I am writing a program to simulate a cache in c++ and am trying to copy addresses that are given in a file into an array. I am struggling to figure out how to copy an array into another array so that I can have an array of memory address arrays. I have read in the addresses into an array called "address" and I want my simulated cache to be an array called "L1_Cache". h is a counter that I am incrementing after I put an address into the L1_Cache. Also, cache size is going to be how many lines of addresses are available in my L1_Cache array, which will be decided by the user of the program. Below is the snippet where I am trying to put the array into the other array.
if(sizeof(L1_Cache) < cachesize)
strcpy(L1_Cache[][h], address);
they are defined as:
const char* address[10];
char* L1_Cache;
If anyone has any suggestions on how to copy one array into another array to make an array of arrays, let me know. I am not sure if anything I am doing is correct, but I am struggling to figure this out.
I want to compare new addresses that I am given to old addresses that are already in the L1_Cache array.
Yes, it is possible to make an array of arrays.
int a[3][3]; // a is an array of integer arrays
You have
a[0]; // this refers to the first integer array
a[1]; // this refers to the second array
Is the following what you are looking for?
#include <iostream>
#include <cstring>
int main()
{
char p[2][256];
strncpy(p[0], "This is my first address", 256);
strncpy(p[1], "This is my second address", 256);
std::cout << p[0] << std::endl << p[1];
return 0;
}
Yes. They are called multidimensional arrays.
They can have any number of dimensions.
For example:
int foo[3][3]; // initialize the 2 dimensional array of integers
foo[0][0] = 1; // change a value
foo[0][1] = 2; // change a value
foo[0][2] = 3; // change a value
foo[1][0] = 4; // change a value
foo[1][1] = 5; // change a value
foo[1][2] = 6; // change a value
foo[2][0] = 7; // change a value
foo[2][1] = 8; // change a value
foo[2][2] = 9; // change a value
for(int i=0;i<3;++i){ // display the 2d array
for(int j=0;j<3;++j){
cout<<foo[i][j];
}
cout<<endl;
}
What's happening:
Values are being assigned in a chart.
Think of it like writing a value on each point of a piece of paper.
Related
I think it is a trivial question, but I couldn't find a specific solution to it. I'm trying to append array into a vector, using push_back() function. Here is the code:
int main()
{
std::vector<int*> matchVector;
int msmTemp[3];
msmTemp[0] = 1;
msmTemp[1] = 2;
msmTemp[2] = 3;
matchVector.push_back(msmTemp);
msmTemp[0] = 4;
msmTemp[1] = 7;
msmTemp[2] = 0;
matchVector.push_back(msmTemp);
for(auto i : matchVector)
{
for(int j = 0; j<3; j++)
{
cout<<i[j]<<", ";
}
cout<<"\n";
}
return 0;
}
The output I'm getting is 4,7,0 two times. I don't understand as to why I'm not able to see the previous values, namely 1,2,3? Is it because of the type of vector matchVector defined above? I think it needs to be array only.
A int* is a pointer to an integer.
An int[3] is an array of 3 integers.
An array of 3 integers "decays" at the drop of a hat to a pointer to the first element.
When you do push_back(msmTemp), you push a pointer to the first element of msmTemp into the vector.
Pointers in C++ do not own what they point to. The vector afte the two push_backs contains two pointers, both to the same array msmTemp.
When you later iterate over the vector, you get two pointers in turn. Each points to msmTemp.
You then use [] to index those pointers. When you have a pointer to the first element of an array, you can use [] to access the other elements of the array. [0] is the first element, [1] the second, etc.
So you look at the 3 elements in msmTemp (luckily it has 3) and look at them twice, because you have two pointers into it in the vector.
You can inject elements like this:
std::vector<int> matchVector;
int msmTemp[3];
msmTemp[0]={1};
msmTemp[1]={2};
msmTemp[2]={3};
matchVector.insert( matchVector.end(), std::begin(msmTemp), std::end(msmTemp) );
etc. This ends up with a vector containing 6 elements, not two arrays.
If you want arrays as values you need std::array:
std::vector< std::array<int,3> > matchVector;
std::array<int, 3> msmTemp;
and then your code works as written. std::array is a library type that acts sort of like a raw array, but it doesn't have the decay-to-pointer problems of a raw array.
Forget that int[3] names a type. C arrays don't behave like sensible values. Arrays are named std::array<type, count>.
