I am stuck on a homework assignment. I have to read text from a file, allocate each word to memory, then user a pointer to send it to a vector<string*>. My program keeps overwriting the vector with the new word from the file instead of just adding it. I can't figure out why this is happening.
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
using namespace std;
void WordFunctions(string *pstr, vector<string*> &words)
{
words.push_back(pstr);
}
int main(){
ifstream file;
vector<string*> a;
string word;
int w =0;
file.open("word.txt");
while (!file.eof())
{
w++;
file >> word;
WordFunctions(&word, a);
}
file.close();
for (int i=0;i<10;i++){
cout<<(*a[i])<<" ";
delete a[i];
}
system ("pause");
}
Either use a vector<string> or allocate the new string on the heap:
void WordFunctions(string *pstr, vector<string*> &words)
{
words.push_back(new string(*pstr));
}
You are pushing the same element into vector which is the address of word. I massage a bit on your code
// pass reference to eliminate copy
void WordFunctions(string &str, vector<string> &words)
{
words.push_back(str);
}
int main(){
ifstream file;
vector<string> a; // you want to store string not the address of the string
string word;
int w =0;
file.open("words.txt");
while (!file.eof())
{
w++;
word.clear(); // clear the content before store something into it
file >> word;
WordFunctions(word, a);
}
file.close();
for (size_t i=0;i<a.size();i++){ // use size instead of hard code magic number
cout<<(a.at(i))<<" "; // use at function instead of []
}
system ("pause");
}
your word string has always the same address in memory, so in the loop you are changing the value of the string, but then you call WordFunctions passing to him always the same address.
If it's a constraint to use vector<string*> instead of vector<string>, you will likely need to allocate memory for new strings in the loop, copy there your word and then pass the new reference to WordFunctions
char *wordPtr
while (!file.eof())
{
w++;
file >> word;
wordPtr = (char *)malloc((strlen(word)+1)*sizeof(char));
strcpy(wordPtr, *word);
WordFunctions(wordPtr, a);
}
Related
I know this is a dumb question!
But I just CAN NOT get my head around how to read my file into an array one word at a time using c++
Here is the code for what I was trying to do - with some attempted output.
void readFile()
{
int const maxNumWords = 256;
int const maxNumLetters = 32 + 1;
int countWords = 0;
ifstream fin;
fin.open ("madLib.txt");
if (!fin.is_open()) return;
string word;
while (fin >> word)
{
countWords++;
assert (countWords <= maxNumWords);
}
char listOfWords[countWords][maxNumLetters];
for (int i = 0; i <= countWords; i++)
{
while (fin >> listOfWords[i]) //<<< THIS is what I think I need to change
//buggered If I can figure out from the book what to
{
// THIS is where I want to perform some manipulations -
// BUT running the code never enters here (and I thought it would)
cout << listOfWords[i];
}
}
}
I am trying to get each word (defined by a space between words) from the madLib.txt file into the listOfWords array so that I can then perform some character by character string manipulation.
Clearly I can read from a file and get that into a string variable - BUT that's not the assignment (Yes this is for a coding class at college)
I have read from a file to get integers into an array - but I can't quite see how to apply that here...
The simplest solution I can imagine to do this is:
void readFile()
{
ifstream fin;
fin.open ("madLib.txt");
if (!fin.is_open()) return;
vector<string> listOfWords;
std::copy(std::istream_iterator<string>(fin), std::istream_iterator<string>()
, std::back_inserter(listOfWords));
}
Anyways, you stated in your question you want to read one word at a time and apply manipulations. Thus you can do the following:
void readFile()
{
ifstream fin;
fin.open ("madLib.txt");
if (!fin.is_open()) return;
vector<string> listOfWords;
string word;
while(fin >> word) {
// THIS is where I want to perform some manipulations
// ...
listOfWords.push_back(word);
}
}
On the suggestion of πάντα ῥεῖ
I've tried this:
void readFile()
{
int const maxNumWords = 256;
int const maxNumLetters = 32 + 1;
int countWords = 0;
ifstream fin;
fin.open ("madLib.txt");
if (!fin.is_open()) return;
string word;
while (fin >> word)
{
countWords++;
assert (countWords <= maxNumWords);
}
fin.clear();
fin.seekg(0);
char listOfWords[countWords][maxNumLetters];
for (int i = 0; i <= countWords; i++)
{
while (fin >> listOfWords[i]) //<<< THIS did NOT need changing
{
// THIS is where I want to perform some manipulations -
cout << listOfWords[i];
}
}
and it has worked for me. I do think using vectors is more elegant, and so have accepted that answer.
