can anyone tell me why this segfaults?
vector<string> vec;
for (int i = 0; i < 1000000; /* 1 million */ i++) {
vec.push_back("string"+i);
}
Compiled in g++ with -std=c++14
As panta-rei correctly pointed out, it looks like you're trying to contain a string of the form
"string" + string form of (i)
but you're actually doing pointer arithmetic which is illogical in this case (you're just passing a pointer incremented i from some location - who knows what's in that memory?).
In order to do what you want, you can use std::to_string, which will translate i to a proper C++ string. The addition of a C-style string with that, is OK.
Change your line to
vec.push_back("string"+to_string(i));
vector<string> vec;
for (int i = 0; i < 1000000; i++) {
vec.push_back("string" + to_string(i));
}
this is not PHP...
Related
I am writing code that finds the number of palindrome strings in a given array of strings. I think I have the right idea, but I am getting weird errors when I run it. What exactly am I doing wrong?
int countPalindromes(string s) {
int size = s.size();
int counter = 0;
string forwardSum = "";
string backwardSum = "";
for(int i = 0; i < size; i++){
for(int j = i; j < size; i++){
forwardSum.push_back(s[j]);
backwardSum.push_back(s[(n - 1)-j]);
if(forwardSum == backwardSum){
counter++;
}
}
}
return counter;
}
string forwardSum[] = {};
This is an array of zero size (which I don't believe is legal but we'll let that pass)
forwardSum[i] = forwardSum[i] + s[j];
This is an attempt to access the ith element of an array which has zero size.
That's bad.
I'm not really following your code (it's late at night), but I think you probably want forwardSum and backwardSum to be strings not arrays of strings. And you probably want to use push_back to add characters from s to those strings. I.e.
string forwardSum;
...
forwardSum.push_back(s[j]); // add s[j] to forwardSum
But if you really do want forwardSum to be an array, then the sensible thing to do would be to use a vector instead.
vector<string> forwardSum(size); // a vector of strings with the given size
Now that should at least not crash with the rest of your code.
Basically, here, I'm trying to reverse an array, and convert the reversed int array into a string (I'm trying to write the equivalent of Java's BigInteger class in C++ - basically turning the input into big endian ordering, breaking down the operations, reversing the result back to little endian format, and returning the string).
And as you can see below, it outputs some strange characters (I think it's an out of range reference?) - but I'm not entirely sure what caused this output?
I would really appreciate if you could take a look at it:
Sample input
int a[] = {1, 2, 3};
int rA[3];
reverseIntArray(a, rA, 3);
string aString = intArrayToString(a, 3);
cout << aString << endl;
Console output
123\216\377
As you can see - it calculates the answer correctly, with the exception of the \277_\377.
I'll post the rest of the relevant functions:
reverseIntArray
void reverseIntArray(int array[], int reversedArray[], int arrayLength) {
for (int i = 0; i < arrayLength; i++) {
reversedArray[i] = array[arrayLength - 1 - i];
}
}
intArrayToString
string intArrayToString(int digits[], int length) {
// convert int array to char array
char digitsChar[length];
for (int i = 0; i < length; i++) {
digitsChar[i] = '0' + digits[i];
}
// convert char array to string
string intString(digitsChar);
return intString;
}
I'm quite sure this is a subtle issue to do with pointers, but I'm still relatively new to C++ (migrating from Java) and I've stared at this for hours but haven't come up with any ideas.
The std::string constructor you are using is assuming that the string you pass is properly terminated, which it isn't and that leads to undefined behavior as the std::string constructor goes beyond the end of the digitsChar array.
Three possible solutions:
Make room for another character in the digitsChar array and terminate it:
char digitsChar[size + 1];
for (...) { ... }
digitsChar[3] = '\0';
string intString(digitsChar);
Use another constructor where you pass the length of the character array:
string intString(digitsChar, length);
Append the characters directly to the string:
string intString;
for (int i = 0; i < length; i++) {
intString += '0' + digits[i];
}
There are of course other solutions as well, like for example using std::ostringstream.
so I'm working on a project that I have to read contents from a file and then analyze them. But I'm having a problem with getting the string out of a pointer that contains the address to what I need.
string lePapel(vector<char> vec){
string *str, s;
int i, j = 0;
vector<char> aux;
aux.resize(6);
for (i = 57; i <= 62; i++){
aux[j] = vec[i];
j++;
}
str = new string[aux.size()];
for (i = 0; i < 6; i++){ str[i] = aux[i]; }
return s;
}
So, the file contains in the array positions from 57 to 62 the word: ABCB4, but when returning the string s my output is A only as expected because of the pointer.
The thing is that I have been trying to find a solution and storing the whole content from vec[57] to vec[64] into the string s and returning it, and the closest that I got to returning anything plausible was using a pointer.
So, now to my question, how can I iterate the *str pointer and copy the whole content to s and return it?
Thanks in advance
I'd suggest you to not use pointers on string in your case. The following code is probably what you want :
#include <iostream>
#include <string>
#include <vector>
using namespace std;
string lePapel(vector<char> vec){
int j = 0;
vector<char> aux;
aux.resize(6);
for (int i = 57; i <= 62; i++){
aux[j] = vec[j];
j++;
}
string str;
str.reserve(6);
for (int i = 0; i < 6; i++){ str.push_back(aux[i]); }
return str;
}
int main() {
char x[5] = {'A', 'B', 'C', 'B', '4'};
vector<char> vec(x, x + 5);
string s = lePapel(vec);
cout << s;
return 0;
}
Tested here : Tested code
About reserving space to your vector : c++ vector::reserve
Same for strings : reserve for strings
The dynamic array of string objects and the whole aux vector seem completely needless here (unless there's some other purpose for them in your code). Additionally, str is currently causing a memory leak because you never delete it when you're finished.
