How can I make an array of n strings using char**?
char** lit;
*lit = (char*)calloc(this->nr_param, sizeof(char*));
for(int i = 0; i < this->nr_param; i++)
lit[i] = (char*) calloc(this->nr_param, sizeof(char));
Is this the way?
If so, how can i access elements? Lets say my array will contain the following elements:
aaab, abba, baab;
I want this structure:
lit[0] = "aaab";
lit[1] = "abba";
lit[2] = "baab";
It's ok how I declared them?
Like this:
// allocate memory for n char pointers dynamically.
char ** lit = static_cast<char**>(::operator new(n * sizeof(char*)));
for (unsigned int i = 0; i != n; ++i)
{
lit[i] = static_cast<char*>(::operator new(length_of_string_i)); // #1
// populate lit[i] with data
}
You need some method of determining the length of the ith string, which you need to paste appropriately in the line marked #1. Note that sizeof(char) == 1, so you don't need to multiply anything in the inner allocation.
(You can use std::malloc instead of ::operator new if you prefer, but then you have to #include <cstdlib>.) Don't forget to clean up when you're done!
This is of course only the literal translation of what you asked for. In C++, you would usually prefer object creation over raw memory allocation, which looks like this:
// construct n char pointers dynamically
char ** lit = new char*[n];
for (unsigned int i = 0; i != n; ++i)
{
lit[i] = new char[length_of_string_i];
// populate lit[i] with data
}
But you should seriously never use array-new. It's not a good concept, and rarely good C++.
So, you shouldn't be doing this at all, and instead you should use:
std::vector<std::string> lit(n);
You can use c++ vector and strings in a similar way:
#include <vector>
#include <string>
#include <iostream>
int main() {
std::vector<std::string> lit;
lit.push_back("aaab");
lit.push_back("aab");
lit.push_back("aabb");
lit[0][0] = 'z';
std::cout << lit[0] << std::endl;
}
Related
I'm trying to implement a void function that takes a c string as its only parameter and reverses it and prints it. Below is my attempt at a solution however I'm not sure how to go about this problem.
void printBackwards(char forward[]) {
int i = 0;
char backwards[];
while (forward[i] != '\0') {
backwards[i] = forward[-i - 1];
i++;
}
cout << backwards;
}
Under such a condition, I guess you are expected to use recursion.
void printBackwards(char forward[]) {
if (!forward[0])
return;
printBackwards(forward + 1);
cout << forward[0];
}
Not being able to use strlen, we'll calculate it ourselves using a simple for loop. Then dynamically allocate a suitable buffer (add one character for the null terminating char, and I "cheated" by using calloc to zero the memory so I don't have to remember to set the null terminator. Then anoher simple loop to copy the original into the result in reverse.
#include <stdlib.h>
#include <stdio.h>
char *rev(char *s) {
size_t i;
char *s2 = s; // A pointer to the beginning as our first loop modifies s
for (i = 0; *s; s++, i++);
char *result = calloc(0, i + 1);
if (!result) return NULL; // In case calloc didn't allocate the requested memory.
for (size_t j = 0; j < i; j++)
result[j] = s2[i - j - 1];
return result;
}
Assuming you want to reverse the string rather than just printing it in reverse order, you first need to find the last character location (actually the position of the null terminator). Pseudo-code below (since this is an educational assignment):
define null_addr(pointer):
while character at pointer is not null terminator:
increment pointer
return pointer
Then you can use that inside a loop where you swap the two characters and move the pointers toward the center of the string. As soon as the pointers become equal or pass each other the string is reversed:
define reverse(left_pointer):
set right_pointer to null_addr(left_pointer)
while right_pointer > left_pointer plus one:
decrement right_pointer
swap character at left_pointer with character at right_pointer
increment left_pointer
Alternatively (and this appears to be the case since your attempt doesn't actually reverse the original string), if you need to print the string in reverse order without modifying it, you still find the last character. Then you run backwards through the string printing each character until you reach the first. That can be done with something like:
define print_reverse(pointer):
set right_pointer to null_addr(pointer)
while right_pointer > pointer:
decrement right_pointer
print character at right_pointer
That's probably better than creating a new string to hold the reverse of the original, and then printing that reverse.
One thing you should keep in mind. This very much appears to be a C-centric question, not a C++ one (it's using C strings rather than C++ strings, and uses C header files). If that's the case, you should probably avoid things like cout.
By using abstractions, like , your code will be much better at communication WHAT it is doing instead of HOW it is doing it.
