Well, as part of learning C++, my project has a restriction on it. I'm not allowed to use any libraries except the basic ones such as <cstring> and a few other necessities.
The project should take in input from a file that is an "n" number of columns of strings and be able to sort the output according to lexicographical ordering of any selected column. So for example, given the input
Cartwright Wendy 93
Williamson Mark 81
Thompson Mark 100
Anderson John 76
Turner Dennis 56
It should sort them by column. And my search around StackOverflow returned a result from someone else who had to do the exact same project a few years ago too Hahaha Qsort based on a column in a c-string?
But in my case I just use a global variable for the column and get on with life. My problem came in when I am implementing the compare function for qsort
In my main method I call
qsort (data, i, sizeof(char*), compare);
where data is a char * data[] and i is the number of lines to compare. (5 in this case)
Below is my code for the compare method
int compare (const void * a, const void * b){
char* line1 = new char[1000]; char* line2 = new char[1000];
strcpy(line1, *((const char**) a));
strcpy(line2, *((const char**) b));
char* left = &(strtok(line1, " \t"))[column-1];
char* right = &(strtok(line2, " \t"))[column-1];
return strcmp(left, right);
}
the 1000s are because I just generalized (and did bad coding on purpose) to overgeneralize that no lines will be longer than 1000 characters.
What confuses me is that when I use the debugger in Eclipse, I can see that it it compares it successfully the first time, then on the second round, it has a segmentation fault when it tries to compare them.
I also tried to change the code for assigning left and right to what is below but that didn't help either
char* left = new char[100];
strcpy(left, &(strtok(line1, " \t"))[column-1]);
char* right = new char[100];
strcpy(right, &(strtok(line2, " \t"))[column-1]);
Please help me understand what is causing this segmentation fault. The first time it compares the two, left = "Williamson" and right = "Thompson". The second time it compares (and crashes trying) left = "Cartwright" and right = "Thompson"
char* line1 = new char[1000]; char* line2 = new char[1000];
This is not good at all. You're never freeing this, so you leak 2000 bytes every time your comparison function is called. Eventually this will lead to low-memory conditions and new will throw. (Or on Linux your process might get killed by the OOM-killer). It's also not very efficient when you could just have said char line1[1000], which is super-quick because it simply subtracts from the stack pointer rather than potentially traversing a free list or asking the kernel for more memory.
But really you could be doing the compare without modifying or copying the strings. For example:
static int
is_end_of_token(char ch)
{
// If the string has the terminating NUL character we consider it the end.
// If it has the ' ' or '\t' character we also consider it the end. This
// accomplishes the same thing as your strtok call, but WITHOUT modifying
// the source buffer.
return (!ch || ch == ' ' || ch == '\t');
}
int
compare(const void *a, const void *b)
{
const char *strA = *(const char**)a;
const char *strB = *(const char**)b;
// Loop while there is data left to compare...
while (!is_end_of_token(*strA) && !is_end_of_token(*strB))
{
if (*strA < *strB)
return -1; // String on left is smaller
else if (*strA > *strB)
return 1; // String on right is smaller
++strA;
++strB;
}
if (is_end_of_token(*strA) && is_end_of_token(*strB))
return 0; // both strings are finished, so they are equal.
else if (is_end_of_token(*strA))
return -1; // left string has ended, but right string still has chars
else
return 1; // right string has ended, but left string still has chars
}
But lastly... You're using std::string you say? Well, if that's the case, then assuming the memory passed to qsort is compatible with "const char **" is a little weird, and I would expect it to crash. In that sense maybe what you should do something like:
int compare(const void *a, const void *b)
{
const char *strA = ((const std::string*)a)->c_str();
const char *strB = ((const std::string*)b)->c_str();
// ...
}
But really, if you are using C++ and not C, you should use std::sort.
Related
I'm currently doing a programming exercise from a C++ book for beginners. The task reads as follows: "Write a function that reverses the characters in a text string by using two pointers. The only function parameter shall be a pointer to the string."