#include <vector>
#include <array>
int main()
{
std::vector<std::array<int, 3>> matchVector;
std::array<int, 3> msmTemp;
msmTemp[0] = 1;
msmTemp[1] = 2;
msmTemp[2] = 3;
matchVector.push_back(msmTemp);
msmTemp[0] = 4;
msmTemp[1] = 7;
msmTemp[2] = 0;
matchVector.push_back(msmTemp);
for(auto & arr : matchVector)
{
for(auto i : arr)
{
std::cout << i <<", ";
}
std::cout<<"\n";
}
return 0;
}
The other answers already explain how to fix your code. I think it's also good to explain why your code behaves the way it does:
Here you tell your compiler to create an std::vector that holds pointers to int:
std::vector<int*> matchVector;
Here you tell your compiler to allocate some space on the stack that fits 3 ints:
int msmTemp[3];
Here you tell your compiler to write the values 1, 2 and 3 into the memory previously allocated:
msmTemp[0] = 1;
msmTemp[1] = 2;
msmTemp[2] = 3;
Here you tell your compiler to take the address of that allocated space, treat it as a pointer and pass it to push_back. This is called array decaying:
matchVector.push_back(msmTemp);
Your matchVector now contains 1 element, which is a pointer to the address of the memory on your stack that was allocated to hold 3 ints.
Here you tell your compiler to write the values 4, 7 and 0 in the memory previously allocated. Note that this is still the same memory block as before:
msmTemp[0] = 4;
msmTemp[1] = 7;
msmTemp[2] = 0;
Here you tell your compiler to again take the address of the allocated space, treat it as a pointer and pass it to push_back:
matchVector.push_back(msmTemp);
Thus matchVector now contains 2 identical values, each a pointer to the same memory location. Specifically the memory location that you last wrote 4, 7 and 0 into.
I would like to have an array variable in private address space and dynamically initialise it with values.
Lets say I have the following code:
__kernel
void sample(__constant float * A){
__private float temp[2]; // Lets assume I know the size upfront.
for(int i = 0; i < 2; i++){
temp[i] = A[i];
printf("%.2f",temp[i]); // This works
}
printf("%.2f",temp[0]); // This does not work
}
What do you think the last printf statement should print assuming vector A contains [1,2,3,4]. I guess it would be reasonable to expect to have [1] printed, but it always returns 0. In my opinion though, after a bit of experiment, I guess the code in some manner associate the actual var 'i' with the value A[i] in array rather than the value (i.e., index) of the var 'i' contains.
Any idea on what I might be missing ?
I have the following piece of code, which is only half on the entire code:
// Declare map elements using an enumeration
enum entity_labels {
EMPTY = 0,
WALL
};
typedef entity_labels ENTITY;
// Define an array of ASCII codes to use for visualising the map
const int TOKEN[2] = {
32, // EMPTY
178 // WALL
};
// create type aliases for console and map array buffers
using GUI_BUFFER = CHAR_INFO[MAP_HEIGHT][MAP_WIDTH];
using MAP_BUFFER = ENTITY[MAP_HEIGHT][MAP_WIDTH];
//Declare application subroutines
void InitConsole(unsigned int, unsigned int);
void ClearConsole(HANDLE hStdOut);
WORD GetKey();
void DrawMap(MAP_BUFFER & rMap);
/**************************************************************************
* Initialise the standard output console
*/
HANDLE hStdOut = GetStdHandle(STD_OUTPUT_HANDLE);
if (hStdOut != INVALID_HANDLE_VALUE)
{
ClearConsole(hStdOut);
// Set window title
SetConsoleTitle(TEXT("Tile Map Demo"));
// Set window size
SMALL_RECT srWindowRect;
srWindowRect.Left = 0;
srWindowRect.Top = 0;
srWindowRect.Bottom = srWindowRect.Top + MAP_HEIGHT;
srWindowRect.Right = srWindowRect.Left + MAP_WIDTH;
SetConsoleWindowInfo(hStdOut, true, &srWindowRect);
// Set screen buffer size
COORD cWindowSize = { MAP_WIDTH, MAP_HEIGHT };
SetConsoleScreenBufferSize(hStdOut, cWindowSize);
}
/*************************************************************************/
/*************************************************************************
* Initialise the tile map with appropriate ENTITY values
*/
MAP_BUFFER tileMap;
for (unsigned int row = 0; row < MAP_HEIGHT; row++)
{
for (unsigned int col = 0; col < MAP_WIDTH; col++)
{
tileMap [row][col] = WALL;
}
}
Essentially the entire code is used to create a tile map and output it to screen but I'm attempting to make tileMap a dynamic array in runtime.
