I am working on a project where a question is displayed in a game window.
Since this question will need to change a lot, I figure it would be easier to have 5 defined lines of text (1 for question, 4 for MC answers) that are simply edited every time the question changes.
I have tried this in the header file:
struct tagQuestion{
int x, y;
const char* qLine[150];
char ansA[150];
char ansB[150];
char ansC[150];
char ansD[150];
}question[1];
then in my main.cpp
question.qLine[150] = "TEST PHRASE";
but it is returning the error "qLine" in "question", which is of non-class type "tagQuestion[1]"
I have tried both char and const char* to no success.
I am trying to follow an example in my textbook and I think I'm misunderstanding it.
Once I declare the character array in the header file, can't I edit its contents in the main file?
That [1] after question doesn't seem to be making any sense. Remove it.
Your answers are arrays of char, but your question is an array of char pointers. I am pretty sure you don't want your question to consist of 150 char pointers.
If you want to change qLine, don't make it const.
qLine[150] = ... This assigns a value to the 151st element of an array with 150 elements. Neither what you want, nor legal code. You probably want qLine =, but that doesn't work with an array, instead you need to use a function like strcpy that writes data into the array, but I highly advise against that. (see next point)
Don't do C in C++. Use std::string, not arrays of char.
If you have strings that regularly change, create a file where your strings are stored, load them into a map, and pull them from the map wherever and whenever you need them, instead of putting hardcoded strings in your code. That way you don't need to change any code, when you change text.
I suggest something like this:
struct Question {
std::string text;
std::vector<std::string> answers;
};
std::map<std::string, Question> questions;
And then something like this:
ifstream questionsFile("questions.txt");
if (questionsFile.is_open())
{
std::string line;
while (std::getline(questionsFile, line))
{
// Split line into some id, text and answers, how exactly depends on the format you chose
std::string id = ...;
Question question;
question.text = ...;
question.answers.push_back(...);
questions[id] = question;
}
questionsFile.close();
}
else
{
// Handle error
}
And wherever in your code:
// Print question with id "q1":
std::cout << questions["q1"].text << std::endl;
// Print answer #2 for question with id "q1":
std::cout << questions["q1"].answers[1] << std::endl;
Even better would be to make Question a class that hides its internals and has accessor methods and a constructor or static method to create new instances from valid lines with proper error handling.
Related
I'm learning to code c++ and I've come to this problem:
I have this struct:
struct storeData
{
string name;
string username;
string fav_food;
string fav_color;
}data[30];
And I need to check if two usernames are equal so I made this statement:
for(i=0;i<c;i++){
if(data[c].username.compare(data[i].username)==0){
cout<<"Username already taken"<<endl;
}
}
And it works well, the problem that I have is that I'm required to make a function let's call it: isTaken that returns the error message, so I can use it whenever I need to for example delete a username so I don't have to copy/paste the code again.
So I began looking for an answer for that, many forums present a way to send the whole struct like this:
void isTaken(struct storeData *data)
which I understand but because I'm using string is not working, so I guess it's because string is an object? I'm using the library <string> I'm sorry if I'm not being that clear at the moment, I'm looking for a way to use isTaken(data[c].user); but I don't know how to declare the function, I think is also because string is not the same as C string but I'm not really sure I've been looking for a solution and could not find it.
I tried: void isTaken(struct storeData *data) but I got an error saying that I can't convert std::string to basic_string which makes sense if I'm correct about string I tried converting string into c string but could not get anywhere. I'm open to suggestions/corrections because I want to improve my code, also I could not find the answer here, so If someone's got a link to a problem like this please let me know.
Thank you so much for you time, have a good day.
Do you mean an array of structs instead of a struct of arrays?
In the example you are giving I see only an array of structs each of which has multiple string objects in it. You see, a string is a class coming from std and I wouldn't call it an array. If you want to know how to pass an array to a function, you should read about it (I'm sure you can find such a question in SO). If you want to have an array within your struct, then the struct will take care of the memory of the array, but you should definitely read about constructors.
You got an error because you are passing an string argument to a function which requires struct pointer
void isTaken(struct storeData *data);
...
isTaken(data[c].user);
but what you actually need is to have a function which takes an array of your users, its size and username you want to check
bool IsUsernameTaken(struct storeData data[], int dataSize, const string &username){
for(int i = 0; i<dataSize; i++){
if(username == data[i].username)
return true;
}
return false;
}
A C string looks like this
data
A C++ string usually looks like this
size
capacity
ptr
|
v
data
or if using short string optimization and the string is short enough
size
data
data
all are zero terminated.
Making a shallow copy a C string only cost the copy of the pointer to it. Where a copy of a might cost just copying the 3 members and possible an allocation of data, which is not ideal, therefor most C++ functions use a reference to a string making the cost equivalent to the C string.
