get id or the , char
}
return is;
}
};
int main() {
// write to file
{
std::ofstream file("C:\\Users\\Amritesh\\Desktop\\student.txt");
person test1{10, "Foo Bar"};
person test2{20, "Apa Bepa"};
test1.write(file);
test2.write(file);
}
// read from file
{
std::ifstream file("C:\\Users\\Amritesh\\Desktop\\student.txt");
person test;
while(test.read(file)) {
std::cout << test.fname << '\n';
}
}
}
流操作符支持的成员函数:
#include <iostream>
#include <string>
#include <fstream>
struct person {
int id;
std::string fname;
std::ostream& write(std::ostream& os) const {
return os << id << ',' << fname << '\n';
}
std::istream& read(std::istream& is) {
if(is >> id && is.peek() == ',') {
is.ignore(); // step over the , char
std::getline(is, fname);
} else {
is.setstate(is.failbit); // we didn't get id or the , char
}
return is;
}
};
// stream operators calling member functions
std::ostream& operator<<(std::ostream& os, const person& p) { return p.write(os); }
std::istream& operator>>(std::istream& is, person& p) { return p.read(is); }
int main() {
// write to file
{
std::ofstream file("C:\\Users\\Amritesh\\Desktop\\student.txt");
person test1{10, "Foo Bar"};
person test2{20, "Apa Bepa"};
file << test1 << test2; // calling operator<< for each person object
}
// read from file
{
std::ifstream file("C:\\Users\\Amritesh\\Desktop\\student.txt");
person test;
while(file >> test) { // extract one person at a time using operator>>
std::cout << test.fname << '\n';
}
}
}
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