I can find length of string, program #29 and I know what's swaping, look at following code

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


int main (void)
{
  char temp;
  char A = '1';
  char B = '2';

  printf(" A = %c \n", A );

  printf(" B = %c \n", B );

  temp = A;
  A = B;
  B = temp;

  printf(" A = %c \n", A );

  printf(" B = %c \n", B );
 
 
   return 0;
}

A = 1
B = 2
A = 2
B = 1

I'm having trouble solving problem and can't figure out how to do it.

Solving what problem?

If you are referring to the string reversal problem, you have all the ingredients already. What else do you need besides outlining the algorithm itself.

You expect us to do everything for you?

 

I'm having trouble solving problem and can't figure out how to do it.

Try something, anything, it doesn't have to work 1st time

 

Just tried the XOR... Amazing... Why didn't I see that.... Also works with add / subtract...

sizeof(); will give the length of string + termination...

 

sizeof(); will give the length of string + termination...

That only works for arrays declared on the stack. For dynamic and/or aliased arrays, something like strlen() would be required. (Better yet: pass the length of the array explicitly.)

I can find length of string, program #29 and I know what's swaping, look at following code

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


int main (void)
{
  char temp;
  char A = '1';
  char B = '2';

  printf(" A = %c \n", A );

  printf(" B = %c \n", B );

  temp = A;
  A = B;
  B = temp;

  printf(" A = %c \n", A );

  printf(" B = %c \n", B );
 
 
   return 0;
}

A = 1
B = 2
A = 2
B = 1

I'm having trouble solving problem and can't figure out how to do it.

Start with this:

void reverse(char* str, int len) {
  if (len == 0)
    len = strlen(str);
  for (int lft = 0, rgt = len; lft <= --rgt; ++lft) {
    printf("str[%d] = %c, str[%d] = %c \n", lft, str[lft], rgt, str[rgt]);
    /*
     ...reverse elements here...
    */
  }
}

Does that help?

 

That only works for arrays declared on the stack. For dynamic and/or aliased arrays, something like strlen() would be required. (Better yet: pass the length of the array explicitly.)

Has always worked for me... In fact the little program I used to "test" said XOR function used it sucessfully with a statically declared array..

In the documentation it states that the sizeof operator can be used on arrays.. BUT!! I do not disbelieve you, only if you have an example I would like to try it out... One of those gotcha type errors...

NOTE**
I have re-read the rules of sizeof() It won't work on dynamically allocated arrays as it calculates at compile time... Which is why it always works for me..

 

Has always worked for me... In fact the little program I used to "test" said XOR function used it sucessfully with a statically declared array..

In the documentation it states that the sizeof operator can be used on arrays.. BUT!! I do not disbelieve you, only if you have an example I would like to try it out... One of those gotcha type errors...

NOTE**
I have re-read the rules of sizeof() It won't work on dynamically allocated arrays as it calculates at compile time... Which is why it always works for me..

Try it with this...

char apple[100] = "Banana";

 

That's different.... No need for a sizeof() if you specify the actual array size..

char apple[] = {"banana"};
len = sizeof(apple); would work for the example given..

 

Try it with this...

char apple[100] = "Banana";

This pseudo-code resolves the banana problem and the HELLO problem

int i, j, maxswaps, maxchars;
char temp;
for ( i=0; i<100; i++) {
 if (apple[i] == 0) break;
}
maxchars=i;
maxswaps=int(float(maxchars))/2.0)
for (int i=0; i < maxswaps; i++) {
j=maxchars-(i +1);
temp=apple[j];
apple[j]=apple[i];
apple[i]=temp;
}

…or something like that.

 

There is no need to use float division as in
maxswaps=int(float(maxchars))/2.0)

Integer divide gives the desired result that works for both odd or even string length:
maxswaps = lenght(string)/2;

 

That's different.... No need for a sizeof() if you specify the actual array size..

char apple[] = {"banana"};
len = sizeof(apple); would work for the example given..

Not if the string changes...

char apple[] = "banana"; 
...
strncpy(apple, "pea", sizeof(apple));  
...
len = sizeof(apple);

...And if you can guarantee that the string won't change, you'd already know the size of the string

In any account sizeof is the wrong tool for the job.

