So I am trying to read string data from a file into a dynamically allocated array, but I cannot seem to get the correct code for doing so. I have below a code using an array of a preset size but that would not be efficient, hence why I want to use dynamic memory allocation. I know I have to use pointers but I am fairly new to the concept so any help would be greatly appreciated.
#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdlib>
#include <algorithm>
#include <cctype>
#include <string>
#define SIZE 100
using namespace std;
void loadData();
int main()
{
loadData();
return 0;
{
string fileName;
std::string wordArray[SIZE];
cout << "Please enter the name of the text file you want to process followed by '.txt': " << endl;
cin >> fileName;
ifstream dataFile(fileName);
if (dataFile.fail()) {
cerr << fileName << " could not be opened." << endl; //error message if file opening fails
exit(-1);
}
while (!dataFile.eof()) {
for (int i = 0; i < SIZE; i ) {
dataFile >> wordArray[I];
for (std::string& s : wordArray) //this for loop transforms all the words in the text file into lowercase
std::transform(s.begin(), s.end(), s.begin(),
[](unsigned char c) { return std::tolower(c); });
}
}
dataFile.close();
}
}
CodePudding user response：
The below program show how you can store the strings read from the input.txt file and store them in lowercase in a std::vector.
Version 1: Stores word by word(in lowercase) into the vector
#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
#include <sstream>
int main()
{
std::ifstream inputFile("input.txt");
//create vector that will contain the words in the file in lowercase
std::vector<std::string> wordVec;
std::string line, individualWord;
if(inputFile)
{
while(std::getline(inputFile, line, '\n'))
{
std::istringstream ss(line);
while(ss >> individualWord)//word by word
{
std::transform(individualWord.begin(), individualWord.end(), individualWord.begin(),
[](unsigned char c)
{ return std::tolower(c);
});
wordVec.push_back(individualWord);
}
}
}
else
{
std::cout<<"file could not be opened"<<std::endl;
}
inputFile.close();
//lets print out the elements of the vector to check if elements are correctly stored in lowercase
for(const std::string &elem: wordVec)
{
std::cout<<elem<<std::endl;
}
return 0;
}
The output of version 1 can be seen here.
Version 2: Stores a complete single line(in lowercase) into the vector
#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
int main()
{
std::ifstream inputFile("input.txt");
//create vector that will contain the words in the file in lowercase
std::vector<std::string> wordVec;
std::string line;
if(inputFile)
{
while(std::getline(inputFile, line, '\n'))
{
std::transform(line.begin(), line.end(), line.begin(),
[](unsigned char c)
{ return std::tolower(c);
});
wordVec.push_back(line);
}
}
else
{
std::cout<<"file could not be opened"<<std::endl;
}
inputFile.close();
//lets print out the elements of the vector to check if elements are correctly stored in lowercase
for(const std::string &elem: wordVec)
{
std::cout<<elem<<std::endl;
}
return 0;
}
The output of the above(version 2) program can be seen here.
The difference between version 1 and 2 is that version 1 reads a complete line and then read word by word and store those words(in lowercase) into the std::vector while version 2 reads a complete line(which ends with '\n') and store that line(in lowercase) into the std::vector.
CodePudding user response：
Sometime life could be easy. By using modern C elements, the implementation will be really simple in the end.
I am not so sure what I should explain for 3 lines of code. It is basically visible with the comments in the code.
Please see the first solution:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <iterator>
#include <cctype>
int main() {
// Open the input file and check, if it could be opened
if (std::ifstream inputStream{ "r:\\input.txt" }; inputStream) {
// Define a vector and read all words from the file
std::vector words(std::istream_iterator<std::string>(inputStream), {});
// Show result to the user
std::copy(words.begin(), words.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
}
}
Then next solution converts the word into lower case. So, I had to write 4 statements. Please see the below second solution.
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <iterator>
#include <cctype>
int main() {
// Open the input file and check, if it could be opened
if (std::ifstream inputStream{ "r:\\input.txt" }; inputStream) {
std::vector<std::string> words{};
// Read all words from the file and convert to lower case
std::transform(std::istream_iterator<std::string>(inputStream), {}, std::back_inserter(words),
[](std::string w) { for (char& c : w) c = std::tolower(c); return w; });
// Show result to the user
std::copy(words.begin(), words.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
}
}
Sometime teachers want the students to learn dynamic memory management using pointers.
But the usage of pointers for owned memory is strongly discouraged. The std::vector has been invented for that reason over a decade ago.
Anyway, I will also show a solution using new. It works, but you should not use it.
int main() {
// Open the input file and check, if it could be opened
if (std::ifstream inputStream{ "r:\\input.txt" }; inputStream) {
// Do some initial allocation of memory
std::string* words = new std::string[1]{};
unsigned int numberOfAvaliableSlotsInDynamicArray{1};
// Now we want to read words. We want also to count the words,so that we can allocate appropriate memory
std::string word{};
unsigned int wordCounter{};
// Read all words in a loop
while (inputStream >> word) {
// Check, if we still have enough space in our dynamic array
if (wordCounter >= numberOfAvaliableSlotsInDynamicArray) {
// Oh, we are running out of space. Get more memory
numberOfAvaliableSlotsInDynamicArray *= 2;
std::string* temp = new std::string[numberOfAvaliableSlotsInDynamicArray]{};
// Copy all existing data into new array
for (unsigned int i{}; i < wordCounter; i)
temp[i] = words[i];
// Delete old memory
delete[] words;
// And assign new storage to words
words = temp;
}
// STore the recently read word at the end of the array
words[wordCounter] = word;
// Count words. Now we have one word more
wordCounter;
}
// Now we have read all words from the file. Show output
for (unsigned int i{}; i < wordCounter; i)
std::cout << words[i] << '\n';
// Release memory
delete[] words;
}
}
And even smart pointers, which shall be used as pointers if at all, are not nice.
Also the following should not be used.
#include <iostream>
#include <fstream>
#include <string>
#include <memory>
int main() {
// Open the input file and check, if it could be opened
if (std::ifstream inputStream{ "r:\\input.txt" }; inputStream) {
// Do some initial allocation of memory
std::unique_ptr<std::string[]> words = std::unique_ptr<std::string[]>(new std::string[1]);
unsigned int numberOfAvaliableSlotsInDynamicArray{ 1 };
// Now we want to read words. We want also to count the words,so that we can allocate appropriate memory
std::string word{};
unsigned int wordCounter{};
// Read all words in a loop
while (inputStream >> word) {
// Check, if we still have enough space in our dynamic array
if (wordCounter >= numberOfAvaliableSlotsInDynamicArray) {
// Oh, we are running out of space. Get more memory
numberOfAvaliableSlotsInDynamicArray *= 2;
std::unique_ptr<std::string[]> temp = std::unique_ptr<std::string[]>(new std::string[numberOfAvaliableSlotsInDynamicArray]);
// Copy all existing data into new array
for (unsigned int i{}; i < wordCounter; i)
temp[i] = std::move(words[i]);
// And assign new storage to words
words = std::move(temp);
}
// STore the recently read word at the end of the array
words[wordCounter] = word;
// Count words. Now we have one word more
wordCounter;
}
// Now we have read all words from the file. Show output
for (unsigned int i{}; i < wordCounter; i)
std::cout << words[i] << '\n';
}
}
Conclusion: Use a std::vector.