#include <iostream>
#include <string>
#include <ranges>
int main()
{
std::string hello{ "!dlrow olleH" };
for (const char c : hello | std::views::reverse)
{
std::cout << c;
}
return 0;
}
Use a template
#include <iostream>
template<int N, int I=2>
void printBackwards(char (&forward)[N]) {
std::cout << forward[N-I];
if constexpr (I<N) printBackwards<N, I+1>(forward);
}
int main() {
char test[] = "elephant";
printBackwards(test);
}
While there seems to be several working answers, I thought I'd throw my hat in the stack (pun intended) since none of them take advantage of a FILO data structure (except #273K's answer, which uses a stack implicitly instead of explicitly).
What I would do is simply push everything onto a stack and then print the stack:
#include <stack>
#include <iostream>
void printBackwards(char forward[]) {
// Create a stack to hold our reversed string
std::stack<char> stk;
// Iterate through the string until we hit the null terminator
int i = 0;
while (forward[i] != '\0'){
stk.push(forward[i]);
++i;
}
// Iterate through the stack and print each character as we pop() it
while (stk.size() > 0){
std::cout << stk.top();
stk.pop();
}
// Don't forget the newline (assuming output lines should be separated)
std::cout << '\n';
}
int main(int argc, char* argv[]){
char s[] = "This is a string";
printBackwards(s);
return 0;
}
Hi guys as promised I have come back to add my own answer. This is my own way using array subscripts and using what I currently know.
#include <iostream>
using namespace std;
void printBackwards(char[]);
int main()
{
char word[] = "apples";
printBackwards(word);
return 0;
}
void printBackwards(char word[]) {
char* temp = word;
int count = 0;
while (*temp++ != '\0') {
count++;
}
for (int i = count - 1; i >= 0; i--) {
cout << word[i];
}
}
You can make a fixed-size buffer and create new ones if needed. Fill it reverse by moving the string offset back with every inserted character. Chars exceeding the buffer are returned to be processed later, so you can make a list of such buffers:
template<int SIZE>
struct ReversedCStr
{
static_assert(SIZE > 10); // just some minimal size treshold
// constexpr
ReversedCStr(char const* c_str, char const** tail = nullptr) noexcept
{
for(buffer[offset] = '\0'; *c_str != '\0';)
{
buffer[--offset] = *c_str++;
if(offset == 0) break;
}
if(tail) *tail = c_str;
}
//constexpr
char const* c_str() const noexcept { return buffer.data()+offset;};
private:
size_t offset = SIZE -1;
std::array<char,SIZE> buffer;
};
The tag is 'C++' so I assume you use C++ not C. The following code is C++11 so it should fit in every modern project. I posted the working example on godbolt.org.
It doesn't allocate memory, and is completely exception-free. The maximum memory wasted is {buffer_size + sizeof(char*)*number_of_chunks}, and can be easily turned into a list of reversed chunks like this:
char const* tail;
std::vector<ReversedCStr<11>> vec;
for(vec.emplace_back(str,&tail); *tail != '\0';)
vec.emplace_back(tail,&tail);
I was at a job interview the other day and I had the following function to implement:
char* Compress (char * text);
Rules were also that you are not allowed to use standard libary functions like strlen, strcpy, string etc... So the function has to compress a given character sequence.
For example if the input text is"11112222333344411" after passing it to the Compress function the returned value is: "12341", or if the text input is:"aaAbbBBcCCa" ---> return: aAbBcCa
I am not sure I did everything properly (with memory handling) here so any suggestions would be great. Am I doing it right that I delete the value of temp every time? Also if there is a simpler way to implement this function (without using the standard library functions of course) I would be really pleased to see it.
#include <iostream>
char* Compress(char* text) {
char* temp;
char* _compText;
int size = 1;
_compText = nullptr;
for (size_t i = 0; text[i] != '\0'; ++i)
{
if (text[i] != text[i + 1]) {
++size;
temp = _compText;
_compText = new char[size];
for (size_t j = 0; j < size-2; ++j)
{
_compText[j] = temp[j];
}
_compText[size-2] = text[i];
_compText[size-1] = '\0';
delete[] temp;
}
}
return _compText;
}
int main()
{
char t[] = "111122222233333444444555555111";
char* compedT;
std::cout << "Before:\n";
std::cout << t;
compedT = Compress(t);
std::cout << "\nAfter: \n";
std::cout << compedT;
delete[] compedT;
return 0;
}
The function initially is implemented incorrectly.