My issue is that I haven't been able to make the characters swap properly, see the output below. (And I also made the assumption that the function parameter doesn't count, hence why I'm technically using three pointers).
I am almost certain that the problem has to do with the for loop. I wrote this pseudocode:
Assign value of element number i in at_front to the 1st element in transfer_back.
Assign value of element number elem in at_back to element number i in at_front.
Assign value of the 1st element in transfer_back to element number elem in at_back.
Increment i, decrement elem. Repeat loop until !(i < elem)
I wasn't sure whether of not I was supposed to take the null terminator into account. I tried writing (elem - 1) but that messed up with the characters even more so I've currently left it as it is.
#include <iostream>
#include <string>
using namespace std;
void strrev(string *at_front) {
string *transfer_back = at_front, *at_back = transfer_back;
int elem = 0;
while(at_back->operator[](elem) != '\0') {
elem++;
}
for(int i = 0; i < elem; i++) {
transfer_back->operator[](0) = at_front->operator[](i);
at_front->operator[](i) = at_back->operator[](elem);
at_back->operator[](elem) = transfer_back->operator[](0);
elem--;
}
}
int main() {
string str = "ereh txet yna";
string *point_str = &str;
strrev(point_str);
cout << *point_str << endl;
return 0;
}
Expected output: "any text here"
Terminal window: "xany text her"
The fact that the 'x' has been assigned to the first element is something I haven't been able to grasp.
Here is the correct answer
void strrev(string *at_front) {
string *at_back = at_front;
char transfer_back;
int elem = 0;
while(at_back->operator[](elem) != '\0') {
elem++;
}
for(int i = 0; i <elem; i++) {
transfer_back = at_front->operator[](i);
at_front->operator[](i) = at_back->operator[](elem);
at_back->operator[](elem) = transfer_back;
elem--;
}
}
Let me explain why you have that error. string *transfer_back = at_front those two are pointed to the same reference, that is why when you change transfer_back->operator[](0) = at_front->operator[](i);this change will reflect in at_front string as well.
"Write a function that reverses the characters in a text string by using two pointers. The only function parameter shall be a pointer to the string."
This sounds to me like the question addresses C strings but not std::string.
Assuming my feeling is right, this could look like:
#include <iostream>
#include <string>
void strrev(char *at_front) {
char *at_back = at_front;
if (!*at_back) return; // early out in edge case
// move at_back to end (last char before 0-terminator)
while (at_back[1]) ++at_back;
// reverse by swapping contents of front and back
while (at_front < at_back) {
std::swap(*at_front++, *at_back--);
}
}
int main() {
char str[] = "ereh txet yna";
strrev(str);
std::cout << str << '\n';
return 0;
}
Output:
any text here
Live Demo on coliru
Note:
I stored the original string in a char str[].
If I had used char *str = "ereh txet yna"; I had assigned an address of a constant string to str. This feels very wrong as I want to modify the contents of str which must not be done on constants.
strrev():
The at_back[1] reads the next char after address in at_back. For a valid C string, this should be always possible as I excluded the empty string (consisting of 0-terminator only) before.
The swapping loop moves at_front as well as at_back. As the pointer is given as value, this has no "destructive" effect outside of strrev().
Concerning std::swap(*at_front++, *at_back--);:
The swapping combines access to pointer contents with pointer increment/decrement, using postfix-increment/-decrement. IMHO, one of the rare cases where the postfix operators are useful somehow.
Alternatively, I could have written:
std::swap(*at_front, *at_back); ++at_front; --at_back;
Please, note that std::string is a container class. A pointer to the container cannot be used to address its contained raw string directly. For this, std::string provides various access methods like e.g.
std::string::operator[]()
std::string::at()
std::string::data()
etc.
Whenever I am comparing string in qsort, the order is completely wrong. For example, the input is
45 4 9 22 2
but my output is
22 45 4 9 2
here is my comparing function
int cmpString(const void *a, const void *b) {
const Node *a1 = *(const Node **)a;
const Node *b1 = *(const Node **)b;
return a1->s.c_str() - b1->s.c_str();
}
and dont tell me to use sort(), I can't for this assignment
This line is your code's major problem.
return a1->s.c_str() - b1->s.c_str();
The reason behind this is that you are subtracting two pointers here which is not what comparator is supposed to do in this case. Comparator does the comparison on the basis of content.
