Im trying to get all values of a dynamic array initialised like this:
string** structure= new string*[nombre_attributs]();
for(int j=0; j<nombre_attributs; j++){
structure[j]= new string[2]();
}
I can fill it with no problem. I then keep a pointer to this array in an array of pointers called adresses_structures.
How can I now acces the data in this array?
This attempt:
string *test = adresses_structures[i];
cout << "Value:" << *(test+0)<<endl;
cout << "Value:" << *(test+1)<<endl;
cout << "Value:" << *(test+2)<<endl;
doesn't work for all values from the second line in the array.
I have tried tons of combinations as
for(int k=0; k<2;k++){
for(int j=0;j<2;j++){
cout << "Truc:" << *(test + k*2 + j)<<endl;
}
}
but nothing seem to be able to get the values on the second line.
Thanks for any advice.
I assume, that string** structure is the same as adresses_structures here.
When You are doing *(test+1) You move by one byte from test address. You should move by size of string object (check sizeof(string)). Like this: *(test+1*sizeof(string)) or *(test+2*sizeof(string))
Related
I am trying to print the list of words from a vector of string (query_list) but I can't get it to work. I know that the current code prints the address.
vector<string>* query_list_ptr;
query_list_ptr = &query_list;
for (i = 0; i < num_words; i++) {
cout << query_list_ptr << " ";
query_list_ptr++;
}
I tried the following:
I get an error when I tried using "*query_list_ptr" to dereference it at the "cout <<" line.
I also get an error when I tried variations of "vector<string>* query_list_ptr" to "vector<string*> query_list_ptr".
Another error is "query_list_ptr = query_list" that's why I ended up adding the ampersand to "query_list_ptr = &query_list;".
Thanks for all the help. I could do without pointers but it's for my CS class about pointers.
The pointer to the first element of std::vector can be obtained via the data() member function.
const string* query_list_ptr; // you want to print words, so I guess the pointer should point to string
query_list_ptr = query_list.data(); // get the pointer to the first element
for (i = 0; i < num_words; i++) {
cout << *query_list_ptr << " "; // dereference the pointer to get the word to print
query_list_ptr++;
}
Trying to insert values into a 2D array, but the output isnt giving my values, instead random letters
int myArr[8][2] = {700,730,760,790,810,840,910,1000}{0.011,0.035,0.105,0.343,0.789,2.17,20,145};
cout << myArr << endl;
system("Pause");
How should I adjust the code, or is it easier to use a text file and insert?
Numerous problems:
the array dimensions are wrong
you don't have outer braces or a comma for the nested arrays
you're trying to store double precision floating point values in an int array
you can't use cout with an entire array.
The array declaration should probably be something like this:
double myArr[2][8] = { {700,730,760,790,810,840,910,1000},
{0.011,0.035,0.105,0.343,0.789,2.17,20,145} };
and to output the contents you could do something like this:
for (int i = 0; i < 2; ++i)
{
for (int j = 0; j < 8; ++j)
{
cout << " " << myArr[i][j];
}
cout << endl;
}
Live Demo
First - you can't print the whole array just by using cout << myArr, you need to iterate over the elements of the array using a for loop.
Second - you are trying to put decimal values into an integer array which will truncate all of the decimals.
Third - Your array should be sized myArr[8][2] not myArr[2][8]. I'm surprised your compiler lets you get away with this. You should probably look into using a different compiler.
You need to iterate through each row and column, otherwise you're just printing out the pointer value of the array handle.
for (int i=0;i<8;i++){
for (int j=0;j<2;j++){
cout << myArr[i][j] << " ";
}
cout << endl;
}
system("Pause");
I have finished writing a program that included reversing, expanding and shifting arrays using the pointer requirement asked by the professor. Everything compiles but the answer from the expand function does not return what I wish: adding 0s after the old user input array which asks for the size of the array and the numbers you wish to put into the array. I think my problem may lie from the fact that I include a pointer on something that might not have a reference in the program. Below is my code:
// *numPtr refers to my old user input array and int tamaño is the size of the array
void expandArray(int *numPtr, int tamaño) {
int *nuevoArray = new int[tamaño *2];
for (int i = 0; i<tamaño; i++) {
nuevoArray[i] = numPtr[i];
}
for (int i = tamaño; i < (tamaño*2); i++) {
nuevoArray[i] = 0;
}
std::cout << nuevoArray << " ";
}
As I said, my theory of the code not compiling the way I wish is because I use the *nuevoArray and it has no reference in my main code, but then again, I am just a beginner with C++. I was thinking of just doing a vector, but I think I would not follow the pointer requirements placed by the professor.