The best option is to just use strlen() from the string.h library, or like in the case of the TS where they can't use a library function, just make your own. i.e. Count how many charactors there are before the '\0' in a loop.

 

I should also add that it is very unusual that the sizeof a buffer where a string is held is the size of the string.

However, there is a case where you are holding a particular constant string, but in that case you probably wouldn't want to reverse it (or modify it in any way)

 

There is no need to use float division as in
maxswaps=int(float(maxchars))/2.0)

Integer divide gives the desired result that works for both odd or even string length:
maxswaps = lenght(string)/2;

Right, I was being over cautious.

 

I know next to nothing about writing code, so this entire thread I find fascinating. Alas, I understood nothing . . .

 

That's different.... No need for a sizeof() if you specify the actual array size..

char apple[] = {"banana"};
len = sizeof(apple); would work for the example given..

void check(char buf[]) {
  printf("sizeof(buf) = %lu\n", sizeof(buf));
}

int main(void) {
  char buf[1024] = "Hello World!";
  printf("sizeof(buf) = %lu\n", sizeof(buf));
  printf("strlen(buf) = %lu\n", strlen(buf));
  check(buf); 
}

Output:

test.c: In function ‘check’:
test.c:49:39: warning: ‘sizeof’ on array function parameter ‘buf’ will return size of ‘char *’ [-Wsizeof-array-argument]
printf("sizeof(buf) = %lu\n", sizeof(buf));
^
test.c:48:17: note: declared here
void check(char buf[]) {
^~~
sizeof(buf) = 1024
strlen(buf) = 12
sizeof(buf) = 8

 

This is why I don't frequent this place as much as I'd like...

My example works for me, but members here try so hard to be "the better" person... All this crap does not help the op. If I use the sizeof(); as shown, it works.. Why spend a lifetime trying to putting others down... If you read the first post, he didn't even want to use library functions anyway..... And as far as I'm concerned, sizeof() is as unuseful as rocking horse poo as it only works at compile time , where (as already stated ) you already know the size.... It will only be used for structs and classes where I can't be arsed working it out.

Done here now..

 

This is why I don't frequent this place as much as I'd like...

My example works for me, but members here try so hard to be "the better" person... All this crap does not help the op. If I use the sizeof(); as shown, it works.. Why spend a lifetime trying to putting others down... If you read the first post, he didn't even want to use library functions anyway..... And as far as I'm concerned, sizeof() is as unuseful as rocking horse poo as it only works at compile time , where (as already stated ) you already know the size.... It will only be used for structs and classes where I can't be arsed working it out.

Done here now..

I'm sorry you feel that way, but there were problems with your suggestion of using 'sizeof' to determine a string's length.

Pointing out such problems is important to help the TS, as they might become confused as to why the suggested solution is not working when they (say) get a random string from the user.

I hope to see you on here again soon

 

TS had indicated that library functions are not to be used.
Fortunately, writing your own strlen( ) function is not difficult and TS has done it.

 

soluation

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

char *reverse(char *buffer, int length)
{
    char *low = buffer;               
    char *high = buffer + length - 1; 
    char  temp;

    while (low < high)
    {
        temp = *low;
        *low = *high;
        *high = temp;

        low++;
        high--;
    }

    return buffer;
}

int main(void)
{
    int i = 0;
    
    char  name[] = "54321";
    
    for (i = 0;  name[i] != '\0'; i++);

    reverse(name, i);

    printf("%s\n", name);

    return 0;
}

 

soluation

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

char *reverse(char *buffer, int length)
{
    char *low = buffer;               
    char *high = buffer + length - 1; 
    char  temp;

    while (low < high)
    {
        temp = *low;
        *low = *high;
        *high = temp;

        low++;
        high--;
    }

    return buffer;
}

int main(void)
{
    int i = 0;
    
    char  name[] = "54321";
    
    for (i = 0;  name[i] != '\0'; i++);

    reverse(name, i);

    printf("%s\n", name);

    return 0;
}

Good work. Looks like you've met some of the original criteria.

 

Good work. Looks like you've met some of the original criteria.

I am thinking of shifting it so right now I am reversing the string but insted of reversing string right now I want to reverse number.