Char pointer objects and respective char array element comparisons - c++

Question

I am programming my custom string class with multiple methods. The issue is that the comparison method does not work as I intend. Instead of doing nothing when the two char arrays differ, an if conditional still proceeds in my main function.
There are no errors given when I compile with g++. The code is syntactically correct, however logically faulty. I know this because I can give the compare method two char arrays which differ in content, and it will not matter whether they differ this way, as the main function will run the if conditional for "s8.compare(s7) == 1" regardless if the result in the compare method is not true.
I will post the entire code below. Any help is greatly appreciated.
string.h
class Str {
private:
char *value;
int length;
int capacity;
//Doubles the size of the string when called.
void growArray();
//If the two strings are uneven, get absolute value of difference in length.
int difference(int a, int b);
//Calculates the size of a character array, passed in as an argument
int getCharArrSize(const char *v);
public:
Str();
explicit Str(const char *STR);
void copy(Str s);
void concatenate(Str s);
bool compare(Str s);
void print();
};
//Str constructor
Str::Str() {
//Assign value, capacity, and length to any new Str object
value = new char[100];
capacity = 100;
length = 0;
}
//Pass STR object as a pointer to string object constructor
Str::Str(const char *STR) {
length = getCharArrSize(STR);
capacity = 100;
value = new char[capacity];
//Copy contents from STR to string object
for (int i = 0; i < length; i++)
value[i] = STR[i];
}
//Doubles the size of the string when called.
void Str::growArray() {
const char *tmp = value;
capacity *= 2;
value = new char[capacity];
for (int i = 0; i < length; i++)
value[i] = tmp[i];
}
//If the two strings are uneven, get absolute value of difference in length.
int Str::difference(int a, int b) {
int d = 0;
if (a > b) d = a - b;
else if (b > a) d = b - a;
return d;
}
//Calculates the size of a character array, passed in as an argument
int Str::getCharArrSize(const char *v) {
int c = 0;
while (v[c] != '\0') {
c++;
}
return c;
}
//Overwrites the data of the string array with the data contained in s
void Str::copy(Str s) {
//Check ability for empty string object to hold Str s contents
if (capacity > s.length) {
//Copy over each element until s length is reached
for (int i = 0; i < s.length ; i++)
value[i] = s.value[i];
//Set string object length to copy's size
length = getCharArrSize(value);
} else { growArray(); }
}
//Concatenate Str s onto string object
void Str::concatenate(Str s) {
//Check ability for string object to hold itself and concatenated chars
if (capacity > length + s.length) {
//Fill string object with s object until end of combined lengths if necessary
for (int i = 0; i < length + s.length; i++)
value[length + i] = s.value[i];
//Set length based on chars in concatenated string object
length = getCharArrSize(value);
} else { growArray(); }
}
//Compare each element in Str s against string for similarities
bool Str::compare(Str s) {
if (length == s.length) {
if (*value == *s.value) {
while ((*value != value[length]) && (*s.value != s.value[s.length])) {
value++;
s.value++;
}
return true;
} else return false;
} else {
difference(length, s.length);
}
}
//Print function
void Str::print() {
std::cout << value << std::endl;
}
main.cpp
#include"string.h"
int main() {
Str s1("Hello ");
Str s2("World");
Str s3(", my ");
Str s4("Name ");
Str s5("is ");
Str s6("Chad!");
Str s7;
s7.copy(s1);
s7.concatenate(s2);
s7.concatenate(s3);
s7.concatenate(s4);
s7.concatenate(s5);
s7.concatenate(s6);
s7.print();
std::cout << "\n\n";
Str s8("Hello World, My Name is Chad!");
if (s8.compare(s7) == 1) {
std::cout << "They Match!" << std::endl;
}
Str s9("I dont match....");
if (s9.compare(s8) == 0) {
std::cout << "I differ by " << s8.compare(s6) << " characters" << std::endl;
}
}
The above code returns a result that appears correct, however changing (s8.compare(s7) == 1) to something like (s8.compare(s5) == 1) returns 'They match!' when I am trying to check each individual element in the char arrays against one another, and only return true if they are the same length and each character matches in the arrays.

Answer 1

Your program has undefined behavior since Str::compare does not have a return statement in one of the branches.
bool Str::compare(Str s) {
if (length == s.length) {
...
} else {
// Missing return statement.
difference(length, s.length);
}
}
Perhaps you want to change that line to:
return (difference(length, s.length) == 0);

Answer 2

Your loop is running without a comparison. You compare the initial values in the char array and then loop through the rest without comparison. So you will return true every time the initial values are equal.
Below the loop runs after the same length is determined then every char is compared. If they are not equal then the function will return false. Otherwise the function will return true.
bool Str::compare(Str s) {
if (length == s.length) {
while ((*value != value[length]) && (*s.value != s.value[s.length])) {
if (*value == *s.value) {
value++;
s.value++;
} else {
return false;//will return false as soon as a comparison is false
}
}
return true;
} else {
difference(length, s.length);
}
}
You also need to return a boolean from the difference function. If you want to return ints from that function switch to a int return on the compare function and use 0 and 1s as their boolean counterparts.