Instead, try this:
int length1 = a1->s.size();
int length2 = b1->s.size();
for (int i = 0; i < min(length1, length2); i++) {
if (a1->s[i] != b1->s[i]) { // if characters are not same, return difference of their ASCII values.
return a1->s[i] - b1->s[i];
}
}
return length1 - length2; // if they are same till now, then shorter string should appear first. That's why it is required.
Suggestion:
If you are coding in C++, then please use STL. There is a nice function sort() given by <algorithm> which allows you to do the same thing without using void *
Update:
As rightly suggested by user4581301, you may use std::string::compare directly.
Like this:
return (a1->s).compare(b1->s);
I'm trying to create this c++ program to perform the description below. I am pretty certain the issue is in the recursive, but uncertain how to fix it. I'm guessing it just keeps iterating through to infinity and crashes. I do not even get an output. I figured I could just compare the previous and current pointer and perform a 3-piece temp swap based on lexicography. I would use a pointer to iterate through the array and decrement it after each swap, then recursively call with that ptr as the parameter. Didn't work, I'm here, help me please :). If there is a simpler solution that would work too, but prefer to understand where I went wrong with this code.
#include <string>
#include <iostream>
using namespace std;
// Given an array of strictly the characters 'R', 'G', and
// 'B', segregate the values of the array so that all the
// Rs come first, the Gs come second, and the Bs come last.
// You can only swap elements of the array.
char* RGBorder(char* c_a)
{
size_t sz = sizeof(c_a)/sizeof(*c_a);
char* ptr_ca = c_a;
char* prv_ptr = ptr_ca;
ptr_ca++;
char temp;
while(*ptr_ca)
{
switch(*ptr_ca)
{
case 'R' :
if( *prv_ptr < *ptr_ca ) {
temp = *prv_ptr; *prv_ptr = *ptr_ca; *ptr_ca = temp;
} else if( *prv_ptr == *ptr_ca ) {
continue;
} else { ptr_ca--; RGBorder(ptr_ca); }
case 'G' :
if( *prv_ptr < *ptr_ca ) {
temp = *prv_ptr; *prv_ptr = *ptr_ca; *ptr_ca = temp;
} else if( *prv_ptr == *ptr_ca ) {
continue;
} else { ptr_ca--; RGBorder(ptr_ca); }
default:
ptr_ca++;
continue;
}
ptr_ca++;
cout << *ptr_ca;
}
return c_a;
}
int main()
{
char ca[] = {'G', 'B', 'R', 'R', 'B', 'R', 'G'};
char *oca =RGBorder(ca);
char *pca = oca;
while(*pca)
{
cout << *pca << endl;
pca++;
}
}
There are many issues with your code.
1) You call the function RGBorder with a character pointer, and then attempt to get the number of characters using this:
size_t sz = sizeof(c_a)/sizeof(*c_a);
This will not get you the number of characters. Instead this will simply get you the
sizeof(char *) / sizeof(char)
which is usually 4 or 8. The only way to call your function using a char array is either provide a null-terminated array (thus you can use strlen), or you have to pass the number of characters in the array as a separate argument:
char *RGBorder(char *c_a, int size)
2) I didn't go through your code, but there are easier ways to do a 3-way partition in an array. One popular algorithm to do this is one based on the Dutch National Flag problem.
Since you want the array in RGB order, you know that the series of G will always come in the middle (somewhere) of the sequence, with R on the left of the sequence, and B always on the right of the sequence.
So the goal is to simply swap R to the left of the middle, and B to the right of the middle. So basically you want a loop that incrementally changes the "middle" when needed, while swapping R's and B's to their appropriate position when they're detected.
