How modify character arrays inside functions?

I want to update a character array by reversing it. The character array is correct when output inside function, but not in main after the function is called.

I believe that the character array is passed by reference, but when updating the array, the character array in the main function is not updated. What am I doing wrong here?

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    ch = sh;

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

I do not want to print the result inside the function, but update the character array 'ch' in the main function by calling the function StringReverse.

edited Jan 2 at 10:59

Holt

25.8k65195

asked Jan 2 at 10:17

sparrow

153

1

"I believe that the character array is ...". Believing is a dangerous thing. I would test anything first and be sure about it, then just go on belief.

– Joey Mallone
Jan 2 at 10:20

2

You want to use strncpy() instead of ch = sh;.

– Joey Mallone
Jan 2 at 10:22

Changing the copy of the pointer in StringReverse doesn't change the pointer to a stack variable in another function. Even if it worked, you still want to use a local stack variable, which is a big "no"! Just use a string.

– Matthieu Brucher
Jan 2 at 10:22

add a comment |

I want to update a character array by reversing it. The character array is correct when output inside function, but not in main after the function is called.

I believe that the character array is passed by reference, but when updating the array, the character array in the main function is not updated. What am I doing wrong here?

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    ch = sh;

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

I do not want to print the result inside the function, but update the character array 'ch' in the main function by calling the function StringReverse.

edited Jan 2 at 10:59

Holt

25.8k65195

asked Jan 2 at 10:17

sparrow

153

1

"I believe that the character array is ...". Believing is a dangerous thing. I would test anything first and be sure about it, then just go on belief.

– Joey Mallone
Jan 2 at 10:20

2

You want to use strncpy() instead of ch = sh;.

– Joey Mallone
Jan 2 at 10:22

Changing the copy of the pointer in StringReverse doesn't change the pointer to a stack variable in another function. Even if it worked, you still want to use a local stack variable, which is a big "no"! Just use a string.

– Matthieu Brucher
Jan 2 at 10:22

add a comment |

I want to update a character array by reversing it. The character array is correct when output inside function, but not in main after the function is called.

I believe that the character array is passed by reference, but when updating the array, the character array in the main function is not updated. What am I doing wrong here?

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    ch = sh;

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

I do not want to print the result inside the function, but update the character array 'ch' in the main function by calling the function StringReverse.

edited Jan 2 at 10:59

Holt

25.8k65195

asked Jan 2 at 10:17

sparrow

153

I want to update a character array by reversing it. The character array is correct when output inside function, but not in main after the function is called.

I believe that the character array is passed by reference, but when updating the array, the character array in the main function is not updated. What am I doing wrong here?

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    ch = sh;

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

I do not want to print the result inside the function, but update the character array 'ch' in the main function by calling the function StringReverse.

c++ arrays char pass-by-reference pass-by-value

edited Jan 2 at 10:59

Holt

25.8k65195

asked Jan 2 at 10:17

sparrow

153

edited Jan 2 at 10:59

Holt

25.8k65195

asked Jan 2 at 10:17

sparrow

153

edited Jan 2 at 10:59

Holt

25.8k65195

edited Jan 2 at 10:59

Holt

25.8k65195

edited Jan 2 at 10:59

Holt

25.8k65195

asked Jan 2 at 10:17

sparrow

153

asked Jan 2 at 10:17

sparrow

153

asked Jan 2 at 10:17

sparrow

153

1

"I believe that the character array is ...". Believing is a dangerous thing. I would test anything first and be sure about it, then just go on belief.

– Joey Mallone
Jan 2 at 10:20

2

You want to use strncpy() instead of ch = sh;.

– Joey Mallone
Jan 2 at 10:22

Changing the copy of the pointer in StringReverse doesn't change the pointer to a stack variable in another function. Even if it worked, you still want to use a local stack variable, which is a big "no"! Just use a string.

– Matthieu Brucher
Jan 2 at 10:22

add a comment |

1

"I believe that the character array is ...". Believing is a dangerous thing. I would test anything first and be sure about it, then just go on belief.

– Joey Mallone
Jan 2 at 10:20

2

You want to use strncpy() instead of ch = sh;.

– Joey Mallone
Jan 2 at 10:22

Changing the copy of the pointer in StringReverse doesn't change the pointer to a stack variable in another function. Even if it worked, you still want to use a local stack variable, which is a big "no"! Just use a string.

