How to find number of characters in dynamic array c++ - c++

Question

I am very much a beginner at C++, and I've been trying for the past 6 hours to try to figure out how to get the number of characters in a dynamic array.
This is the code I have:
using namespace std;
int array_size;
typedef char* charPtr;
charPtr a = new char[array_size];
char *p=a;
int len(char p[]) {
int count = 0;
while (*p != '\0'); {
count++;
p++;
return count;
}
}
int main() {
cout << "Type a phrase" << endl;
cin >> array_size;
cout << "Your phrase was " << len(p) << " characters long." << endl;
return 0;
}
Any help would be greatly appreciated!

Answer 1

You have to remove the semicolon (;) after the temination condition of your while loop. ; is the end of an statement, after that a new statment beginns. Apart from this your return statement would be inside the while loop. Remove it outside the loop. Adapt your code like this:
int len(char p[]) {
int count = 0;
while (*p != '\0')
{
count++;
p++;
}
return count;
}
Further you have to read a string from input into your dynamicly allocated array of char:
int main() {
int array_size = 100;
char *p=new char[array_size]; // allocate memory
cout << "Type a phrase" << endl;
cin >> p; // read string into allocated memory
cout << "Your phrase was " << len(p) << " characters long." << endl;
delete p; // free memory
return 0;
}
The function strlen gives you the length of a \0-terminated string.
#include <string.h>
int main() {
int array_size = 100;
char *p=new char[array_size]; // allocate memory
cout << "Type a phrase" << endl;
cin >> p; // read string into allocated memory
cout << "Your phrase was " << strlen(p) << " characters long." << endl;
delete p; // free memory
return 0;
}
But I recommend to use std::string:
#include <string>
int main() {
std::string str;
cout << "Type a phrase" << endl;
cin >> str;
cout << "Your phrase was " << str.size() << " characters long." << endl;
return 0;
}
std::string represents a sequences of characters with dynamic length.

Answer 2

Getting the size of dynamically allocated array
A dynamically allocated array does not have any information about its size that is available in a standards compliant way.
We are able to compute the length of C style strings since there is sentinel element, '\0', to mark the end of the string. There are no such elements for other types.
Even then, you cannot compute the size of an array of characters allocated using heap memory like you have.
Problems with posted code
You have the following lines outside all functions.
int array_size;
typedef char* charPtr;
charPtr a = new char[array_size];
char *p=a;
They are executed before anything in main gets executed. When these lines are executed, array_size gets initialized to 0. Then you allocate memory for charPtr using 0 as the value of array_size.
Function len has an error due to a typo, which could lead to either (a) a hanging program or an incorrect return value.
int len(char p[])
{
int count = 0;
while (*p != '\0');
// ^^^ The semicolon is a problem
{
count++;
p++;
return count;
}
}
If *p is not equal to '\0', the program will never get out of the while statement. It will hange.
If *p is equal to '\0', the program will get out of the while statement but it will still execute the the lines after that. As a consequence, you will end up returning 1 as the length where 0 is the right answer.
In main, you have:
cout << "Type a phrase" << endl;
cin >> array_size;
When the user sees the output, they will try to enter a phrase. However, array_size is an int. There is a mismatch between the prompt to the user and the line to read the data.
You could change it them to:
cout << "Type array size" << endl;
cin >> array_size;
cout << "Type a phrase" << endl;
That is one step better but that still does not change the fact that memory for a was allocated using a size of 0.
Using uninitialized memory
You are calling len(p) in the cout line but the elements of p have not been initialized.
It's not clear from your post what the program is supposed to do. I am guessing that you want to read a phrase from stdin and write it out to stdout. You can use the following simplified version for that.
int main()
{
std::string phrase;
cout << "Type a phrase" << endl;
// Get the entire line as a phrase.
std::getline(std::cin, phrase);
cout << "Your phrase is " << phrase << endl;
cout << "It is " << phrase.size() << " characters long." << endl;
return 0;
}

Answer 3

First problem: you cannot use array_size till it has been initialized.
char* a = new char [array_size]
since array_size has not been initialized and has garbage data in it, you have no idea how big that array is going to be.
that's why first you have to initialize array_size, and only after allocate your char array.
like
int array_size = 0 //no reason to have this variable as global, but that's up to you
char *a = nullptr; //same
int main()
{
cin >> array_size;
if (array_size <= 0) //make sure the input is valid and not negative
return 0;
a = new char[array_size];
//now you can work on your newly allocated array of characters
//the number of characters the array has equals to the array_size variable.
delete[] a;
return 0;
}
your len function checks for a '\0' character, but again, you have not initialized that char array either, so it contains garbage data. therefore sometimes you may get a result of len 5, 10, 502043, anything can happen really (undefined behavior).