All code is untested.
bool Find(const std::string& target);
Making a deep copy of a C string would also cost an allocation.
In C++ you have many options to do a search, for your struct it could look like this. In case your member variables are private you must use an access function
auto found = std::find(std::begin(data), std::begin(data)+c, [&target](const storeData& auser) { return auser.GetName() == target });
return (found != std::begin(data)+c);
The first two parameters are the range that is search, not including the 2nd. A lambda is used to check the name, a free function with the right declaration would also do.
std::string& GetName() { return name; }
The higher C++ protection schemes would advice adding 2 consts to that in case you don't need to change name.
const std::string& GetName() const { return name; }
Meaning the returned string cant be changed and the 2nd says it wont change anything in your class. This const version would be required as I used a const storeData& auser in the lambda to satisfy the constness of the struct.
I've been using:
string letters = THESAMELENGTH; // Assign for allocation purposes.
Reason being, if I:
string letters[THESAMELENGTH.length()];
I get a non constant expression complaint.
But if I:
string letters[12];
I'm at risk of needing to change every instance if the guide const string changes size.
But it seems foolish to assign a string when I won't use those entries, I only want my newly assigned string to be the same length as the previously assigned const string, then fill with different values.
How do you recommend I do this gracefully and safely?
You can
string letters(THESAMELENGTH.length(), ' '); // constructs the string with THESAMELENGTH.length() copies of character ' '
BTW: string letters[12]; doesn't mean the same as you expected. It declares a raw array of string containing 12 elements.
I only want my newly assigned string to be the same length as the previously assigned const string, then fill with different values.
Part of the reason the string class/type exists is so you don't have to worry about trying to manage its length. (The problem with arrays of char.)
If you have a const std::string tmp then you can't just assign anything to it after it has already been initialized. E.g.:
const std::string tmp = "A value"; // initialization
tmp = "Another value"; // compile error
How do you recommend I do this gracefully and safely?
If you really want to keep strings to a specific size, regardless of their contents, you could always resize your string variables. For example:
// in some constants.h file
const int MAX_STRING_LENGTH = 16;
// in other files
#include "constants.h"
// ...
std::string word = ... // some unknown string
word.resize(MAX_STRING_LENGTH);
Now your word string will have a length/size of MAX_STRING_LENGTH and anything beyond the end gets truncated.
This example is from C++ Reference
// resizing string
#include <iostream>
#include <string>
int main ()
{
std::string str ("I like to code in C");
std::cout << str << '\n';
unsigned sz = str.size();
str.resize (sz+2,'+');
std::cout << str << '\n';
str.resize (14);
std::cout << str << '\n';
return 0;
}
// program output
I like to code in C
I like to code in C++
I like to code
You can't just ask a string variable for its length at compile-time. By definition, it's impossible to know the value of a variable, or the state of any given program for that matter, while it's not running. This question only makes sense at run-time.
Others have mentioned this, but there seems to be an issue with your understanding of string letters[12];. That gives you an array of string types, i.e. you get space for 12 full strings (e.g. words/sentences/etc), not just letters.
In other words, you could do:
for(size_t i = 0; i < letters.size(); ++i)
letters[i] = "Hello, world!";
So your letters variable should be renamed to something more accurate (e.g. words).
If you really want letters (e.g. the full alphabet on a single string), you could do something like this:
// constants.h
const std::string ALPHABET_LC = "abc...z";
const std::string ALPHABET_UC = "ABC...Z";
const int LETTER_A = 0;
const int LETTER_B = 1;
// ...
// main.cpp, etc.
char a = ALPHABET_LC[LETTER_A];
char B = ALPHABET_UC[LETTER_B];
// ...
It all depends on what you need to do, but this might be a good alternative.
Disclaimer: Note that it's not really my recommendation that you do this. You should let strings manage their own length. For example, if the string value is actually shorter than your limit, you're causing your variable to use more space/memory than needed, and if it's longer, you're still truncating it. Neither side-effect is good, IMHO.
The first thing you need to do is understand the difference between a string length and an array dimension.
std::string letters = "Hello";
creates a single string that contains the characters from "Hello", and has length 5.
In comparison
std::string letters[5];
creates an array of five distinct default-constructed objects of type std::string. It doesn't create a single string of 5 characters. The reason for the non-constant complaint when doing
std::string letters[THESAMELENGTH.length()];
is that construction of arrays in standard C++ is required to use a length known to the compiler, whereas the length of a std::string is determined at run time.
If you have a string, and what to create another string of the same length, you can do something like
std::string another_string(letters.length(), 'A');
which will create a single string containing the required number of letters 'A'.