The following code illustrates this:
#include <algorithm>
char *RGBorder(char *c_a, int num)
{
int middle = 0; // assume we only want the middle element
int low = 0; // before the G's
int high = num - 1; // after the G's
while (middle <= high)
{
if ( c_a[middle] == 'R' ) // if we see an 'R' in the middle, it needs to go before the middle
{
std::swap(c_a[middle], c_a[low]); // swap it to a place before middle
++middle; // middle has creeped up one spot
++low; // so has the point where we will swap when we do this again
}
else
if (c_a[middle] == 'B') // if we see a 'B' as the middle element, it needs to go after the middle
{
std::swap(c_a[middle], c_a[high]); // place it as far back as you can
--high; // decrease the back position for next swap that comes here
}
else
++middle; // it is a 'G', do nothing
}
return c_a;
}
Live Example
Here is another solution that uses std::partition.
#include <algorithm>
#include <iostream>
char *RGBorder(char *c_a, int num)
{
auto iter = std::partition(c_a, c_a + num, [](char ch) {return ch == 'R';});
std::partition(iter, c_a + num, [](char ch) {return ch == 'G';});
return c_a;
}
Live Example
Basically, the first call to std::partition places the R's to the front of the array. Since std::partition returns an iterator (in this case, a char *) to the end of where the partition occurs, we use that as a starting position in the second call to std::partition, where we partition the G values.
Note that std::partition also accomplishes its goal by swapping.
Given this solution, we can generalize this for an n-way partition by using a loop. Assume we want to place things in RGBA order (4 values instead of 3).
#include <algorithm>
#include <iostream>
#include <cstring>
char *RGBorder(char *c_a, int num, char *order, int num2)
{
auto iter = c_a;
for (int i = 0; i < num2 - 1; ++i)
iter = std::partition(iter, c_a + num, [&](char ch) {return ch == order[i];});
return c_a;
}
int main()
{
char ca[] = "AGBRRBARGGARRBGAGRARAA";
std::cout << RGBorder(ca, strlen(ca), "RGBA", 4);
}
Output:
RRRRRRRGGGGGBBBAAAAAAA
Sorry to put it blunt, but that code is a mess. And I don't mean the mistakes, those are forgivable for beginners. I mean the formatting. Multiple statements in one line make it super hard to read and debug the code. Short variable names that carry no immediate intrinsic meaning make it hard to understand what the code is supposed to do. using namespace std; is very bad practise as well, but I can imagine you were taught to do this by whoever gives that course.
1st problem
Your cases don't break, thus you execute all cases for R, and both G and default for G. Also your code will never reach the last 2 lines of your loop, as you continue out before in every case.
2nd problem
You have an endless loop. In both cases you have two situations where you'll end up in an endless loop:
In the else if( *prv_ptr == *ptr_ca ) branch you simply continue; without changing the pointer.
In the else branch you do ptr_ca--;, but then in default you call ptr_ca++; again.(Note that even with breaks you would still call ptr_ca++; at the end of the loop.)
In both cases the pointer doesn't change, so once you end up in any of those conditions your loop will never exit.
Possible 3rd problem
I can only guess, because it is not apparent from the name, but it seems that prv_ptr is supposed to hold whatever was the last pointer in the loop? If so, it seems wrong that you don't update that pointer, ever. Either way, proper variable names would've made it more clear what the purpose of this pointer is exactly. (On a side note, consistent usage of const can help identify such issues. If you have a variable that is not const, but never gets updated, you either forgot to add const or forgot to update it.)
How to fix
Format your code:
Don't use using namespace std;.
One statement per line.
Give your variables proper names, so it's easy to identify what is what. (This is not 1993, really, I'd rather have a thisIsThePointerHoldingTheCharacterThatDoesTheThing than ptr_xy.)
Fix the aforementioned issues (add breaks, make sure your loop actually exits).
Then debug your code. With a debugger. While it runs. With breakpoints and stepping through line by line, inspecting the values of your pointers as the code executes. Fancy stuff.
Good luck!
just count the number of 'R', 'G' and 'B' letters and fill the array from scratch.
much easier, no recursions.
