Why it can't run I use pointer(I want to compare names)

Question

1.00/5 (1 vote)

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#include <stdio.h>
#include <stdlib.h>

void swap(char *ap,char *bp);
void compare(char *ap,char *bp);
int main()
{
    int b,x,y,j,damn;
    int i=0;
    char info[13][20]=
    {
        "christina",
        "victor",
        "chris",
        "chester",
        "elta",
        "kezia",
        "bew",
        "grace",
        "mavis",
        "tony",
        "oat",
        "adonique",
        "ploy"
    };
    char *ap;
    ap=info[i];
    char *bp;
    bp=info[i+1];
    for(b=0; b<13; b++)
    {
        while(i<13)
        {
            compare(ap,bp);
            i++;
        }
        for(y=0; y<13; y++)
        {
            printf("%s\n", info[y]);
        }
        return 0;
    }
}

void compare(char *ap,char *bp)
{
    int i=0;
    int x;
    int damn;
    if(*ap==*bp)
    {
        int j=1;
        while (*(ap+j)==*(bp+j))
        {
            if (*(ap+j) == '\0' && *(bp+j) == '\0')
            {
                damn=0;
                break;
            }
            j++;
        }
        if (*(ap+j)<*(bp+j))
        {
            damn=1;
        }
        if (*(ap+j)>*(bp+j))
        {
            damn=-1;
        }
    }
        if (*ap<*bp)
        {
            damn=1;
        }
        if (*ap>*bp)
        {
            damn=-1;
        }
        if (damn==-1)
        {
            while(*(ap+x) != '\0' || *(bp+x) != '\0')
            {
                swap(ap,bp);
                x++;
            }
        }
}

void swap(char *ap,char *bp)
{
    char tmp[20];
    int x;
    for(x=0; x<=19; x++)
    {
        tmp[20]=*(ap+x);
        *(ap+x)=*(bp+x);
        *(bp+x)=tmp[20];
    }
}

What I have tried:

I change some part and make a new function compare but it didn't work.

Posted 18-Dec-17 21:00pm

victor hsu 1B

Updated 19-Dec-17 1:08am

v3

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CPallini · Answer 1 · 2017-12-18T21:17:00

Solution 1

The correct implementation of your swap function would be

C

void swap( char pa[], char pb[], int size)
{
  char t;
  int n;
  for (n=0; n<size; ++n)
  {
    t = pa[n];
    pa[n] = pb[n];
    pb[n] = t;
  }
}

Please note: copying character arrays is very inefficient compared to just swap character pointers (the option suggested at your previous post).

Posted 18-Dec-17 21:17pm

CPallini

Comments

victor hsu 1B 19-Dec-17 3:28am

i change it but it still can't work.

Jochen Arndt · Answer 2 · 2017-12-18T21:28:00

Your info is a two dimensional array of chars where info[index] accesses an array of NULL terminated strings of type char[] resp. char*.

These are commonly called string pointers. When comparing these uing the == operator, you compare the pointers (the addresses). To compare the content (the strings), there is a C standard library function: strcmp - C++ Reference[^].

If you want to access characters from your info, you have to use two indexes: info[string_index][char_index].

When ap is pointing to one of the strings in info, you can also use ap[char_index] or *(ap + char_index). I suggest to use the first method because it is not so obscure as the second. The second from is used when there is no char index (just *ap); e.g. when incrementing the pointer within a loop.

So use strcmp to compare strings or do it in your own code by accessing characters as described above.

Patrice T · Answer 3 · 2017-12-18T21:30:00

C is zero based, it mean that in an array of 20 elements, their positions are from 0 to 19.

C++

void swap(char *ap, char *bp)
{
    printf("swap");
    char tmp[20];
    int x;
    for(x=0;x<=19;x++)
          {
               tmp[20]=*(ap+x);
               *(ap+x)=*(bp+x);
               *(bp+x)=tmp[20];
          }
}

tmp[20] is outside of array tmp.
Note that tmp do not need to be an array, a single char is enough, as outlined in solution 1.

You really need to learn debugger to help you learn what works and what don't.

[Update]

Quote:
someone say my swap have problem ,but i don't know which part have problem can u help me.

I don't know what is the exact problem, there was many versions of your code with many corrections suggested.

By learning programming with C, you are doing it the hard way because with C you have to handle all the details.
My advice: Learn to program with Visual Basic, it will handle most of details and you can concentrate on learning programming.
Once you know programming, you can learn specifics of C.

Here is links to references books on C and C++ by the authors of the languages. Note than C is the ancestor of C++, so knowing C is always useful with C++.
The C Programming Language - Wikipedia, the free encyclopedia[^]
https://hassanolity.files.wordpress.com/2013/11/the_c_programming_language_2.pdf[^]
http://www.ime.usp.br/~pf/Kernighan-Ritchie/C-Programming-Ebook.pdf[^]

C++ Programing Language[^]

Why it can't run I use pointer(I want to compare names)

3 solutions

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