It is largely pointless to do what you are seeking as a std::string can dynamically change its length anyway, as needed. There is also nothing stopping a std::string from allocating more than it needs (e.g. to make provision for multiple increases in its length).
Ps: This is more of a conceptual question.
I know this makes things more complicated for no good reason, but here is what I'm wondering. If I'm not mistaken, a const char* "like this" in c++ is pointing to l and will be automatically zero terminated on compile time. I believe it is creating a temporary variable const char* to hold it, unless it is keeping track of the offset using a byte variable (I didn't check the disassembly). My question is, how would you if even possible, add characters to this string without having to call functions or instantiating strings?
Example (This is wrong, just so you can visualize what I meant):
"Like thi" + 's';
The closest thing I came up with was to store it to a const char* with enough spaces and change the other characters.
Example:
char str[9];
strcpy(str, "Like thi")
str[8] = 's';
Clarification:
Down vote: This question does not show any research effort; it is unclear or not useful
Ok, so the question has been highly down voted. There wasn't much reasoning on which of these my question was lacking on, so I'll try to improve all of those qualities.
My question was more so I could have a better understanding of what goes on when you simply create a string "like this" without storing the address of that string in a const char* I also wanted to know if it was possible to concatenate/change the content of that string without using functions like strcat() and without using the overloaded operator + from the class string. I'm aware this is not exactly useful for dealing with strings in C++, but I was curious whether or not there was a way besides the standard ways for doing so.
string example = "Like thi" + "s"; //I'm aware of the string class and its member functions
const char* example2 = "Like this"; //I'm also aware of C-type Strings (CString as well)
It is also possible that not having English as my native language made things even worst, I apologize for the confusion.
Instead of using a plain char string, you should use the string library provided by the C++ library:
#include <string>
#include <iostream>
using namespace std;
int main()
{
string str = "Like thi";
cout << str << endl;
str = str + "s";
cout << str << endl;
return 0;
}
Normally, it's not possible to simply concatenate plain char * strings in C or C++, because they are merely pointers to arrays of characters. There's almost no reason you should be using a bare character array in C++ if you intend on doing any string manipulations within your own code.
Even if you need access to the C representation (e.g. for an external library) you can use string::c_str().
First, there is nothing null terminated, but the zero terminated. All char* strings in C end with '\0'.
When you in code do something like this:
char *name="Daniel";
compiler will generate a string that has a contents:
Daniel\0
and will initialize name pointer to point at it at a certain time during program execution depending on the variable context (member, static, ...).
Appending ANYTHING to the name won't work as you expect, since memory pointed to by name isn't changeable, and you'll probably get either access violation error or will overwrite something else.
Having
const char* copyOfTheName = name;
won't create a copy of the string in question, it will only have copyOfTheName point to the original string, so having
copyOfTheName[6]='A';
will be exactly as
name[6]='A';
and will only cause problems to you.
Use std::strcat instead. And please, do some investigating how the basic string operations work in C.
Hey everyone, thanks for taking the time to address my problem. I've looked at so much material at this point that I've just started getting more and more confused. Basically, I'm writing a simple segment of code that parses a string. Please understand that my question is NOT about parsing. I am using C++ and have never used C before and possess a little bit of c++ experience (introductory experience, I'm still a newbie).
struct parsedString{
char chunk1[50];
char chunk2[10];
char chunk3[50];
};
main(char* bigstring)
{
parsedString ps;
ps = parseMe(bigString)
cout << ps.chunk1 << endl;
cout << ps.chunk2 << endl;
cout << ps.chunk3 << endl;
}
parsedString parseMe(char* stringToParse)
{
char* parseStr = stringToParse;
parsedString ps;
ps.chunk1 = first x chars;
ps.chunk2 = next y chars;
ps.chunk3 = last z chars;
return ps;
}
Obviously this is not working code, I didn't want to throw up all the extra stuff since it would be tougher to read through and I'm pretty sure my problem is a newbie c/c++ problem and something about memory allocation or something like that...
Basically when the main function gets to printing the strings from the parsedString it prints out exactly what I want it to, plus garbage characters. I'm entering the values for each array ps.chunk using
ps.chunk1[i] = *parseStr
since parseStr gets me each individual character. I can't figure out where the garbage characters are coming from, does it have something to do with how I am accessing the big string? Originally I used char in the struct instead of arrays and when I printed from within the parseMe() function they would come out great but they would turn into gibberish when I accessed it from the main function. Any help is appreciated, thanks so much.
If something is unclear please let me know I'll do my best to elaborate.
It's not clear why you're trying to do this with '\0' terminated
strings, when C++ has a perfectly usable string class, but
supposing some pedagogical reasons: are your strings '\0'
terminated? How do you extract and copy the first x chars into
ps.chunk1? How do you ensure that it has a '\0'?