I have tried creating one down where the tileMap is being created.
I've tried creating one just after "entity_lables" are given the typedef "ENTITY".
I've tried creating one after the "MAP_BUFFER" and "GUI_BUFFER" become aliases.
But still I'm at a loss, I have no idea on how to successfully implement a dynamic array to tileMap, and I certainly don't know the best spot to put it.
Any help would be greatly appreciated.
The syntax you are using for defining your array is for a constant sized C array. In general you should shy away from C arrays unless the size of the data is determined at compile time(and never needs to change) and the array never leaves the scope(because a C array does not retain information on its own size.)
In place of constant or dynamically sized C arrays I would suggest to use the Vector container. The Vector is a dynamically sized container that fills up from the back, the last element you have added to
std::vector<std::vector<ENTITY>>
To add the vector container to your project add the line
#include <vector>
To fill the container your loop could look like:
MAP_BUFFER tileMap;
for (unsigned int row = 0; row < MAP_HEIGHT; row++)
{
std::vector<ENTITY> column; // A column of the tile map
for (unsigned int col = 0; col < MAP_WIDTH; col++)
{
column.push_back(WALL); // Add one element to the column
}
tileMap.push_back(column); // Add the column to the tile map
}
or you could initialize the Vector to the size you want at the beginning and use your current loop to assign the tile values:
using TILE_MAP = vector<vector<ENTITY>>;
// MAP_WIDTH x MAP_HEIGHT multidimensional vector
TILE_MAP tileMap(MAP_WIDTH, vector<ENTITY>(MAP_HEIGHT));
for (unsigned int row = 0; row < MAP_HEIGHT; row++)
{
for (unsigned int col = 0; col < MAP_WIDTH; col++)
{
tileMap [row][col] = WALL;
}
}
Calling an element of a vector after it has been filled has the same syntax as an array.
tileMap[2][4]
You can also check the length of the vector:
int rows = tileMap.size();
if( rows > 0 )
int columnsInRow0 = tileMap[0].size()
While you are at it you should look into other containers like Maps and Sets since they make your life easier.
Edit:
Since you want to know how to make a dynamic array not using a vector I will give you an answer: std::vector is the C++ defined dynamically sized array. C arrays will not change size after they are defined, vector will.
However I think you are asking about the ability to define runtime constant sized arrays. So I will explain what they are and why you should not use them.
When you define the C array you are probably getting a warning saying that the expression needs to be constant.
A C array is a pointer to the stack. And the implementation of the compiletime C array is that it needs to be a constant size at compile time.
int compiletimeArray[] = { 1, 2, 3 };
// turns out c arrays are pointers
int* ptr = compiletimeArray;
// prints 2
std::cout << compiletimeArray[1];
// prints 2
std::cout << ptr[1];
// prints 2
std::cout << *(compiletimeArray + 1);
// also prints 2
std::cout << *(ptr + 1); //move pointer 1 element and de-reference
Pointers are like a whiteboard with a telephone number written on it. The same kind of issues occur as with telephone numbers; number on whiteboard has been erased, number on whiteboard has changed, recipient does not exist, recipient changed their number, service provider running out of available numbers to give new users... Keep that in mind.
To get create a runtime constant sized array you need to allocate the array on the heap and assign it to a pointer.
int size = 4;
int* runtimeArray = new int[size]; // this will work
delete[] runtimeArray; // de-allocate
size = 8; // change size
runtimeArray = new int[size]; // allocate a new array
The main difference between the stack and heap is that the stack will de-allocate the memory used by a variable when the program exits the scope the variable was declared in, on the other hand anything declared on the heap will still remain in memory and has to be explicitly de-allocated or you will get a memory leak.
// You must call this when you are never going to use the data at the memory address again
// release the memory from the heap
delete[] runtimeArray; // akin to releasing a phone number to be used by someone else
If you do not release memory from the heap eventually you will run out.
// Try running this
void crashingFunction() {
while(true)
{
// every time new[] is called ptr is assigned a new address, the memory at the old address is not freed
// 90001 ints worth of space(generally 32 or 64 bytes each int) is reserved on the heap
int* ptr = new int[90001]; // new[] eventually crashes because your system runs out of memory space to give
}
}
void okFunction() {
// Try running this
while(true)
{
// every time new[] is called ptr is assigned a new address, the old is not freed
// 90001 ints worth of space is reserved on the heap
int* ptr = new int[90001]; // never crashes
delete[] ptr; // reserved space above is de-allocated
}
}
Why use std::vector? Because std::vector internally manages the runtime array.