I am writing a class called Word, that handles a c string and overloads the
<, >, <=, >= operators.
word.h:
friend bool operator<(const Word &a, const Word &b);
word.cc:
bool operator<(const Word &a, const Word &b) {
if(a == NULL && b == NULL)
return false;
if(a == NULL)
return true;
if(b == NULL)
return false;
return strcmp(a.wd, b.wd) < 0; //wd is a valid c string, EDIT: changed to strcmp
}
main:
char* temp = NULL; //EDIT: i was mistaken, temp is a char pointer
Word a("blah"); //a.wd = [b,l,a,h]
cout << (temp<a);
I get a segmentation error before the first line of the operator< method
after the last line in the main. I can correct the problem by writing
cout << (a>temp);
where the operator> is similarly defined and I get no errors but my
assignment requires (temp < a) to work so this is where I ask for help.
EDIT: I made a mistake the first time and i said temp was of type Word,
but it is actually of type char*. So I assume that the compiler converts
temp to a Word using one of my constructors. I dont know which one it would
use and why this would work since the first parameter is not Word.
here is the constructor I think is being used to make the Word using temp:
Word::Word(char* c, char* delimeters="\n") {
char *temporary = "\0";
if(c == NULL)
c = temporary;
check(stoppers!=NULL, "(Word(char*,char*))NULL pointer"); // exits the program if the expression is false
if(strlen(c) == 0)
size = DEFAULT_SIZE; //10
else
size = strlen(c) + 1 + DEFAULT_SIZE;
wd = new char[size];
check(wd!=NULL, "Word(char*,char*))heap overflow");
delimiters = new char[strlen(stoppers) + 1]; // EDIT: changed to []
check(delimiters!=NULL,"Word(char*,char*))heap overflow");
strcpy(wd,c);
strcpy(delimiters,stoppers);
count = strlen(wd);
}
wd is of type char*
thanks for looking at this big question and trying to help. let me know if you
need more code to look at
I'm almost positive you did not mean to construct a char on the heap with an initial value of some integer based on the size of stoppers:
delimiters = new char(strlen(stoppers) + 1); // Should use [] not ()
Also you are using C++ and I would never tell you what to do, but please, unless you know exactly that there is no danger, do not use strcpy. For exactly this reason.
It is a blind copy of strings, and when the destination does not have enough space (as is the case from your typo-ed allocation), things go BAD.
EDIT:
I also see in your overload of operator< that you use
a.wd < b.wd
and claim that .wds are valid C strings. If that is the case, you cannot apply a simple < operator to them and must use strcmp, strncmp or some other full compare function
Cutting out the other bits of the constructor:
Word::Word(char* c, char* delimeters=NULL) {
check(stoppers!=NULL, "(Word(char*,char*))NULL pointer"); //exits the program if the expression is false
delimiters = new char[strlen(stoppers) + 1];
check(delimiters!=NULL,"Word(char*,char*))heap overflow");
strcpy(delimiters,stoppers);
}
You are allocating and copying to the input parameter (delimiters) not the member variable (stoppers). Therefore, when you call:
delimiters = new char[strlen(stoppers) + 1];
Here, stoppers == NULL (infered from the check call) so strlen(NULL) crashes.
Also, in:
bool operator<(const Word &a, const Word &b)
You check things like a == NULL. This is not needed as a and b are references, so the objects are non-null.
If wd can be null, you will need to change these to check a.wd and b.wd.