The type of the function is
char* Compress (char * text);
^^^^^^^
that is its parameter is not const char *, This means that the function should update the source string in place and return pointer to its first character. There is no need to allocate dynamically memory to perform the task.
The function can be defined as it is shown in the demonstrative program.
#include <iostream>
char * Compress( char *s )
{
for ( char *p = s, *q = s; *q; )
{
if ( *++q != *p ) *++p = *q;
}
return s;
}
int main()
{
char s[] = "11112222333344411";
std::cout << Compress( s ) << '\n';
}
Its output is
12341
Or the function can look also the following way
char * Compress( char *s )
{
for ( char *p = s, *q = s; *q; )
{
if ( ( *++q != *p ) and ( ++p != q ) ) *p = *q;
}
return s;
}
As for your function implementation then you should read warnings as for example
warning: comparison of integer expressions of different signedness: 'size_t' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
34 | for (size_t j = 0; j < size-2; ++j)
| ~~^~~~~~~~
And your function returns nullptr for an empty string. This looks logically inconsistent. And the function is very inefficient.:)
And do not use names that start with underscore.
Does my code have memory leak?
As far as I can see, no; there is no memory leak.
That said, the use of bare owning pointers makes it difficult to spot memory leaks. They are a bad design choice, especially when transferring ownership to outside of the function. At the very least, there should be a comment near the function declaration that should document how the caller must clean up the allocation. If you need a dynamic array, a better solution is to use a container.
Am I doing it right that I delete the value of temp everytime?
As far memory leaks are concerned yes, you do need to delete every allocation. But reallocating memory on every iteration is unnecessary, and quite slow. In fact, there doesn't appear to be need for any dynamic allocation (See Vlad's answer).
I need to pass a char pointer to function, then change the value that it points to inside the function and print values outside the function.
The problem I have is that I'm losing it when I leave function and try to print it outside. What can I do to avoid this?
This is an code example:
char array[] = "Bada boom";
char *pText = array;
reverseText(pText);
cout << (pText);
cout should print
moob adaB
When I print inside the function, everything is fine(it prints reversed).
My task is to print It out outside the function (as you can see in a 4th line of code)
This is the full of code which have the bug (printing inside func works, outside didn't work)
#include <iostream>
#include <string>
#include <string.h>
using namespace std;
char reverseText(char *text);
int main(){
char array[] = "Bada boom";
char *pTekst = array;
reverseText(pTekst);
cout << (pTekst); //in here it doesn't work
}
char reverseText(char *text){
char befRev[100]; int lenght=-1;
/*until *text doesn't meet '\0' */
for(int i=0;*text!='\0';i++){
befRev[i]=(*text);
text++;
lenght++;
}
/*reversing*/
int j=0;
for(int i=lenght;i>=0;i--){
*(text+j)=befRev[i];
j++;
}
for(int i=0;i<=lenght;i++) //in here it does print the right value
cout << text[i];
};
Just re-arrange the array in-place. The pointer itself doesn't need to change:
#include <cstring>
#include <algorithm>
void reverseText(char* array)
{
auto len = std::strlen(array);
std::reverse(array, array+len);
}
int main()
{
char array[] = "Bada boom";
char *pText = array;
reverseText(pText);
std::cout << pText << std::endl;
}
Output:
moob adaB
If you really wanted to provide a pointer that points to a different address to the caller, you could simply return it:
char* foo(char* stuff)
{
char* tmp = ....;
...
// do some stuff
...
return tmp;
}
Alternatively, you could pass the pointer by reference, but the intent is less clear than in the previous version:
void foo(char*& stuff)
{
stuff = something_else;
}
But in both cases, you must make absolutely sure the thing the new pointer points to is valid outside of the function. This might require some dynamic memory allocation. For your case, it seems the best and simplest option is to re-arrange the array in place.
To answer your question, you have an error in logic. Notice that in your first loop in reverseText you increment the local pointer text. In your second loop you did not reset text to it's original value so beforeRev is being copied over starting at location text+offset.
If you were to look at pText on return from call to reverseText you would find it contains:
"Bada boom\0moob adaB"
Your reverseText should be renamed palindrome :)
This is pretty straightforward. Some points to note:
An array decays to a pointer when you pass it to a function.
You are passing in a null terminated string. So the length of the char array you are passing in is the length of the string (including white space) +1.
Because you are using a pointer there is no need to assign a temp variable to hold everything.
Here is some code in C that is easy to translate to C++. Working out the actual reverse algorithm is left for you as an exercise.