// allocates for you
vector(int size) {
// ...
runtimeArray = new runtimeArray[size];
}
// When the vector exits scope the deconstructor is called and it deletes allocated memory
// So you do not have to remember to do it yourself
~vector() {
// ...
delete[] runtimeArray;
}
So if you had the same scenario as last time
void vectorTestFunction() {
// Try running this
while(true)
{
std::vector<int> vec(9001); // internally allocates memory
} // <-- deallocates memory here because ~vector is called
}
If you want to use a runtime constant array I suggest the std:array container. It is like vector in that it manages its internal memory but is optimized for if you never need to add new elements. It is declared just like vector but does not contain resizing functions after its constructor.
so I'm having an issue passing an entire array of histograms into a function in C++
the arrays are declared like this
TH1F *h_Energy[2];
h_Energy[0] = new TH1F("h1", "h1", 100, 0, 100);
h_Energy[1] = new TH1F("h2", "h2", 100, 0, 100);
And here is what I'm trying to do in the function:
void overlayhists(TH1 *hists, int numhists) {
int ymax = 0;
for (int i=0; i<numhists; i++) {
if (hist[i].GetMaximum() > ymax) {
ymax = (hist[i].GetMaximum())*1.05;
}
}
}
And I'm passing the function an array like this
overlayhists(*h_Energy, 2);
Where h_Energy is an 1D array with 2 elements. The code will run through the first histogram in the loop but as soon as it starts the second loop and tries to access hist[i].GetMaximum() on the second try it segfaults.
What gives?
This creates an array of pointers to type TH1F
TH1F *h_Energy[2]; //edited after OP changed
If you want to use this, and subsequently pass it as an argument
You must first initialize it, and then create your function prototype to accommodate:
void overlayhists(TH1F **hists, int numhists);
^^
From what you have shown above, you would call it like this: (after your initializations)
h_Energy[0] = new TH1F("h1", "h1", 100, 0, 100);
h_Energy[1] = new TH1F("h2", "h2", 100, 0, 100);
overlayhists(h_Energy, 2);
1. Passing any array to function in c++ to change the content:
Refer to this code snippet:
//calling:
int nArr[5] = {1,2,3,4,5};
Mul(nArr, 5);
Whenever you pass an array to function you actually pass the pointer to first element of the array. This is implicit to C++ and C. If you pass normal value(non array) it will be considered as pass by value though.
// Function Mul() declaration and definition
void MUl(int* nArr, size_t nArrSize){
size_t itr = 0;
for(;itr<nArrSize; itr++)
nArr[i] = 5*nArr;// here we've coded to multiply each element with 5
}
2. Passing any Ptr to function in c++ to change what pointer is pointing to:
Now let us suppose we want to copy nArr (from above code snippet) to another array, say nArrB
The best way for a beginner would be to use reference to the pointer.
You can pass reference to the pointer to your function
//so we had
int nArr[5] = {1,2,3,4,5};
int *nArrB;
Here we don't know the gonnabe size of nArrB.
to copy nArr to nArrB we have to pass nArr, address of pointer to nArrB(or reference to pointer of nArrB or pointer to pointer of nArrB) and size of array.
Here is the implementation.
//Calling
CopyNArr(nArr, &nArrB, 5);
//Function implementation
void CopyNArr(int* nArr, int* & nArrB, size_t nArrSize) {
// dymanically allocating memory size for array. Assuming 4 byte int size
nArrB = new int[nArrSize*4];
size_t itr = 0;
//Copying values
for(;itr<nArrSize; itr++)
nArrB[i] = nArr[i];
}
//After copy nArrB is pointing to first element of 5 element array.
I hope it helped. Write for any further clarification.
You have an array of size 2, but you've created only one element. And that one with a wrong index. Array indexing starts with 0.
The elements should be at h_histogram[0] and h_histogram[1].
I am sorry if this answer is completely irrelevant but
I am tempted to post it. These is an experiment I have
done after seeing your question.
#include<iostream>
using namespace std;
main()
{
int e[2]={0,1};
int *p[2];
int i;
/*
Printing the array e content using one pointer
from an array of pointers. Here I am not using p[2]
at all.