I just took an exam where I was asked the following:
Write the function body of each of the methods GenStrLen, InsertChar and StrReverse for the given code below. You must take into consideration the following;
How strings are constructed in C++
The string must not overflow
Insertion of character increases its length by 1
An empty string is indicated by StrLen = 0
class Strings {
private:
char str[80];
int StrLen;
public:
// Constructor
Strings() {
StrLen=0;
};
// A function for returning the length of the string 'str'
int GetStrLen(void) {
};
// A function to inser a character 'ch' at the end of the string 'str'
void InsertChar(char ch) {
};
// A function to reverse the content of the string 'str'
void StrReverse(void) {
};
};
The answer I gave was something like this (see bellow). My one of problem is that used many extra variables and that makes me believe am not doing it the best possible way, and the other thing is that is not working....
class Strings {
private:
char str[80];
int StrLen;
int index; // *** Had to add this ***
public:
Strings(){
StrLen=0;
}
int GetStrLen(void){
for (int i=0 ; str[i]!='\0' ; i++)
index++;
return index; // *** Here am getting a weird value, something like 1829584505306 ***
}
void InsertChar(char ch){
str[index] = ch; // *** Not sure if this is correct cuz I was not given int index ***
}
void StrRevrse(void){
GetStrLen();
char revStr[index+1];
for (int i=0 ; str[i]!='\0' ; i++){
for (int r=index ; r>0 ; r--)
revStr[r] = str[i];
}
}
};
I would appreciate if anyone could explain me roughly what is the best way to have answered the question and why. Also how come my professor closes each class function like " }; ", I thought that was only used for ending classes and constructors only.
Thanks a lot for your help.
First, the trivial }; question is just a matter of style. I do that too when I put function bodies inside class declarations. In that case the ; is just an empty statement and doesn't change the meaning of the program. It can be left out of the end of the functions (but not the end of the class).
Here's some major problems with what you wrote:
You never initialize the contents of str. It's not guaranteed to start out with \0 bytes.
You never initialize index, you only set it within GetStrLen. It could have value -19281281 when the program starts. What if someone calls InsertChar before they call GetStrLen?
You never update index in InsertChar. What if someone calls InsertChar twice in a row?
In StrReverse, you create a reversed string called revStr, but then you never do anything with it. The string in str stays the same afterwords.
The confusing part to me is why you created a new variable called index, presumably to track the index of one-past-the-last character the string, when there was already a variable called StrLen for this purpose, which you totally ignored. The index of of one-past-the-last character is the length of the string, so you should just have kept the length of the string up to date, and used that, e.g.
int GetStrLen(void){
return StrLen;
}
void InsertChar(char ch){
if (StrLen < 80) {
str[StrLen] = ch;
StrLen = StrLen + 1; // Update the length of the string
} else {
// Do not allow the string to overflow. Normally, you would throw an exception here
// but if you don't know what that is, you instructor was probably just expecting
// you to return without trying to insert the character.
throw std::overflow_error();
}
}
Your algorithm for string reversal, however, is just completely wrong. Think through what that code says (assuming index is initialized and updated correctly elsewhere). It says "for every character in str, overwrite the entirety of revStr, backwards, with this character". If str started out as "Hello World", revStr would end up as "ddddddddddd", since d is the last character in str.
What you should do is something like this:
void StrReverse() {
char revStr[80];
for (int i = 0; i < StrLen; ++i) {
revStr[(StrLen - 1) - i] = str[i];
}
}
Take note of how that works. Say that StrLen = 10. Then we're copying position 0 of str into position 9 of revStr, and then position 1 of str into position 9 of revStr, etc, etc, until we copy position StrLen - 1 of str into position 0 of revStr.
But then you've got a reversed string in revStr and you're still missing the part where you put that back into str, so the complete method would look like
void StrReverse() {
char revStr[80];
for (int i = 0; i < StrLen; ++i) {
revStr[(StrLen - 1) - i] = str[i];
}
for (int i = 0; i < StrLen; ++i) {
str[i] = revStr[i];
}
}
And there are cleverer ways to do this where you don't have to have a temporary string revStr, but the above is perfectly functional and would be a correct answer to the problem.
By the way, you really don't need to worry about NULL bytes (\0s) at all in this code. The fact that you are (or at least you should be) tracking the length of the string with the StrLen variable makes the end sentinel unnecessary since using StrLen you already know the point beyond which the contents of str should be ignored.
int GetStrLen(void){
for (int i=0 ; str[i]!='\0' ; i++)
index++;
return index; // *** Here am getting a weird value, something like 1829584505306 ***
}
You are getting a weird value because you never initialized index, you just started incrementing it.
