Hey I know I can just use a string to read from a text file. However I need to use a char array. Like If I was using a string I would do this
while (!input.eof()){
input >> s;
}
I am unsure how I would go about this if I don't know the length of the string. I know I can use getLine, but I'll prefer to use input.
I'm thinking that maybe I can use a loop to check until it reaches "\0"?
Anyway I have a feeling this question has been asked before, but if it has I can't find it. So sorry if that is the case.
You can consider istream::getline. Note that it can be use for C++ string and it must have a length limit for C string.
I think you should avoid check eof directly in while condition. It only returns true it reach end-of-file. So if you have multiple line, you read it, then do some calculate, the consequence can be unexpected when it reach the end-of-file right at reading step. So the check of EOF should be placed right after reading from stream like my example.
int main()
{
ifstream input("filename.txt");
const int MAX = 10000;
char characters[MAX];
while (true) {
input.getline(characters, MAX - 1, '\n');
if (input.eof())
break;
}
}
Related
I'm new to c++ and I'm trying to solve the exercise 6 from chapter 4 out of Bjarne Stroustrups book "Programming Principles and Practise Using C++ and don't understand why my code doesn't work.
The exercise:
Make a vector holding the ten string values "zero", "one", ...,
"nine". Use that in a program that converts a digit to its
corresponding spelled-out value: e.g., the input 7 gives the output
seven. Have the same program, using the same input loop, convert
spelled-out numbers into their digit form; e.g., the input seven gives
the output 7.
My loop only executes one time for a string and one time for an int, the loop seems to continue but it doesn't matter which input I'm giving, it doesn't do what it's supposed to do.
One time it worked for multiple int inputs, but only every second time. It's really weird and I don't know how to solve this in a different way.
It would be awesome if someone could help me out.
(I'm also not a native speaker, so sorry, if there are some mistakes)
The library in this code is a library provided with the book, to make the beginning easier for us noobies I guess.
#include "std_lib_facilities.h"
int main()
{
vector<string>s = {"zero","one","two","three","four","five","six","seven","eight","nine"};
string input_string;
int input_int;
while(true)
{
if(cin>>input_string)
{
for(int i = 0; i<s.size(); i++)
{
if(input_string == s[i])
{
cout<<input_string<<" = "<<i<<"\n";
}
}
}
if(cin>>input_int)
{
cout<<input_int<<" = "<<s[input_int]<<"\n";
}
}
return 0;
}
When you (successfully) read input from std::cin, the input is extracted from the buffer. The input in the buffer is removed and can not be read again.
And when you first read as a string, that will read any possible integer input as a string as well.
There are two ways of solving this:
Attempt to read as int first. And if that fails clear the errors and read as a string.
Read as a string, and try to convert to an int. If the conversion fails you have a string.
if(cin >> input) doesn't work properly in while loop?
A possible implementation of the input of your program would look something like:
std::string sentinel = "|";
std::string input;
// read whole line, then check if exit command
while (getline(std::cin, input) && input != sentinel)
{
// use string stream to check whether input digit or string
std::stringstream ss(input);
// if string, convert to digit
// else if digit, convert to string
// else clause containing a check for invalid input
}
To discriminate between int and string value you could use peek(), for example.
Preferably the last two actions of conversion (between int and string) are done by separate functions.
Assuming the inclusion of the headers:
#include <iostream>
#include <sstream>
I am facing a problem with reading and writing a string from and to a file respectively.
Purpose:
To enter a string into a text file as a complete sentence, read the string from the text file and separate all words that start from a vowel using a function and display them as a sentence. (The sentence just needs to consist of the words from the string that start with a vowel.)
Problem:
The code is working as intended but as i have used the getline() function to obtain the string from the txt file when i withdraw a substring from it, it includes the entire file after the vowel instead of just the word. I cannot understand how to make the substring only include words.
Code:
#include <fstream>
#include <string>
#include <iostream>
#include <cstring>
using namespace std;
string vowels(string a)
{
int c=sizeof(a);
string b[c];
string d;
static int n;
for(int i=1;i<=c;i++)
{
if (a.find("a")!=-1)
{
b[i]=a.substr(a.find("a",n));
d+=b[i];
n=a.find("a")+1;
}
else if (a.find("e")!=-1)
{
b[i]=a.substr(a.find("e",n));
d+=b[i];
n=a.find("e")+1;
}
else if (a.find("i")!=-1)
{
b[i]=a.substr(a.find("i",n));
d+=b[i];
n=a.find("i")+1;
}
else if (a.find("o")!=-1)
{
b[i]=a.substr(a.find("o",n));
d+=b[i];
n=a.find("o")+1;
}
else if (a.find("u")!=-1)
{
b[i]=a.substr(a.find("u",n));
d+=b[i];
n=a.find("u")+1;
}
}
return d;
}
int main()
{
string input,lne,e;
ofstream file("output.txt", ios::app);
cout<<"Please input text for text file input: ";
getline(cin,input);
file << input;
file.close();
ifstream myfile("output.txt");
getline(myfile,lne);
e=vowels(lne);
cout<<endl<<"Text inside file reads: ";
cout<<lne;
cout<<endl;
cout<<e<<endl;
system("pause");
myfile.close();
return 0;
}
I haven't read your code VERY carefully, but several things stand out:
Look up find_first_of - it'll simplify your code A LOT.
