Up

Reading and Writing on a Text File in C++

The following is a sample program in C++ to read and write on a text file.

#include <iostream>

#include <fstream>

using namespace std;

int main(){

cout << "Input File Name = ";

char nm[100];

cin >> nm;

ifstream fin;

fin.open(nm);

cout << "Output File Name = ";

cin >> nm;

ofstream fout;

fout.open(nm);

char str[1000];

while(!fin.fail()){

char str[1000];

fin.getline(str, 1000);

fout << str << endl;

}

fin.close();

fout.close();

return 0;

}

Let’s run the above program “RdLN.cpp” with input text file “Data.txt” and output text file “Check.txt.” The name of the output file is arbitrary.

Figure 1 shows Data.txt.

Figure 1. (Line numbers 1 to 10 are shown on the left )

Figure 2 shows the terminal display, on which the program was run.

Figure 2

Command g++ built an executable file “prgLN” from the source code file “RdLN.cpp.”

In the case of Figure 2, Data.txt (the input file) and CheckLN.txt (the output file) are in the same folder as the program, i.e., in the current directory.

Figure 3 shows output file ‘CheckLN.txt.”

Figure 3

Change the following code of the above program

fin.getline(str, 1000);

to

fin.getline(str, 5);

Then, we get the following program.

#include <iostream>

#include <fstream>

using namespace std;

int main(){

cout << "Input File Name = ";

char nm[100];

cin >> nm;

ifstream fin;

fin.open(nm);

cout << "Output File Name = ";

cin >> nm;

ofstream fout;

fout.open(nm);

char str[1000];

fin.getline(str, 5);

while(!fin.fail()){

fout << str << endl;

fin.getline(str, 5);

}

fin.close();

fout.close();

return 0;

}

Run this program, then we get the output file (Figure 4).

Figure 4

When the following code

fin.getline(str, 5);

reads the 4^th character, i.e., the number of characters read becomes the value of the 2^nd parameter – 1,

the 1^st to 4^th characters are set in str, and the value of the function

fin.fail()

is set to be true.

In the first example of a program, the value of function

fin.fail()

is set to be true, after the last character of the file is read by execution of code

fin.getline(str, 1000);

The timing when the value is set to be true is different from that of Visual C++.

The following program reads values written in a text file one by one.

#include <iostream>

#include <fstream>

using namespace std;

int main(){

cout << "Input File Name = ";

char nm[100];

cin >> nm;

ifstream fin(nm);

cout << "Output File Name = ";

cin >> nm;

ofstream fout(nm);

int v;

fin >> v;

while (!fin.fail()){

fout << v << endl;

fin >> v;

}

fin.close();

fout.close();

return 0;

}

File streams fin of stream class ifstream and fout of stream class ofstream are used in the same way as standard I/O cin and cout, respectively.

Prepare a text file shown in Figure 5.

Figure 5

Run the above program as shown in Figure 6.

Figure 6

We get the output file shown in Figure 7.

Figure 7

How to extract words separated by a blank character or the like

Figure 8 shows a text file, which contains a line with multiple words separated by blank or tab characters.

Figure 8

Consider how to extract words, e.g., “aa”, “f”, and “gg” from the 3^rd line.

Function SeekStr of the following program extracts the (pos)th word from (char *) cstr.

#include <iostream>

#include <fstream>

#include <sstream>

using namespace std;

string SeekStr(char * cstr, int pos){

stringstream strfile(cstr);

string vs;

for (int i = 0; i <= pos; i++){

if (strfile.eof()){

vs = "";

break;

}

else{

strfile >> vs;

if (i < pos) vs = "";

}

}

return vs;

}

int main(){

cout << "Input File = ";

char nm[100];

cin >> nm;

ifstream fin(nm);

cout << "Output File = ";

cin >> nm;

ofstream fout(nm);

while (!fin.fail()){

char vcs[1000];

fin.getline(vcs, 1000);

for (int i = 0; i < 10; i++)

fout << i << ":<" << SeekStr(vcs, i) << "> ";

fout << endl;

}

return 0;

}

When the value of pos is too large for the corresponding word to be found, cstr is set to be “”.