Your GetStrLen() function doesn't work because the str array is uninitialized. It probably doesn't contain any zero elements.
You don't need the index member. Just use StrLen to keep track of the current string length.
There are lots of interesting lessons to learn by this exam question. Firstly the examiner is does not appear to a fluent C++ programmer themselves! You might want to look at the style of the code, including whether the variables and method names are meaningful as well as some of the other comments you've been given about usage of (void), const, etc... Do the method names really need "Str" in them? We are operating with a "Strings" class, after all!
For "How strings are constructed in C++", well (like in C) these are null-terminated and don't store the length with them, like Pascal (and this class) does. [#Gustavo, strlen() will not work here, since the string is not a null-terminated one.] In the "real world" we'd use the std::string class.
"The string must not overflow", but how does the user of the class know if they try to overflow the string. #Tyler's suggestion of throwing a std::overflow_exception (perhaps with a message) would work, but if you are writing your own string class (purely as an exercise, you're very unlikely to need to do so in real life) then you should probably provide your own exception class.
"Insertion of character increases its length by 1", this implies that GetStrLen() doesn't calculate the length of the string, but purely returns the value of StrLen initialised at construction and updated with insertion.
You might also want to think about how you're going to test your class. For illustrative purposes, I added a Print() method so that you can look at the contents of the class, but you should probably take a look at something like Cpp Unit Lite.
For what it's worth, I'm including my own implementation. Unlike the other implementations so far, I have chosen to use raw-pointers in the reverse function and its swap helper. I have presumed that using things like std::swap and std::reverse are outside the scope of this examination, but you will want to familiarise yourself with the Standard Library so that you can get on and program without re-inventing wheels.
#include <iostream>
void swap_chars(char* left, char* right) {
char temp = *left;
*left = *right;
*right = temp;
}
class Strings {
private:
char m_buffer[80];
int m_length;
public:
// Constructor
Strings()
:m_length(0)
{
}
// A function for returning the length of the string 'm_buffer'
int GetLength() const {
return m_length;
}
// A function to inser a character 'ch' at the end of the string 'm_buffer'
void InsertChar(char ch) {
if (m_length < sizeof m_buffer) {
m_buffer[m_length++] = ch;
}
}
// A function to reverse the content of the string 'm_buffer'
void Reverse() {
char* left = &m_buffer[0];
char* right = &m_buffer[m_length - 1];
for (; left < right; ++left, --right) {
swap_chars(left, right);
}
}
void Print() const {
for (int index = 0; index < m_length; ++index) {
std::cout << m_buffer[index];
}
std::cout << std::endl;
}
};
int main(int, char**) {
Strings test_string;
char test[] = "This is a test string!This is a test string!This is a test string!This is a test string!\000";
for (char* c = test; *c; ++c) {
test_string.InsertChar(*c);
}
test_string.Print();
test_string.Reverse();
test_string.Print();
// The output of this program should look like this...
// This is a test string!This is a test string!This is a test string!This is a test
// tset a si sihT!gnirts tset a si sihT!gnirts tset a si sihT!gnirts tset a si sihT
return 0;
}
Good luck with the rest of your studies!
void InsertChar(char ch){
str[index] = ch; // *** Not sure if this is correct cuz I was not given int index ***
}
This should be something more like
str[strlen-1]=ch; //overwrite the null with ch
str[strlen]='\0'; //re-add the null
strlen++;
Your teacher gave you very good hints on the question, read it again and try answering yourself. Here's my untested solution:
class Strings {
private:
char str[80];
int StrLen;
public:
// Constructor
Strings() {
StrLen=0;
str[0]=0;
};
// A function for returning the length of the string 'str'
int GetStrLen(void) {
return StrLen;
};
// A function to inser a character 'ch' at the end of the string 'str'
void InsertChar(char ch) {
if(StrLen < 80)
str[StrLen++]=ch;
};
// A function to reverse the content of the string 'str'
void StrReverse(void) {
for(int i=0; i<StrLen / 2; ++i) {
char aux = str[i];
str[i] = str[StrLen - i - 1];
str[StrLen - i - 1] = aux;
}
};
};
When you init the char array, you should set its first element to 0, and the same for index. Thus you get a weird length in GetStrLen since it is up to the gods when you find the 0 you are looking for.