sizeof(a) certainly doesn't do what you think it does [unless you think it gives you the size of the std::string class type - which makes it rather strange as a use-case, why not use either 12 or 24?]
find (and find_first_of), technically speaking, doesn't return -1 when the function isn't finding what you want. It returns std::string::npos [which may appear to be -1, but a) is not guaranteed to be, and b) is unsingned so can't be negative].
Your program only reads one line.
x.substr(n) will give you the string of x from position n - is that what you want?
Don't repeat find, use p = x.find("X"); and then do x.substr(p) [assuming that is what you want].
There are various problems with your code.
int c = sizeof( a );
This is the number of bytes that a string takes up in memory. And you certainly don't want to create an array of this many strings as it makes no sense for what you're trying to achieve. Don't do this to yourself. You're only copying one string inside the loop, all you need is one string and you already have string d.
To get the actual size of a string, you have to call
str.size()
The string.substr(..) has a couple overloads, one of them takes only one argument, an index. This will return sub string starting at that index in the original string. (The string starting at the vowel all the way through to the end of the string)
What you are maybe looking for is the overload that takes two arguments, the start index (beginning of the word and the end of the word).
The string input will not take the newline that you enter to flush cin. And then you add it to the file in append mode, so after running the program a few times your file is a huge one-liner. Did you really intend to do this?
Maybe you should explicitly add a new line to the file after entering the input. Something like file << std::endl;
Also, the conditions in the ifs
if (a.find("a")!=-1)
Don't match what you do next,
b[i]=a.substr(a.find("a",n));
Then you use a static int,
static int n;
This is bad, because this function will only work once. You're lucky that static initializes its values to zero, but you should always initialize explicitly. In your case, you don't need this to be static.
Finally: "so i was unsure of how many loops to run"
When you don't know how many loops you have to run, then a for loop is not adequate.
You should use a while loop or a do while.
You shouldn't try to learn C++ by guessing, because that's what it looks like you're doing. You're trying to do more than you know and making some very silly mistakes. Find a good book to learn from, or at the very least google the functions you're using to see what they do and how to use them properly. (ie: http://www.cplusplus.com/reference/string/string/substr/ )
Here's a list of books from stackoverflow's FAQ: The Definitive C++ Book Guide and List
The last thing is about finding vowels. When you find a vowel, you have to make sure it's at the beginning of a word. Then you want to read it until the word ends, that is when you find a character that is not part of a word. (a whitespace, certain punctuation, ... ) This should mark the beginning and end of the word.
Ok, just to be up front, this IS homework, but it isn't due for another week, and I'm not entirely sure the final details of the assignment. Long story short, without knowing what concepts he'll introduce in class, I decided to take a crack at the assignment, but I've run into a problem. Part of what I need to do for the homework is read individual characters from an input file, and then, given the character's position within its containing word, repeat the character across the screen. The problem I'm having is, the words in the text file are single words, each on a different line in the file. Since I'm not sure we'll get to use <string> for this assignment, I was wondering if there is any way to identify the end of the line without using <string>.
Right now, I'm using a simple ifstream fin; to pull the chars out. I just can't figure out how to get it to recognize the end of one word and the beginning of another. For the sake of including code, the following is all that I've got so far. I was hoping it would display some sort of endl character, but it just prints all the words out run together style.
ifstream fin;
char charIn;
fin.open("Animals.dat");
fin >> charIn;
while(!fin.eof()){
cout << charIn;
fin >> charIn;
}
A few things I forgot to include originally:
I must process each character as it is input (my loop to print it out needs to run before I read in the next char and increase my counter). Also, the length of the words in 'Animals.dat' vary which keeps me from being able to just set a number of iterations. We also haven't covered fin.get() or .getline() so those are off limits as well.
Honestly, I can't imagine this is impossible, but given the restraints, if it is, I'm not too upset. I mostly thought it was a fun problem to sit on for a while.