The suggestion was also made to post this as a self answer rather than as an edit - which I kind of agree is sensible so I've gone ahead and done so.
The most simple way to do that is using the STL algorithm... Here is an example:
#include <iostream>
#include <iomanip>
#include <iterator>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
vector<string> words;
auto beginStream = istream_iterator<string>{cin};
auto eos = istream_iterator<string>{};
copy(beginStream, eos, back_inserter(words));
// print the content of words to standard output
copy(begin(words), end(words), ostream_iterator<string>{cout, "\n"});
}
Instead of cin of course, you can use any istream object (like file)
I'm having slight trouble creating a 2D Vector of String that's created by reading values from a text file. I initially thought I needed to use an array. however I've come to realise that a vector would be much more suited to what I'm trying to achieve.
Here's my code so far:
I've initialised the vector globally, but haven't given it the number of rows or columns because I want that to be determined when we read the file:
vector<vector<string>> data;
Test data in the file called "test" currently looks like this:
test1 test2 test3
blue1 blue2 blue3
frog1 frog2 frog3
I then have a function that opens the file and attempts to copy over the strings from text.txt to the vector.
void createVector()
{
ifstream myReadFile;
myReadFile.open("text.txt");
while (!myReadFile.eof()) {
for (int i = 0; i < 5; i++){
vector<string> tmpVec;
string tmpString;
for (int j = 0; j < 3; j++){
myReadFile >> tmpString;
tmpVec.push_back(tmpString);
}
data.push_back(tmpVec);
}
}
}
However, when I attempt to check the size of my vector in my main function, it returns the value '0'.
int main()
{
cout << data.size();
}
I think I just need a pair of fresh eyes to tell me where I'm going wrong. I feel like the issues lies within the createVector function, although I'm not 100% sure.
Thank you!
You should use std::getline to get the line of data first, then extract each string from the line and add to your vector. This avoids the while -- eof() issue that was pointed out in the comments.
Here is an example:
#include <string>
#include <iostream>
#include <vector>
#include <sstream>
typedef std::vector<std::string> StringArray;
std::vector<StringArray> data;
void createVector()
{
//...
std::string line, tempStr;
while (std::getline(myReadFile, line))
{
// add empty vector
data.push_back(StringArray());
// now parse the line
std::istringstream strm(line);
while (strm >> tempStr)
// add string to the last added vector
data.back().push_back(tempStr);
}
}
int main()
{
createVector();
std::cout << data.size();
}
Live Example
I am trying to replace arrays with vectors but I can't figure out how.
Replace this function to dynamically allocate memory for vectors:
string readFile(string filename, string** list, int size){
*list = new string[size];
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
*(*list + i) = line;
}
file.close();
return **list;
}
And here's my attempt to change it to vectors with no luck. Any feedback is greatly appreciated:
string processFile(string filename, vector<string>** list, int size){
*list = new vector<string>(size);
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
*list[i] = line; // error
}
file.close();
return **list; // error
}
You will need some proper error handling, but basically, you need neither pointers nor fixed sizes if you use containers:
std::vector<std::string> readLinesFromFile(const std::string& filename)
{
std::vector<std::string> result;
std::ifstream file(filename);
for (std::string line; std::getline(file, line); )
{
result.push_back(line);
}
return result;
}
There are several problems:
You don't need to use vector**, vector is equivalent to the list in previous code.
The return type is string, but you are returning vector**
This code should work, not tested though:
void processFile(string filename, vector<string>& list, int size){
//list = new vector<string>(size); // no need if you get a vector reference
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
list.push_back(line); //the error was because you are assigning string to a vector<string>*
}
file.close();
// you dont have to return, as vector is passed by reference
}
If you still need to use pointer of vector
void processFile(string filename, vector<string>** list, int size){
*list = new vector<string>(size); // bad practice
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
(*list)->push_back(line);
}
file.close();
// you dont have to return, as vector is passed by pointer
}
Change *list[i] = line to *list->push_back(line) and you should be okay for the first error.
The second error is going to depend on what your intent is for the return value. I think return *list->front(); will give the same result as the first example, but if you are planning on returning more than just the first line then you will need to do some concatenation. You can just create a local string and append each line as you read them.
Hopefully your teacher knows using new vector is almost always a code smell in C++ and is using this for a specific reason with a plan to fix it later.
here is a working example. enjoy :)
BTW - you don't need to pass the length, just instantiate the memory and use the push_back method.
