ch2-Files

Chapter_2     ch2-Strings                                                                           ch2-vector 

Contents:

Scopy.cpp     FillString.cpp     Fillvector.cpp     GetWords.cpp     Exercise_2-3     Exercise_2-4     Exercise_2-5     Exercise_2-6     Exercise_2-7

Scopy.cpp     TCP1, p. 111     (Copy files using a string)     download

// Copy one file to another, a line at a time

#include <string>

#include <fstream>

using std::string;

using std::ifstream;

using std::ofstream;

int main()

{

  ifstream in("Scopy.cpp"); // Open for reading

  ofstream out("Scopy2.cpp"); // Open for writing

  string s;

  while(getline(in, s)) // Discards newline char

  {out << s << "\n";} // must add it back

}

/*

g++ Scopy.cpp -o Scopy

./Scopy

g++ Scopy2.cpp -o Scopy2

./Scopy2

rm Scopy2* // clean

*/

Notes:  It is good coding practice to enclose in braces the single statement of a loop, like while() above. It is easier to read, to delimit from other parts of the code, and you may decide to add other statements later on.

You may compile and run Scopy2.cpp, with the same result (Scopy2.cpp will be overwritten with exactly the same content):

g++ Scopy2.cpp -o Scopy2

./Scopy2

At the end we remove the created files Scopy2.cpp and Scopy2:

rm Scopy2*

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FillString.cpp     TCP1, p. 112     (Read file into a string)     download

// Read an entire file into a single string

#include <iostream>

#include <string>

#include <fstream>

using std::cout;

using std::string;

using std::ifstream;

using std::ofstream;

int main()

{

  ifstream in("FillString.cpp");

  string s, line;

  while(getline(in, line)) // getline() removes '\n'

  {s += line + '\n';} // char '\n' is converted into string "\n"

  cout << s;

}

/*

g++ FillString.cpp -o FillString

./FillString

*/

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Fillvector.cpp     TCP1, p. 115-116     (Read file into a vector of strings)     download

// Copy an entire file into a vector of string

#include <iostream>

#include <string>

#include <vector>

#include <fstream>

using std::cout;

using std::endl;

using std::string;

using std::vector;

using std::ifstream;

int main()

{

  vector<string> v;

  ifstream in("Fillvector.cpp");

  string line;

  while(getline(in, line)) // getline() removes newline char

  {v.push_back(line);} // Add the line to the end

  int size = v.size();

  // Add line numbers:

  for(int i = 0; i < size; i++)

  {cout << i+1 << ": " << v[i] << endl;} // first line is line 1, not 0

}

/*

g++ Fillvector.cpp -o Fillvector

./Fillvector

*/

Notes:  v.push_back(line); adds each new line after the last read one, keeping the lines in the same order as in the original file.

We print the line numbers starting at 1, so we write  cout << i+1  instead of  cout << i .

The function call v.size() is expensive inside a loop, so we store the size of v in an int variable and then refer to this variable inside the loop:

int size = v.size();

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

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GetWords.cpp     TCP1, p. 117     (Break file into words)     download

// Break a file into whitespace-separated words

#include <iostream>

#include <string>

#include <vector>

#include <fstream>

using std::cout;

using std::endl;

using std::string;

using std::vector;

using std::ifstream;

int main()

{

  vector<string> words;

  ifstream in("GetWords.cpp");

  string word;

  while(in >> word)

  {words.push_back(word);}

  int size = words.size();

  for(int i = 0; i < size; i++)

  {cout << words[i] << endl;} // one word per line

}

/*

g++ GetWords.cpp -o GetWords

./GetWords

*/

Note:  We can also iterate the vector with the for-each_syntax:

for (string word : words) // for all strings in vector<string> words

{cout << word << endl;} // one word per line

In this case, we no longer need the size variable:

int size = words.size();

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Exercise 2-3     TCP1, p. 120     (Count words in a file)

Exercise 2-3. Create a program that opens a file and counts the whitespace-separated words in that file.

CountWords.cpp          download

// Counts the whitespace-separated words in the file

#include <iostream>

#include <string>

#include <fstream>

using std::cout;

using std::endl;

using std::string;

using std::ifstream;

int main()

{

  int words = 0;

  ifstream in("CountWords.cpp");

  string word;

  while(in >> word)

  {words++;}

  cout << "Number of whitespace-separated words: "

       << words << endl;

}

/*

g++ CountWords.cpp -o CountWords

./CountWords

Number of whitespace-separated words: 61

*/

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Exercise 2-4     TCP1, p. 120     (CountWord)

Exercise 2-4. Create a program that counts the occurrence of a particular word in a file (use the string class' operator '==' to find the word).