Why not use an array of chars? You can try it as follow:
#define MAX_WORD_NUM 20
#define MAX_STR_LEN 40 //I think 40 is big enough to hold one word.
char words[MAX_WROD_NUM][MAX_STR_LEN];
Then you can input a word to the words.
cin >> words[i];
The >> operator ignores whitespace, so you'll never get the newline character. You can use c-strings (arrays of characters) even if the <string> class is not allowed:
ifstream fin;
char animal[64];
fin.open("Animals.dat");
while(fin >> animal) {
cout << animal << endl;
}
When reading characters from a c-string (which is what animal is above), the last character is always 0, sometimes represented '\0' or NULL. This is what you check for when iterating over characters in a word. For example:
c = animal[0];
for(int i = 1; c != 0 && i < 64; i++)
{
// do something with c
c = animal[i];
}
I'm trying to read till the end of a file for a phonebook app that im converting from C to C++. When I print the the results from the file i get this:
johnny smith
(Home)3
(Cell)4
x☺> x☺>
(Home)4
(Cell)4
it should print:
johnny smith
(Home)3
(Cell)4
Right now I'm using while(!infile.eof()) which i've read is a poor practice, but when I use infile.getline() I get a repeat of the first and last name, and the format is all jacked up. Is there anyway(or another way) to get rid of the junk at the end of the input or another way to read till the end of file in C++ that fixes this. I've been reading about different solutions, but the one a lot of sites seem to agree on is fgets, which is what I had with the original C version, but obviously fgets doesn't work with ifstream which is what I'm using. here is the code:
void contacts:: readfile(contacts*friends ,int* counter, int i,char buffer[],char user_entry3[])
{
ifstream read;
read.open(user_entry3,ios::in);
int len;
contacts temp;
*counter=0;
i=0;
while (!read.eof()) {
temp.First_Name=(char*)malloc(36);
temp.Last_Name=(char*)malloc(36);
read>>temp.First_Name>>temp.Last_Name;
read>>buffer;
len=strlen(buffer);
if(buffer[len-1]=='\n')
buffer[len-1]='\0';
temp.home=(char*)malloc(20);
strcpy(temp.home, buffer);
read>>buffer;
len=strlen(buffer);
if(buffer[len-1]=='\n')
buffer[len-1]='\0';
temp.cell=(char*)malloc(20);
strcpy(temp.cell, buffer);
friends[i].First_Name=(char*)malloc(MAXNAME);
friends[i].Last_Name=(char*)malloc(MAXNAME);
friends[i].home=(char*)malloc(MAXPHONE);
friends[i].cell=(char*)malloc(MAXPHONE);
//adds file content to the structure
strcpy(friends[*counter].First_Name,temp.First_Name);
strcpy(friends[*counter].Last_Name,temp.Last_Name);
strcpy(friends[*counter].home,temp.home);
strcpy(friends[*counter].cell,temp.cell);
(*counter)++;
i++;
}
//closes file and frees memory
read.close();
free(temp.Last_Name);
free(temp.First_Name);
free(temp.home);
free(temp.cell);
}
Don't use !eof(). It checks whether the last read failure was due to reaching the end of the file. It does not predict the future.
Don't use malloc in C++. If you do, check the return value for errors!
Don't use operator>> for char *. There's no size check so that's just asking for buffer overflows.
The '\n' check on buffer is useless. operator>> for strings stops at whitespace.
You're blindly strcpying a string of unknown length into temp.home of size 20. That's another buffer overflow.
... I kind of stopped reading there. If you want to read stuff from a file but stop on eof/error, you can do something like this:
.
string a, b, c;
while (true) {
if (!(in >> a)) break;
if (!(in >> b)) break;
if (!(in >> c)) break;
do_stuff_with(a, b, c);
}
Do not use eof() to determine if you reached end of file. Instead, read what you want to read and then check if you successfully read the data. Obce reading failed you may use eof() to determine if the error is down to having reached the end of the file before producing an error report about a format error.
Since you mentioned that you read that using !infile.eof() is good practice: Can you point us at the source of this wrong information? This information need correction.
I'm trying to read an ifstream into a string, where I can set the number of characters being read. I've read the documentation for ifstream.get() and ifstream.getline(), but neither of those accomplish what I want.
Given the following string:
asdfghjklqwertyuiop
I want to read in varying number of characters at a time into a string. I've started like this, but I'm getting an error that there's no function that will take a string as the first parameter:
string destination;
int numberOfLettersToGet = 1;
while (inputstream.get(destination, numberOfLettersToGet)){
//Do something.
}
What can I use instead of inputstream.get()?
You may like to use read and gcount member-functions of std::istream. get appends a zero-terminator, which is unnecessary when you read into std::string.
std::string destination;
int numberOfLettersToGet = 1;
destination.resize(numberOfLettersToGet);
std::streamsize n = inputstream.gcount();
inputstream.read(&destination[0], numberOfLettersToGet);
destination.resize(inputstream.gcount() - n); // handle partial read
istream::get returns the character as an integer, so you simply need to append the returned character as the next character of the string. e.g.
while (string.push_back(inputstream.get()))
{ //...
}