Figure 9 shows the terminal on which the program was run.

Figure 9

Figure 10 shows output file Check.txt.

Figure 10

Conversion of a string to the numerical value

In the following program, execution times of two ways of conversion of a string value to the represented numerical value are compared. One is by C functions atoi and strtod, the other is by string stream class stringstream.

#include <iostream>

#include <sstream>

#include <time.h>

#include <stdlib.h>

using namespace std;

// Conversion of string to int

int str2i(string str){

stringstream sstrm(str);

int iv;

sstrm >> iv;

return iv;

}

// Conversion of string to double

double str2d(string str){

stringstream sstrm(str);

double dv;

sstrm >> dv;

return dv;

}

int main(){

cout << "Start..." << endl;

const int n = 1000000;

string * vstr = new string[n], * vstrd = new string[n];

time_t * t0 = new time_t;

time(t0);

//

// Set string data

//

for (int i = 0; i < n; i++){

ostringstream sstrm, sstrmd;

sstrm << i;

vstr[i] = sstrm.str();

sstrmd << 0.1 * i;

vstrd[i] = sstrmd.str();

}

time_t * t1 = new time_t;

time(t1);

double diff_time = difftime(*t1, *t0);

cout << "string values..." << endl;

for (int i = n - 3; i < n; i++)

cout << i << ": " << vstr[i] << " " << vstrd[i] << endl;

cout << "diff time = " << diff_time << "s for " << n * 2 << "values" << endl;

//

// Convert string to char*

//

char ** cstr = new char*[n], ** cstrd = new char*[n];

for (int i = 0; i < n; i++){

cstr[i] = new char[100];

for (int j = 0; j < vstr[i].length(); j++)

cstr[i][j] = vstr[i][j];

cstr[i][vstr[i].length()] = 0;

cstrd[i] = new char[100];

for (int j = 0; j < vstrd[i].length(); j++)

cstrd[i][j] = vstrd[i][j];

cstrd[i][vstrd[i].length()] = 0;

}

cout << endl << "(char *) values..." << endl;

for (int i = n - 3; i < n; i++)

cout << cstr[i] << " " << cstrd[i] << endl;

//

// Convert char* to int or double

//

int * ivalues = new int[n];

double * dvalues = new double[n];

char * ept;

int gn_rep = 20;

cout << endl << "Conversion by C functions..." << endl;

time(t0);

for (int k = 0; k < gn_rep; k++)

for (int i = 0; i < n; i++){

ivalues[i] = atoi(cstr[i]); // C function

dvalues[i] = strtod(cstrd[i], &ept); // C function

}

time(t1);

diff_time = difftime(*t1, *t0);

cout << "diff time/C = " << diff_time << "s for atoi and strtod functins." << endl;

cout << "Converted values..." << endl;

for (int i = n - 3; i < n; i++)

cout << ivalues[i] << " " << dvalues[i] << endl;

//

// Convert string to int or double

//

cout << endl << "Conversion by stringstream objects..." << endl;

time(t0);

for (int k = 0; k < gn_rep; k++)

for (int i = 0; i < n; i++){

ivalues[i] = str2i(vstr[i]); // Defined above

dvalues[i] = str2d(vstrd[i]); // Defined above

}

time(t1);

diff_time = difftime(*t1, *t0);

cout << "diff time/S = " << diff_time << "s for stringstream objects." << endl;

cout << "Converted values..." << endl;

for (int i = n - 3; i < n; i++)

cout << ivalues[i] << " " << dvalues[i] << endl;

cout << endl << "Deleting objects..." << endl;

delete [] vstr;

delete [] vstrd;

delete t0;

delete t1;

for (int i = 0; i < n; i++){

delete [] cstr[i];

delete [] cstrd[i];

}

delete [] cstr;

delete [] cstrd;

delete [] ivalues;

delete [] dvalues;

return 0;

}

Figure 11 shows the terminal on which the above program was run.

Figure 11

A sample program, which shows how to use a multidimensional array of string type data, is prepared at this website.

Up