– Matthieu Brucher
Jan 2 at 10:22

"I believe that the character array is ...". Believing is a dangerous thing. I would test anything first and be sure about it, then just go on belief.

– Joey Mallone
Jan 2 at 10:20

You want to use strncpy() instead of ch = sh;.

– Joey Mallone
Jan 2 at 10:22

Changing the copy of the pointer in StringReverse doesn't change the pointer to a stack variable in another function. Even if it worked, you still want to use a local stack variable, which is a big "no"! Just use a string.

– Matthieu Brucher
Jan 2 at 10:22

add a comment |

2 Answers
2

active

oldest

votes

You want to use strncpy() instead of ch = sh;. As other people have kindly pointed, you are not modifying ch. You are modifying the local variable sh only.

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    strncpy(ch, sh, 100);

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

answered Jan 2 at 10:26

Joey Mallone

2,26561933

OP should use string instead.

– Matthieu Brucher
Jan 2 at 10:30

@MatthieuBrucher, Indeed. The question is tagged c++ but then again he is using char arrays and including the cstring header file. It is a bit of both worlds. :P

– Joey Mallone
Jan 2 at 10:32

Why do strncpy() work but not ch = sh ?

– sparrow
Jan 2 at 11:00

Yes. Using the strings would make it a lot easier I know. I am just clearing out my concepts :P @MatthieuBrucher

– sparrow
Jan 2 at 11:01

1

@sparrow ch is actually passed by value, i.e., you are passing a pointer by value (because array decay to pointers), so ch in StringReverse is actually a copy of the pointer ch in the main function. When you do ch = sh, you modify ch, the copy, and not the original ch. strncpy works on the "pointed" array, which is why it works because both the original and the copy points to the same array.

– Holt
Jan 2 at 11:03

|
show 2 more comments

The ch in StringReverse function is local to that function. And that's why when assign ch = sh;, it doesn't change the ch in main.
While you can copy sh, a better approach is to use reverse in-place so that you wouldn't need a local copy at all.

For example:

void StringReverse(char *ch, int size){

    for(int i = 0, j= size - 1 ; i < j ; i++, j--){

        int t = ch[i];

        ch[i] = ch[j];

        ch[j] = t;

    }

    cout << ch << endl;

}

If you use std::string instead of a plain char array, it would be a lot easier to do this. You could for example use std::reverse.

answered Jan 2 at 10:34

usr

14.4k32143

You can use std::reverse on a simple array, just use ch and ch + size for the iterator.

– Holt
Jan 2 at 11:01

That's a way better solution. Thanks :)

– sparrow
Jan 2 at 11:03

@sparrow If it's an exercise then you're probably expected to implement it rather than use C++ standard library to do everything (so plain array is OK). But you could still go for a better algorithm.

– usr
Jan 2 at 11:10

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

You want to use strncpy() instead of ch = sh;. As other people have kindly pointed, you are not modifying ch. You are modifying the local variable sh only.

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    strncpy(ch, sh, 100);

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

answered Jan 2 at 10:26

Joey Mallone

2,26561933

OP should use string instead.

– Matthieu Brucher
Jan 2 at 10:30

@MatthieuBrucher, Indeed. The question is tagged c++ but then again he is using char arrays and including the cstring header file. It is a bit of both worlds. :P

– Joey Mallone
Jan 2 at 10:32

Why do strncpy() work but not ch = sh ?

– sparrow
Jan 2 at 11:00

Yes. Using the strings would make it a lot easier I know. I am just clearing out my concepts :P @MatthieuBrucher

– sparrow
Jan 2 at 11:01

1

@sparrow ch is actually passed by value, i.e., you are passing a pointer by value (because array decay to pointers), so ch in StringReverse is actually a copy of the pointer ch in the main function. When you do ch = sh, you modify ch, the copy, and not the original ch. strncpy works on the "pointed" array, which is why it works because both the original and the copy points to the same array.

– Holt
Jan 2 at 11:03

|
show 2 more comments

You want to use strncpy() instead of ch = sh;. As other people have kindly pointed, you are not modifying ch. You are modifying the local variable sh only.

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    strncpy(ch, sh, 100);

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

answered Jan 2 at 10:26

Joey Mallone

2,26561933

OP should use string instead.