A much simpler approach is just to append the characters one-at-a-time to the s string object (assuming it's a std::string):
string lePapel(vector<char> vec) {
string s;
for (int i = 57; i <= 62; i++) {
s += vec[i];
}
return s;
}
There are various ways to make the code even shorter (and more efficient) than that though, if you really want to.
EDIT: If you still need/want to iterate your dynamic array and concatenate the contents into s, here's how you could do it:
for (i = 0; i < 6; i++) s += str[i];
delete [] str; //<-- very important!
Short answer, you don't want a string * you want a char *. What you created is a string array. String objects contain a pointer to the char * data you are trying to capture. Also, the sizeof(std::string) (8 bytes in size) is a lot bigger than sizeof(char) (1 byte in size) the second character you store is 8 bytes away from the first character instead of being adjacent.
There are a lot of other C++ style and safety concerns, but I'll stick with the question. ;)
I have a simple table called mytable2 with only one column, name as varchar2(20).
I now have a list of names stored as vector of std::string to be inserted into the table.
I want to use executeArrayUpdate, so I must do the setDataBuffer first.
However, as I could see, people always use char[][20] to set databuffer.
This leaves me a big headache, since I have two issues here, first is to convert from vector to array, second is to convert the string to char.
1st, I tired to use vector of char[20], and this doesn't compile. Googled and they say that vector can't take char[], so I changed my vector of std::string to vector of char*.
2nd, I tried to turn the vector to arrray by using "void* p=&names[0]", as some people say this way we can use vectors just as array.
I used stmt->setDataBuffer(1,mystring,OCCI_SQLT_STR,20,NULL), and the program compiled and executed alright, but when I "select name from mytable2", it showed only some strange charaters.
Anyone has had a similiar issue before? what should I do?
My code is simple as below:
count=2;
vector<char*> mystring;
for(int i=0;i<count;i++)
{
char my[20];
strcpy_s(my,"Michael");
mystring.insert(mystring.end(),my);
}
stmt->setDataBuffer(1,&mystring[0],OCCI_SQLT_STR,20,NULL);
stmt->setBatchErrorMode (true);
stmt->executeArrayUpdate(count);
You'd need to dynamically create the char array you're putting into the vector for it to have a chance of working correctly.
I have not used OCCI, but if I had to use API that asked for char[][20], I would give it char[][20]
If you have your existing data in vector, why not just copy it across into the 2D char array? Eg.
// intialise vector with nonsense
unsigned int const VEC_SIZE = 2 ;
vector<string> v;
for (unsigned int i = 0; i < VEC_SIZE; ++i) {
stringstream s;
s << "Jon Paul " << i << endl;
v.push_back ( s.str() ) ;
}
// create the required 2D char array
unsigned int const STR_LEN = 20 ;
char c [VEC_SIZE][STR_LEN];
// copy the data from the vector of strings to the 2D char array
for (unsigned int i = 0; i < VEC_SIZE; ++i) {
string s = v[i].substr(0,STR_LEN);
char const * const cc = s.c_str();
unsigned int j = 0;
for (; j < STR_LEN && j < s.size(); ++j) {
c[i][j] = cc[j];
}
c[i][ j==STR_LEN ? 20 : j ] = 0; // write NULL character
}
I take it you've simplified your example to be a fixed size vector, so my response is going to be simplified to, with the thorny issue of dynamic allocation of 2D arrays left as an exercise for the reader...
As part of a homework assignment I need to concatenate certain values in an array in C++. So, for example if I have:
int v[] = {0,1,2,3,4}
I may need at some point to concatenate v[1] -> v[4] so that I get an int with the value 1234.
I got it working using stringstream, by appending the values onto the stringstream and then converting back to an integer. However, throughout the program there will eventually be about 3 million different permutations of v[] passed to my toInt() function, and the stringstream seems rather expensive (at least when dealing with that many values). it's working, but very slow and I'm trying to do whatever I can to optimize it.
Is there a more optimal way to concatenate ints in an array in C++? I've done some searching and nearly everywhere seems to just suggest using stringstream (which works, but seems to be slowing my program down a lot).
EDIT: Just clarifying, I do need the result to be an int.
Pseudo code for a simple solution:
int result = 0;
for (int i=0; i < len(v); i++)
{
result = result*10 + v[i];
}
Large arrays will bomb out due to int size overflow.
How about:
int result = (((v[1])*10+v[2])*10+v[3])*10+v[4];
If the number of elements is variable rather than a fixed number, I'm sure you can spot a pattern here that can be applied in a loop.
Remember ASCII codes?
char concat[vSize+1];
concat[vSize] = 0;
for(int i = 0; i < vSize; i++) {
concat[i] = (v[i] % 10) & 0x30;
}
All are integers. Shouldn't you do the following.
//if you want to concatenate v[1] and v[4]
int concatenated;
concatenated = v[1]*10+v[4];
//If you want to concatenate all
concatenated = 0;
for(int i=1;i<=4;i++)
concatenated = concatenated*10+v[i];
the output would be an integer ( not a string)
Things you can do:
Make sure that you compile with -O3 (Or equivalent compiler optimization).
Do you generate the values in the vector yourself? If so, try changing toInt() function to accept a simple pointer type.
Write the conversion yourself (Browser code : may not even compile - u get the idea though):
char* toInt(int* values, size_t length)
{
int *end = values + sizeof(int)*length;
int *cur = values;
char* buf = new char[length + 1]
for(char* out = buf;cur < end;++cur, ++buf)
{
*out = (char)*cur + '0';
}
*buf = '\0';
return buf;
}