CountWord.cpp          download

// Counts the occurrences of a whitespace-separated word in the file

#include <iostream>

#include <string>

#include <fstream>

using std::cout;

using std::cin;

using std::endl;

using std::string;

using std::ifstream;

int main()

{

  int occurrences = 0;

  string word, lookfor;

  cout << "Word to look for in the file: ";

  cin >> lookfor;

  ifstream in("CountWord.cpp");

  while(in >> word)

  {

    if (word == lookfor)

    {occurrences++;}

  }

  cout << "Number of occurrences of '" << lookfor

       << "': " << occurrences << endl;

}

/*

g++ CountWord.cpp -o CountWord

./CountWord

Word to look for in the file: word

Number of occurrences of 'word': 2

./CountWord

Word to look for in the file: word,

Number of occurrences of 'word,': 2

./CountWord

Word to look for in the file: word)

Number of occurrences of 'word)': 2

./CountWord

Word to look for in the file: (word

Number of occurrences of '(word': 2

./CountWord

Word to look for in the file: ==

Number of occurrences of '==': 2

./CountWord

Word to look for in the file: >>

Number of occurrences of '>>': 3

./CountWord

Word to look for in the file: <<

Number of occurrences of '<<': 7

./CountWord

Word to look for in the file: occurrences

Number of occurrences of 'occurrences': 13

*/

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Exercise 2-5     TCP1, p. 120     (Fillvector, reverse order)

Exercise 2-5. Change Fillvector.cpp so that it prints the lines (backwards) from last to first.

RevFillvector.cpp          download

// Copy an entire file into a vector of string (in reverse order of lines)

#include <iostream>

#include <string>

#include <vector>

#include <fstream>

using std::cout;

using std::endl;

using std::string;

using std::vector;

using std::ifstream;

int main()

{

  vector<string> v;

  ifstream in("RevFillvector.cpp");

  string line;

  while(getline(in, line))

  {v.push_back(line);} // Add line to the end, preserving line order

  // Add line numbers and print in reverse, from last to first line:

  for(int i = v.size()-1; i >= 0; i--)

  {cout << i+1 << ": " << v[i] << endl;}

}

/*

g++ RevFillvector.cpp -o RevFillvector

./RevFillvector

*/

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Exercise 2-6     TCP1, p. 120     (Concatenate vectors into a string)

Exercise 2-6. Change Fillvector.cpp so that it concatenates all the elements in the vector into a single string before printing it out, but don’t try to add line numbering.

CatFillvector.cpp          download

// Copy an entire file into a vector of string, concatenate strings

#include <iostream>

#include <string>

#include <vector>

#include <fstream>

using std::cout;

using std::string;

using std::vector;

using std::ifstream;

int main()

{

  vector<string> v;

  ifstream in("CatFillvector.cpp");

  string line;

  while(getline(in, line))

  {v.push_back(line);} // Add the line to the end

  line = ""; // reset line

  int size = v.size();

  // Concatenation:

  for(int i = 0; i < size; i++)

  {line += v[i] + "\n";}

  cout << line;

}

/*

g++ CatFillvector.cpp -o CatFillvector

./CatFillvector

*/

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Exercise 2-7     TCP1, p. 120     (less - Display a file a line at a time)

Exercise 2-7. Display a file a line at a time, waiting for the user to press the "Enter" key after each line.

DisplayFile.cpp          download

// Displays the file one line at a time. Press Enter to continue.

// Press Enter after each line is displayed.

#include <cstdio> // for getchar()

#include <string>

#include <iostream>

#include <fstream>

using std::cout;

using std::cin;

using std::string;

using std::ifstream;

int main()

{

  char c;

  ifstream in("DisplayFile.cpp");

  string line;

  while(getline(in, line))

  {

    cout << line; // no newline here

    cin.ignore(); // getchar(); // Press Enter to continue

  }

}

/*

g++ DisplayFile.cpp -o DisplayFile

./DisplayFile

*/

Notes:  There are two ways (presented here) to wait for the user to hit a key before continuing (similar to the UNIX_less command). One is cin.ignore(); and the other one is getchar();

You should only hit ENTER (not other key) for the file display to work as intended. Each line is displayed without newline at the end, whose place it taken by hitting ENTER.

User input/output is normally buffered and the buffer is cleared after hitting ENTER (or printing newline). As such, if you hit the space bar two times and then ENTER, three lines will then be displayed (with no newlines ending the first two of them).

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