– Matthieu Brucher
Jan 2 at 10:30

@MatthieuBrucher, Indeed. The question is tagged c++ but then again he is using char arrays and including the cstring header file. It is a bit of both worlds. :P

– Joey Mallone
Jan 2 at 10:32

Why do strncpy() work but not ch = sh ?

– sparrow
Jan 2 at 11:00

Yes. Using the strings would make it a lot easier I know. I am just clearing out my concepts :P @MatthieuBrucher

– sparrow
Jan 2 at 11:01

1

@sparrow ch is actually passed by value, i.e., you are passing a pointer by value (because array decay to pointers), so ch in StringReverse is actually a copy of the pointer ch in the main function. When you do ch = sh, you modify ch, the copy, and not the original ch. strncpy works on the "pointed" array, which is why it works because both the original and the copy points to the same array.

– Holt
Jan 2 at 11:03

|
show 2 more comments

You want to use strncpy() instead of ch = sh;. As other people have kindly pointed, you are not modifying ch. You are modifying the local variable sh only.

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    strncpy(ch, sh, 100);

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

answered Jan 2 at 10:26

Joey Mallone

2,26561933

You want to use strncpy() instead of ch = sh;. As other people have kindly pointed, you are not modifying ch. You are modifying the local variable sh only.

#include <iostream>

#include <cstring>

using namespace std;



void StringReverse(char *ch, int size){

    char sh[100] = {0};

    for(int i=0 ; i<size ; i++){

        sh[i] = ch[size-1-i];

    }

    sh[size] = '';

    strncpy(ch, sh, 100);

    cout<<ch<<endl;

}



int main(){

    char ch[100];

    cin.getline(ch, 100);

    int size = strlen(ch);

    StringReverse(ch,size);

    cout<<ch;

}

answered Jan 2 at 10:26

Joey Mallone

2,26561933

answered Jan 2 at 10:26

Joey Mallone

2,26561933

answered Jan 2 at 10:26

Joey Mallone

2,26561933

answered Jan 2 at 10:26

Joey Mallone

2,26561933

OP should use string instead.

– Matthieu Brucher
Jan 2 at 10:30

@MatthieuBrucher, Indeed. The question is tagged c++ but then again he is using char arrays and including the cstring header file. It is a bit of both worlds. :P

– Joey Mallone
Jan 2 at 10:32

Why do strncpy() work but not ch = sh ?

– sparrow
Jan 2 at 11:00

Yes. Using the strings would make it a lot easier I know. I am just clearing out my concepts :P @MatthieuBrucher

– sparrow
Jan 2 at 11:01

1

@sparrow ch is actually passed by value, i.e., you are passing a pointer by value (because array decay to pointers), so ch in StringReverse is actually a copy of the pointer ch in the main function. When you do ch = sh, you modify ch, the copy, and not the original ch. strncpy works on the "pointed" array, which is why it works because both the original and the copy points to the same array.

– Holt
Jan 2 at 11:03

|
show 2 more comments

OP should use string instead.

– Matthieu Brucher
Jan 2 at 10:30

@MatthieuBrucher, Indeed. The question is tagged c++ but then again he is using char arrays and including the cstring header file. It is a bit of both worlds. :P

– Joey Mallone
Jan 2 at 10:32

Why do strncpy() work but not ch = sh ?

– sparrow
Jan 2 at 11:00

Yes. Using the strings would make it a lot easier I know. I am just clearing out my concepts :P @MatthieuBrucher

– sparrow
Jan 2 at 11:01

1

@sparrow ch is actually passed by value, i.e., you are passing a pointer by value (because array decay to pointers), so ch in StringReverse is actually a copy of the pointer ch in the main function. When you do ch = sh, you modify ch, the copy, and not the original ch. strncpy works on the "pointed" array, which is why it works because both the original and the copy points to the same array.

– Holt
Jan 2 at 11:03

OP should use string instead.

– Matthieu Brucher
Jan 2 at 10:30

@MatthieuBrucher, Indeed. The question is tagged c++ but then again he is using char arrays and including the cstring header file. It is a bit of both worlds. :P

– Joey Mallone
Jan 2 at 10:32

Why do strncpy() work but not ch = sh ?