*/
p[1]=e;
cout<<"Elements of e are : \n";
for(i=0;i<2;i++)
{
cout<<*(p[1]+i)<<endl;
/*
In the above line both *((*p)+i) and *(p+i)
won't serve the purpose of printing the array values.
*/
}
/*Printing the array e content using pointer to array*/
cout<<"Elements of e are : \n";
for(i=0;i<2;i++)
{
cout<<*(e+i)<<endl;
}
/*Note that pointer to array is legal but array TO pointer
(don't confuse with array OF pointers) is not.*/
}
Hope this will refresh your understanding.
I am trying to make an own simple string implementation in C++. My implementation is not \0 delimited, but uses the first element in my character array (the data structure I have chosen to implement the string) as the length of the string.
In essence, I have this as my data structure: typedef char * arrayString; and I have got the following as the implementation of some primal string manipulating routines:
#include "stdafx.h"
#include <iostream>
#include "new_string.h"
// Our string implementation will store the
// length of the string in the first byte of
// the string.
int getLength(const arrayString &s1) {
return s1[0] - '0';
}
void append_str(arrayString &s, char c) {
int length = getLength(s); // get the length of our current string
length++; // account for the new character
arrayString newString = new char[length]; // create a new heap allocated string
newString[0] = length;
// fill the string with the old contents
for (int counter = 1; counter < length; counter++) {
newString[counter] = s[counter];
}
// append the new character
newString[length - 1] = c;
delete[] s; // prevent a memory leak
s = newString;
}
void display(const arrayString &s1) {
int max = getLength(s1);
for (int counter = 1; counter <= max; counter++) {
std::cout << s1[counter];
}
}
void appendTest() {
arrayString a = new char[5];
a[0] = '5'; a[1] = 'f'; a[2] = 'o'; a[3] = 't'; a[4] = 'i';
append_str(a, 's');
display(a);
}
My issue is with the implementation of my function getLength(). I have tried to debug my program inside Visual Studio, and all seems nice and well in the beginning.
The first time getLength() is called, inside the append_str() function, it returns the correct value for the string length (5). When it get's called inside the display(), my own custom string displaying function (to prevent a bug with std::cout), it reads the value (6) correctly, but returns -42? What's going on?
NOTES
Ignore my comments in the code. It's purely educational and it's just me trying to see what level of commenting improves the code and what level reduces its quality.
In get_length(), I had to do first_element - '0' because otherwise, the function would return the ascii value of the arithmetic value inside. For instance, for decimal 6, it returned 54.
This is an educational endeavour, so if you see anything else worth commenting on, or fixing, by all means, let me know.
Since you are getting the length as return s1[0] - '0'; in getLength() you should set then length as newString[0] = length + '0'; instead of newString[0] = length;
As a side why are you storing the size of the string in the array? why not have some sort of integer member that you store the size in. A couple of bytes really isn't going to hurt and now you have a string that can be more than 256 characters long.
You are accessing your array out of bounds at couple of places.
In append_str
for (int counter = 1; counter < length; counter++) {
newString[counter] = s[counter];
}
In the example you presented, the starting string is "5foti" -- without the terminating null character. The maximum valid index is 4. In the above function, length has already been set to 6 and you are accessing s[5].
This can be fixed by changing the conditional in the for statement to counter < length-1;
And in display.
int max = getLength(s1);
for (int counter = 1; counter <= max; counter++) {
std::cout << s1[counter];
}
Here again, you are accessing the array out of bounds by using counter <= max in the loop.
This can be fixed by changing the conditional in the for statement to counter < max;
Here are some improvements, that should also cover your question:
Instead of a typedef, define a class for your string. The class should have an int for the length and a char* for the string data itself.
Use operator overloads in your class "string" so you can append them with + etc.
The - '0' gives me pain. You subtract the ASCII value of 42 from the length, but you do not add it as a character. Also, the length can be 127 at maximum, because char goes from -128 to +127. See point #1.
append_str changes the pointer of your object. That's very bad practice!
Ok, thank you everyone for helping me out.
The problem appeared to be inside the appendTest() function, where I was storing in the first element of the array the character code for the value I wanted to have as a size (i.e storing '5' instead of just 5). It seems that I didn't edit previous code that I had correctly, and that's what caused me the issues.
As an aside to what many of you are asking, why am I not using classes or better design, it's because I want to implement a basic string structure having many constraints, such as no classes, etc. I basically want to use only arrays, and the most I am affording myself is to make them dynamically allocated, i.e resizable.