#include<stdio.h>
void reverseText(char* text)
{
// Hint: It can be done in one loop!
int i;
for(i = 0; i < 9; i++)
{
// Your algorithm to reverse the text. I'm not doing it for you! ;)
*(text + i) = 'r';
}
}
int main()
{
char array[] = "Bada boom";
reverseText(array);
printf("The text reversed: %s\n", array);
return 0;
}
My final code:
#include <iostream>
void reverseText(char* text){
int length=-1; char tmp;
/*Length = sign from 0 to 8 without counting explicit NUL terminator*/
for(int i=0;*(text+i)!='\0';i++){
length++;
}
int j=0; int i=length;
while(j<i){
tmp=*(text+j); //tmp=first
*(text+j)=*(text+i); //first=last
*(text+i)=tmp; //last=tmp
j++;
i--;
}
}
int main(){
char array[] = "Bada boom";
char *pText = array;
reverseText(pText);
std::cout << pText;
}
I should have read more about pointers before I started this exercise.
You can either return a pointer or pass a pointer to pointer as a function argument.
//pointer to pointer
void reverseText(char** textPtr) {
char* newText = ...; //initialize;
...
*textPtr = newText; //assign newText
}
//return pointer
char* reverseText(char* text) {
char* newText = ...; //initialize
return newText;
}
Remember that if you allocate memory in this function you must do it dynamically (with new or malloc) and you have to free it afterwards (with delete or free respectively). Memory allocation in a function like this is probably a bad practice and should be avoided.
So I'm making a function that is similar to SubStr. This is an assignment so I cannot use the actual function to do this. So far I have created a function to take a string and then get the desired substring. My problem is returning the substring. In the function when I do Substring[b] = AString[b]; the substring is empty, but if I cout from inside the function I get the desired substring. So what is wrong with my code?
Here is a working demo: http://ideone.com/4f5IpA
#include <iostream>
using namespace std;
void subsec(char AString[], char Substring[], int start, int length);
int main() {
char someString[] = "abcdefg";
char someSubString[] = "";
subsec(someString, someSubString, 1, 3);
cout << someSubString << endl;
return 0;
}
void subsec(char AString[], char Substring[], int start, int length) {
for (int b = start; b <= length; b++) {
Substring[b] = AString[b];
}
}
Maybe this does what you're looking for? It's hard to say as your initial implementation used the length parameter as more of an end position.
#include <iostream>
using namespace std;
void subsec(char AString[], char Substring[], int start, int length)
{
const int end = start + length;
int pos = 0;
for(int b = start; b < end; ++b)
{
Substring[pos++] = AString[b];
}
Substring[pos] = 0;
}
int main()
{
char someString[50] = "abcdefghijklmnopqrstuvwxyz";
char someSubString[50];
subsec(someString, someSubString, 13, 10);
cout << someSubString << endl;
return 0;
}
There are several problems with the code:
1) The char arraysomeSubString has size 1 which cannot hold the substring.
2) The subsec is not correctly implemented, you should copy to the Substring from index 0.
Also remember to add \0 at the end of the substring.
void subsec(char AString[], char *Substring, int start, int length) {
int ii = 0;
for (int jj = start; jj <= length; jj++, ii++) {
Substring[ii] = AString[jj];
}
Substring[ii] = '\0';
}
You need to allocate more than 1 byte for someSubString i.e.
char someSubString[] = "xxxxxxxxxxxxxxxxxx";
or just
char someSubString[100];
if you know the max size you'll ever need.
Either would allocate enough space for the string you're copying to it. Then, you're not doing anything about the terminating 0 either. At the end of a C-style string there needs to be a terminating null to signify end of string. Otherwise cout will print something like;
abcdefgxxxxxxx
if you initialized with x's as I indicated.
There are a few problems with your code as it stands. Firstly, as your compiler is no doubt warning you, in C++ a string literal has type const char[], not just char[].
Secondly, you need to have enough space to store your substring. A good way to do this is for your function to allocate the space it needs, and then pass back a pointer to this memory. This is the way things are typically done in C code. The only thing is that you have to remember to delete the allocated array when you're done with it. (There are other, better ways to do this in C++, with things like smart pointers and wrapper objects, but those come later :-) ).
Thirdly, you'll have a problem if you request a length which is actually longer than the passed-in string -- you'll run off the end and start copying random memory (or just crash), which is definitely not what you want. C strings are terminated with a "nul byte" -- so you need to check whether you've come across this.
