Reverse a sentence in C??

Question

Hi, I had just written a programme which reverses a sentence whatever the user gives. for eg:If the user gives the sentence as "How are you",my programm generates "uoy era woH". The programme which i had written is shown below.I just have a wild intution that there can be a smarter programe than the one which i had written.So valuable inputs from your side is most appreciated or any better programme than this is also most welcome.

int ReverseString(char *);

main()
{
    char *Str;
    printf("enter any string\n");
    gets(Str);
    ReverseString(Str);
    getch();
}

int ReverseString(char *rev)
{
    int len = 0;
    char p;
    while(*rev!='\0')
    {
        len++;
        rev++;
    }
    rev--;
    while(len>0)
    {
        p = *rev;
        putchar(p);
        rev--;
        len--;
    }
}

Thanks a lot

Answer 1

This wont work. Should allocate memory for your sentence.

    char *Str;
   printf("enter any string\n");
   gets(Str);

should be>

char str[81]={0};
printf("Enter any string up to 80 characters\n");
scanf("%80s\n",str);
ReverseString(str)

Besides, you should avoid gets function. It leads to buffer overflows

Answer 2

You could use recursion.

         int ReverseString(char *rev)
         {
            if(*rev!='\0')
            {
               ReverseString(rev + 1);
               putchar(*rev);
            }

            return 1;
         }

Answer 3

void ReverseString( char* str, int len )
{
    if( len > 1 )
    {
        swap( &str[0], &str[len - 1] );
        ReverseString( ++str, len - 2 );
    }
}

Or, unrolling the tail-recursion:

void ReverseString( char* str, int len )
{
    while( len > 1 )
    {
        swap( &str[0], &str[len - 1] );
        ++str;
        len -= 2;
    }
}

Where swap is defined as:

void swap( char* a, char* b )
{
    *a ^= *b;
    *b ^= *a;
    *a ^= *b;
}

If you use this though, your TA's will definitely know you didn't figure this out yourself :)

Answer 4

Okay, here is my function. I wrote it a while ago, just for practice.

char* reverse(char *string){

    int length = 0;
    int half = 0;

    length = strlen(string);
    half = (length/2) - 1;
    --length;

    int i = 0;
    register char interim;
    for(; i<=half; ++i){
        interim = string[i];
        string[i] = string[length - i];
        string[length - i] = interim;
    }

    return string;

}

now that I look at it, I'm less proud of it than when I got it to work. I'm just posting it because you asked me to post it when I found it--and for completeness' sake.

After looking at some other answers I realize that the calculation of half the string is unnecessary and I could have just decremented length until i and length were equal. Oh well--here it is.

Also, please don't bash me for the use of the register keyword :P

Answer 5

If you don't know the length of the string:

void reverse_string(char* str)
{
    char* p2 = str;
    while (*p2 != '\0')
    {
     /* assumes the string is null-terminated, will fail otherwise */
     ++p2;
    }
    --p2;
    char* p1 = str;
    while (p1 < p2)
    {
     char tmp = *p1;
     *p1 = *p2;
     *p2 = tmp;
     ++p1;
     --p2;
    } 
}

If you do:

void reverse_string(char* str, const size_t len)
{
    if (len <= 1)
    {
     return;
    } 
    char* p2 = str + len - 1; 
    char* p1 = str;
    while (p1 < p2)
    {
     char tmp = *p1;
     *p1 = *p2;
     *p2 = tmp;
     ++p1;
     --p2;
    } 
}

Answer 6

The following program prints its arguments in reverse character order:

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

char * reverse(char * string)
{
    char * a = string;
    char * b = string + strlen(string) - 1;

    for(; a < b; ++a, --b)
        *a ^= *b, *b ^= *a, *a ^= *b; // swap *a <-> *b

    return string;
}

int main(int argc, char * argv[])
{
    for(int i = 1; i < argc; ++i)
        puts(reverse(argv[i]));
}

Nothing new here, but imo more readable than most other answers.

Answer 7

void revstr(TCHAR *str)
{
    if( *str == '\0' )
    {
        return;
    }

    TCHAR *start = str;
    TCHAR *end = start + strlen(str) - 1;

    while(start < end)
    {
        *start ^= *end;
        *end ^= *start;
        *start ^= *end;
        *start++;
        *end-–;
        /*

        could also use *start ^= *end ^= *start++ ^= *end–-; if you want to get fancy
        */
    }
}

Stolen from the 2005 version of myself, but screw that guy, he slept with my wife.

Yes, I know I don't need some of the '*'s, but I wrote the one-liner first and just converted it, and the one-liner does require them.