If you really want to get exactly n characters, you have to:
assert(strlen(parseStr) >= n);
, copy them into the target (which must contain at least n+1 char's),
memcpy(ps.chunk1, parseStr, n);
, and add the final '\0':
ps.chunk1[n] = '\0';
(And, of course, advance parseStr: parseStr += n;)
Or you can simply use std::string everywhere, and write
something like:
ps.chunk1(stringToParse.substr(startPosition, length));
As pointed out by others, you should use std::string to represent strings, and save yourself all the trouble. This could look like this:
struct parsedString{
std::string chunk1;
std::string chunk2;
std::string chunk3;
};
parsedString parseMe(const std::stirng & stringToParse) {
parsedString result;
// just an example - this assigns the first two characters of stringToParse
// to chunk1
result.chunk1 = stringToParse.substr(0, 2);
// get the other chunks
return result; // return the result
}
The above code should illustrate the usage. You can find more information on std::string here.
This could be happening due to a couple of reasons.
When you declare parsedString ps; it would be good to initialize the structure and make sure that you are receiving clean memory blocks to work with.parsedString ps = {0}; Some platforms don't do this and it's up to you to zero the memory before using it.
char* strings must end with the \0 character. This character signals the end of a char*. This is mandatory! Not inserting it at the end of the string will most probably make the print operation (for instance) display contents beyond the limits of the array giving you garbage characters. This is automatically achieved by zeroing all the struct as I suggest above.
I'm trying to get a contiguous line with values separated by "&" to load into a multi-dimensional array. Here's the way I'm trying to do it - Everything checks out in the code, except the string "str" which contains my separated values in the format "value1, value2, value3, etc..." just loads that whole string into array[0][0]. I know there are better ways of doing this, but what I would like to know is why C++ won't treat "str" as if I had typed out the individual values and hard coded "array".
Here is the code:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;
int main(int argc, char* argv[])
{
string str, strTotal;
ifstream in;
in.open("Desktop/01_001.PAC");
getline(in,str);
while ( in ) {
strTotal += str;
getline(in,str);
}
string searchString( "&" );
string replaceString( ", " );
assert( searchString != replaceString );
string::size_type pos = 0;
while ( (pos = str.find(searchString, pos)) != string::npos ) {
str.replace( pos, searchString.size(), replaceString );
pos++;
}
string array[4][5] = {str};
cout << array[0][0];
return(0);
}
And here is the external file ("Desktop/01_001.PAC"):
void&void&void&void&a&a1&a2&a3&b&b1&b2&b3&c&c1&c2&c3&d&d1&d2&d3
Thanks in advance!
Because code and data are different things. Your code is compiled before it runs.
It sounds as if this is what you expect:
The string contains the text "foo, bar, baz".
The statement string[] whatever = {str}; is run.
Since "str" contains "foo, bar, baz", you want it to have the same effect as if the line of code were actually string[] whatever = {"foo", "bar", "baz"}.
Asking something like this implies a complete misunderstanding of how programming works.
Nothing this magical will ever happen in C++. It cannot, because (a) what if you actually wanted to put 'str' into the array? (b) what if 'foo', 'bar' and 'baz' were also variables in your program - should they be interpreted the same way?
Variable names are not text. They no longer exist, for all practical purposes, at the time that your code runs. They are only there so that you, as the programmer, can say "the value that is used over here should be the same one that is used over there".
Further, array initializations in C++ do not care how many elements are actually in the initialization vs. the declared size of the array. Any additional elements will be default-initialized (i.e., assigned empty strings).
A string cannot be treated like an array of strings, because it isn't one. If you want an array of strings, then build it, using the individual string elements as you determine them.
But since you don't know in advance how many elements there are, you should use std::vector instead of an array. And why are you trying to arrange the data into a 2-dimensional structure? How are you expecting to know how "wide" it should be?
If I'm reading your code correctly, you appear to be searching through the string (loaded from file), and only assigning the very last result to an array index (x=4, y=5). So your code is doing something like this:
while (have not found last variable)
search for next variable in string
assign variable to (4,5) in matrix
So that last assignment might even work, but since you only assign at the end, the array is not going to be filled the way I think you want it to be filled.
I'm going to assume the matrix you want is always the same size, otherwise things get more complicated. In this case, you could use something like this:
let xMax = 4
let yMax = 5
for (x from 0 to xMax)
for (y from 0 to yMax)
find the next variable in the string
assign it to the current (x,y) location in matrix
Debug statements are your friend here! Try the above solution without saving it to an array, and instead print out each term, to see if it is working correctly.
I would also point out that the string "void" is not the C++ keyword void, and so will not work if you want an array index to be void. Try getting your code to work without voids at first.