Speaking of the nul, you need to make sure that your substring ends with one.
Lastly, it's not really a problem but there's no need for the start parameter, you can just pass a pointer to the middle of the array if you want to.
char* substring(const char* str, int length)
{
// Allocate memory for substring;
char* subs = new char[length+1];
// Copy characters from given string
int i = 0;
while (i < length && str[i] != '\0') {
subs[i] = str[i];
i++;
}
// Append the nul byte
subs[i] = '\0';
return subs;
}
int main()
{
const char someString[] = "foobarbaz"; // Note -- must be const in C++
char* subs = substring(someString + 3, 3);
assert(strcmp(subs, "bar") == 0);
delete subs;
}
I have been pulling my hair out on this particular issue and would like some advice. I have the following struct:
struct MqlStr // MQL String Array
{
int len;
char *string;
};
this is being passed to a function as a pointer from an external application as such:
MT4_EXPFUNC double __stdcall CheckExecutionRequests(MqlStr* RequestInfo)
within the function i am generating a number of string values that i need to assign to varies elements of the MqlStr array. the following works fine:
RequestInfo[1].string = "1";
RequestInfo[2].string = "2";
but when i use strcpy to get my generated string value into the array, it overwrites the entire array with the value i copied. for example:
string field1 = value.substr(Demark + 1, Demark2 - Demark - 1);
strncpy(RequestInfo[1].string, field1.c_str(), field1.size());
string field2 = value.substr(Demark + 1, Demark2 - Demark - 1);
strncpy(RequestInfo[2].string, field2.c_str(), field2.size());
if field1 = 1 and field2 = 2 then the entire RequestInfo[] array would be equal to 2 (the last value copied)
can someone point me in the right direction?
RequestInfo[1] = "1";
is not doing what you think. It's either
RequestInfo[1].string = "1";
if RequestInfo is a vector of MqlStr objects containing at least 2 elements, or
RequestInfo->string = "1";
if RequestInfo is a pointer to a single MqlStr object.
Have you allocated enough space for the .string pointers in your RequestInfo elements? strncpy is not allocating the space for you, use strdup for that.
You need to manage MqlStr memory in a safe manner, this can happen by using a standard container like std::string or by writing methods to allocate and deallocate the internal memory.
Here is an example of a simple class that manages its internal memory:
#include <cstdlib>
#include <iostream>
#include <string.h>
#include <sstream>
struct MqlStr
{
public:
int len;
char *string;
MqlStr() { init (); }
~MqlStr() { dealloc(); }
void assign(std::string& str) {
dealloc();
string = new char[str.length() + 1];
strncpy(string, str.c_str(), str.length());
string[str.length()] = 0;
len = str.length();
}
void dealloc() {
if(string != 0) delete [] string; init();
}
private:
void init() { string = 0; len = 0; }
MqlStr(const MqlStr &);
void operator= (const MqlStr &);
};
double CheckExecutionRequests(MqlStr* RequestInfo)
{
static int callCount = 0;
std::ostringstream stringstream; stringstream<<"callCount: "<<callCount++;
std::string field1 = stringstream.str();
RequestInfo->assign(field1);
return 1.0;
}
int main(int argc, char** argv)
{
MqlStr s[5];
std::cout<<"First call"<<std::endl;
for(unsigned i = 0; i < sizeof(s)/sizeof(s[0]); ++i)
CheckExecutionRequests(s + i);
for(unsigned i = 0; i < sizeof(s)/sizeof(s[0]); ++i)
std::cout<<"MqlStr["<<i<<"].string = "<<s[i].string<<std::endl;
std::cout<<"Second call"<<std::endl;
for(unsigned i = 0; i < sizeof(s)/sizeof(s[0]); ++i)
CheckExecutionRequests(s + i);
for(unsigned i = 0; i < sizeof(s)/sizeof(s[0]); ++i)
std::cout<<"MqlStr["<<i<<"].string = "<<s[i].string<<std::endl;
return EXIT_SUCCESS;
}
The second execution of CheckExecutionRequests with the same MqlStr instances will not corrupt the memory.
An extension to the code can be preallocation of the string size, and only reallocating the memory in the assign method if the new str.length > this.maxLength (preallocated length different from the string size).
The copy constructor and assignment operator are currently disabled, because they can cause problems if not implemented properly while managing internal memory on the heap.
A simpler solution would be to write your struct using a standard container as follows:
struct MqlStr
{
public:
std::string string;
}
And then just assign the string you get for the fields to MqlStr string.