If you want to print the contents of nuevoarray, just use a for loop like this:
for (int i = 0; i < (tamaño*2); i++) {
std::cout << nuevoArray[i] << " ";
}
std::cout << "\n";
Also, since you are using new[] to create the array, you should not forget to delete[] it!
you can print your array by using
for (int i = 0 ; i < tamano * 2 ; ++i) {
std::cout << nuevoArray[i] << " ";
}
std::cout << std::endl;
or in c++11
for (auto i : nuevoArray) {
std::cout << i << " ";
}
std::cout << std::endl;
PS: The std::endl will return to the start of the new line and flush the cout buffer.
Your code does appear to be allocating a larger array and correctly copying data from numPtr into the new array and also correctly filling the remainder of the new array with zeros.
You don't explicitly say what you expect this function to output, but I'm guessing you expect it to print out the contents of the new array, and that you believe there's a problem because instead of that, you're seeing it print something like "0x7fb46be05d10".
You're not correctly printing the array out. Instead you're printing the memory address of the first element out. If you want to see the contents, then you need to loop over the elements of the array and print each one out individually.
Here's a function showing one way of doing that:
#include <algorithm>
#include <iterator>
void printArray(int *arr, int n) {
std::copy(arr, arr + n, std::ostream_iterator<int>(std::cout, " "));
}
Now you can replace the line std::cout << nuevoArray << " "; in your existing code with printArray(nuevoArray, tamaño*2);
(Also it sounds like whoever is teaching you C++ should take a look at this presentation from the recent C++ conference, CppCon 2015: Stop Teaching C)
I am basically trying to store everything after a certain index in the array.
For example, I want to store a name which is declared as char name[10]. If the user inputs in say 15 characters, it will ignore the first five characters and store the rest in the char array, however, my program crashes.
This is my code
char name[10];
cout<< "Starting position:" << endl;
cin >> startPos;
for(int i= startPos; i< startPos+10; i++)
{
cout << i << endl; // THIS WORKS
cout << i-startPos << endl; // THIS WORKS
name[i-startPos] = name[i]; // THIS CRASHES
}
For example, if my name was McStevesonse, I want the program to just store everything from the 3rd position, so the end result is Stevesonse
I would really appreciate it if someone could help me fix this crash.
Thanks
Suppose i is equal to 3. In the last iteration of the loop, i is now equal to 12, so substituting 12 in for i, your last line reads
name[12-startPos] = name[12];
name[12] is out of bounds of the array. Based on what you have shown so far, there is nothing but garbage stored in name anyway before you start doing this assignment, so all you're doing is reorganizing garbage in the array.
Please in future: post full compilable example.
A simple answer is that your array maybe is out of bound, since you don't provide full example its hard to know exactly.
Here is a working example:
#include <iostream>
using namespace std;
int main() {
int new_length, startPos;
int length = 15;
char name[15]= "McStevesonse";
cout<< "Starting position:" << endl;
cin >> startPos;
if(new_length <1){ // you need to check for negative or zero value!!!
cout << "max starting point is " <<length-1 << endl;
return -1;
}
new_length=length-startPos;
char newname[new_length];
for(int i= 0; i<new_length; i++){
newname[i] = name[i+startPos]; // THIS CRASHES
}
cout << "old name: " << name << " new name: " << newname << endl;
return 0 ;
}
To put it simply, change this:
for(int i= startPos; i< startPos+10; i++)
To this:
for(int i= startPos; i<10; i++)
You should be fine with that.
Explanation:
At some point, when you use the your old loop, this name[i-startPos] = name[i] would eventually reach an array index out of bounds and causes the crash.
Don't forget to clean up/hide the garbage:
Doing so, would cause the output to produce some kind of garbage outputs. If you got a character array of 'ABCDEFGHIJ', and have chosen 3 as the starting position, the array would be arranged to 'DEFGHIJHIJ'. In your output, you should atleast hide the excess characters, or remove by placing \0's
I am doing some exercises to figure out how to access values in an array after they are changed with pointers. Can someone point out why the first output does not show the desired output? I am trying to get both cout to print 1234, one by using the new pointer and one by using the position in the array
int main()
{
char myArray[50]={0};
short* sizeOfAlloc=(short*)(myArray+5);
*sizeOfAlloc=1234;
cout << (short*)(myArray+5) <<endl;
cout << *sizeOfAlloc <<endl;
system("pause");
}
cout << (short*)(myArray+5) <<endl;
Prints the pointer. Not the value pointed by it.
cout << *((short*)(myArray+5)) <<endl;
^^ ^^
Will print the value pointed to by (short*)(myArray+5)