[Update] In C/C++ if you do not explicitly initialize your variables, you usually get them filled with random garbage (the content of the raw memory allocated to them). There are some exceptions to this rule, but the best practice is to always initialize your variables explicitly. [/Update]
In InsertChar, you should (after checking for overflow) use StrLen to index the array (as the comment specifies "inser a character 'ch' at the end of the string 'str'"), then set the new terminating 0 character and increment StrLen.
You don't need index as a member data. You can have it a local variable if you so please in GetStrLen(): just declare it there rather than in the class body. The reason you get a weird value when you return index is because you never initialized it. To fix that, initialize index to zero in GetStrLen().
But there's a better way to do things: when you insert a character via InsertChar() increment the value of StrLen, so that GetStrLen() need only return that value. This will make GetStrLen() much faster: it will run in constant time (the same performance regardless of the length of string).
In InsertChar() you can use StrLen as you index rather than index, which we already determined is redundant. But remember that you must make sure the string terminates with a '\0' value. Also remember to maintain StrLen by incrementing it to make GetStrLen()'s life easier. In addition, you must take the extra step in InsertChar() to avoid a buffer overflow. This happens when the user inserts a character to the string when the length of the string is alreay 79 characters. (Yes, 79: you must spend one character on the terminating null).
I don't see an instruction as to how to behave when that happens, so it must be up to your good judgment call. If the user tries to add the 80th character you might ignore the request and return, or you might set an error flag -- it's up to you.
In your StrReverse() function you have a few mistakes. First, you call GetStrLen() but ignore its return value. Then why call it? Second, you're creating a temporary string and work on that, rather than on the string member of the class. So your function doesn't change the string member, when it should in fact reverse it. And last, you could reverse the string faster by iterating through half of it only.
Work on the member data string. To reverse a string you can swap the first element (character) of the string with its last (not the terminating null, the character just before that!), the second element with the second-to-last and so on. You're done when you arrive at the middle of the string. Don't forget that the string must terminate with a '\0' character.
While you were solving the exam it would also be a good opportunity to teach your instructor a think or two about C++: we don't say f(void) because that belongs to the old days of C89. In C++ we say f(). We also strive in C++ to use class initializer lists whenever we can. Also remind your instructor how important const-correctness is: when a function shouldn't change the object is should be marked as such. int GetStrLen(void) should be int GetStrLen() const.
You don't need to figure out the length. You already know it it is strLen. Also there was nothing in the original question to indicate that the buffer should contain a null terminated string.
int GetStrLen(void){
return strLen;
}
Just using an assertion here but another option is to throw an exception.
void InsertChar(char ch){
assert(strLen < 80);
str[strLen++] = ch;
}
Reversing the string is just a matter of swapping the elements in the str buffer.
void StrRevrse(void){
int n = strLen >> 1;
for (int i = 0; i < n; i++) {
char c = str[i];
str[i] = str[strLen - i];
str[strLen - i] = c;
}
}
I would use StrLen to track the length of the string. Since the length also indicates the end of the string, we can use that for inserting:
int GetStrLen(void) {
return StrLen;
}
int InsertChar(char ch)
{
if (strLen < sizeof(str))
{
str[StrLen] = ch;
++strLen;
}
}
void StrReverse(void) {
for (int n = 0; n < StrLen / 2; ++n)
{
char tmp = str[n];
str[n] = str[StrLen - n - 1];
str[StrLen - n - 1] = tmp;
}
}
first of all why on you use String.h for the string length?
strlen(char[] array) returns the Lenght or any char array to a int.
Your function return a werid value because you never initialize index, and the array has zero values, first initilize then execute your method.