– sparrow
Jan 2 at 11:00

Yes. Using the strings would make it a lot easier I know. I am just clearing out my concepts :P @MatthieuBrucher

– sparrow
Jan 2 at 11:01

@sparrow ch is actually passed by value, i.e., you are passing a pointer by value (because array decay to pointers), so ch in StringReverse is actually a copy of the pointer ch in the main function. When you do ch = sh, you modify ch, the copy, and not the original ch. strncpy works on the "pointed" array, which is why it works because both the original and the copy points to the same array.

– Holt
Jan 2 at 11:03

|
show 2 more comments

For example:

void StringReverse(char *ch, int size){

    for(int i = 0, j= size - 1 ; i < j ; i++, j--){

        int t = ch[i];

        ch[i] = ch[j];

        ch[j] = t;

    }

    cout << ch << endl;

}

If you use std::string instead of a plain char array, it would be a lot easier to do this. You could for example use std::reverse.

answered Jan 2 at 10:34

usr

14.4k32143

You can use std::reverse on a simple array, just use ch and ch + size for the iterator.

– Holt
Jan 2 at 11:01

That's a way better solution. Thanks :)

– sparrow
Jan 2 at 11:03

@sparrow If it's an exercise then you're probably expected to implement it rather than use C++ standard library to do everything (so plain array is OK). But you could still go for a better algorithm.

– usr
Jan 2 at 11:10

add a comment |

For example:

void StringReverse(char *ch, int size){

    for(int i = 0, j= size - 1 ; i < j ; i++, j--){

        int t = ch[i];

        ch[i] = ch[j];

        ch[j] = t;

    }

    cout << ch << endl;

}

If you use std::string instead of a plain char array, it would be a lot easier to do this. You could for example use std::reverse.

answered Jan 2 at 10:34

usr

14.4k32143

You can use std::reverse on a simple array, just use ch and ch + size for the iterator.

– Holt
Jan 2 at 11:01

That's a way better solution. Thanks :)

– sparrow
Jan 2 at 11:03

@sparrow If it's an exercise then you're probably expected to implement it rather than use C++ standard library to do everything (so plain array is OK). But you could still go for a better algorithm.

– usr
Jan 2 at 11:10

add a comment |

For example:

void StringReverse(char *ch, int size){

    for(int i = 0, j= size - 1 ; i < j ; i++, j--){

        int t = ch[i];

        ch[i] = ch[j];

        ch[j] = t;

    }

    cout << ch << endl;

}

If you use std::string instead of a plain char array, it would be a lot easier to do this. You could for example use std::reverse.

answered Jan 2 at 10:34

usr

14.4k32143

For example:

void StringReverse(char *ch, int size){

    for(int i = 0, j= size - 1 ; i < j ; i++, j--){

        int t = ch[i];

        ch[i] = ch[j];

        ch[j] = t;

    }

    cout << ch << endl;

}

If you use std::string instead of a plain char array, it would be a lot easier to do this. You could for example use std::reverse.

answered Jan 2 at 10:34

usr

14.4k32143

answered Jan 2 at 10:34

usr

14.4k32143

answered Jan 2 at 10:34

usr

14.4k32143

answered Jan 2 at 10:34

usr

14.4k32143

You can use std::reverse on a simple array, just use ch and ch + size for the iterator.

– Holt
Jan 2 at 11:01

That's a way better solution. Thanks :)

– sparrow
Jan 2 at 11:03

@sparrow If it's an exercise then you're probably expected to implement it rather than use C++ standard library to do everything (so plain array is OK). But you could still go for a better algorithm.

– usr
Jan 2 at 11:10

add a comment |

You can use std::reverse on a simple array, just use ch and ch + size for the iterator.

– Holt
Jan 2 at 11:01

That's a way better solution. Thanks :)

– sparrow
Jan 2 at 11:03

@sparrow If it's an exercise then you're probably expected to implement it rather than use C++ standard library to do everything (so plain array is OK). But you could still go for a better algorithm.

– usr
Jan 2 at 11:10

You can use std::reverse on a simple array, just use ch and ch + size for the iterator.

– Holt
Jan 2 at 11:01

That's a way better solution. Thanks :)

– sparrow
Jan 2 at 11:03

@sparrow If it's an exercise then you're probably expected to implement it rather than use C++ standard library to do everything (so plain array is OK). But you could still go for a better algorithm.

– usr
Jan 2 at 11:10

add a comment |

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