#include <vector>
#include <fstream>
#include <string>
using namespace std;
void processFile(string filename, vector<string>** list, int size);
void main()
{
vector<string>* list = NULL;
processFile("C:\\temp.txt", &list, 13);
int i = 1;
}
void processFile(string filename, vector<string>** list, int size){
*list = new vector<string>();
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
(**list).push_back(line); // error
}
file.close();
}
I am new to c++. I am learning fast, but i dont know much yet.
I cannot see the problem with index in this function:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;
void get_rows(string filepath, vector<string> &rows);
int main() {
vector<string> rows;
get_rows("ninja.txt", rows);
for (int i = 0; i < rows.size(); i++) {
cout << rows[i] << endl;
}
}
void get_rows(string filepath, vector<string> &rows) {
ifstream file;
file.open(filepath);
string str;
int index = 0;
while (!file.eof()) {
getline(file, str);
rows[index] = str;
index++;
}
}
Any help will be appreciated.
You have constructed an std::vector<string> object:
vector<string> rows;
and then later you are trying to access its elements although there are no elements in this vector yet:
rows[index] = str;
You should push new elements into the vector using push_back method:
rows.push_back(str);
Also note that using while (!file.eof()) is wrong becuase getline might fail inside the loop:
while (!file.eof()) {
getline(file, str);
...
}
Your loop should look the following way:
while (std::getline(file, str)) {
if (str.empty()) continue; // we skip empty lines
rows.push_back(str); // and push non-empty lines at the end
}
vector<string> rows;
^
size() is 0
get_rows("ninja.txt", rows);
void get_rows(string filepath, vector<string> &rows) {
//...
int index = 0;
rows[index] = str; // but there is no rows[0] yet
//...
}
you should either use push_back to add new elements into vector or create a vector with specified size at the beginning (if it is known)
vector<string> rows(160);
which has advantage over the former as you can avoid potential reallocation (which may invalidate pointers to vector elements i.e)
I'm trying to load lines of a text file containing dictionary words into an array object. I want an array to hold all the words that start with "a", another one for "b" ... for all the letters in the alphabet.
Here's the class I wrote for the array object.
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
class ArrayObj
{
private:
string *list;
int size;
public:
~ArrayObj(){ delete list;}
void loadArray(string fileName, string letter)
{
ifstream myFile;
string str = "";
myFile.open(fileName);
size = 0;
while(!myFile.eof())
{
myFile.getline(str, 100);
if (str.at(0) == letter.at(0))
size++;
}
size -= 1;
list = new string[size];
int i = 0;
while(!myFile.eof())
{
myFile.getline(str, 100);
if(str.at(0) == letter.at(0))
{
list[i] = str;
i++;
}
}
myFile.close();
}
};
I'm getting an error saying:
2 IntelliSense: no instance of overloaded function "std::basic_ifstream<_Elem, _Traits>::getline [with _Elem=char, _Traits=std::char_traits<char>]" matches the argument list d:\champlain\spring 2012\algorithms and data structures\weeks 8-10\map2\arrayobj.h 39
I guess it's requiring me to overload the getline function, but I'm not quite certain how to go about or why it's necessary.
Any advice?
the function for streams that deals with std::string is not a member function of istream but rather a free function it is used like so. (the member function version deals with char*).
std::string str;
std::ifstream file("file.dat");
std::getline(file, str);
It is worth noting there are better safer ways to do what you are trying to do like so:
#include <fstream>
#include <string>
#include <vector>
//typedeffing is optional, I would give it a better name
//like vector_str or something more descriptive than ArrayObj
typedef std::vector<std::string> > ArrayObj
ArrayObj load_array(const std::string file_name, char letter)
{
std::ifstream file(file_name);
ArrayObj lines;
std::string str;
while(std::getline(file, str)){
if(str.at(0)==letter){
lines.push_back(str);
}
}
return lines;
}
int main(){
//loads lines from a file
ArrayObj awords=load_array("file.dat", 'a');
ArrayObj bwords=load_array("file.dat", 'b');
//ao.at(0); //access elements
}
don't reinvent the wheel; checkout vectors they are standard and will save you a lot of time and pain.
Final try not to put in using namespace std that is bad for a whole host of reasons I wont go into; instead prefix std objects with std:: so like std::cout or std::string.
http://en.cppreference.com/w/cpp/container/vector
http://en.cppreference.com/w/cpp/string/basic_string/getline
http://en.cppreference.com/w/cpp/string