Reverse a sentence in C? - c

Question

I have just written a program which reverses a sentence whatever the user gives. For example: if the user enters "How are you", my program generates "uoy era woH".
The programme which I wrote is shown below. I just have a wild intution that there can be a smarter program than this. So valuable input from your side is most appreciated or any better program 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

You could use recursion.
int ReverseString(char *rev) {
if(*rev!='\0') {
ReverseString(rev + 1);
putchar(*rev);
}
return 1;
}

Answer 2

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 3

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 4

Yet another variation...
void ReverseString( char *str, int len ) {
int i;
for(i=0; i < len/2; i++) {
swap( &str[i], &str[len -1 -i] );
}
}
void swap( char *a, char *b ) {
char tmp = *a;
*a = *b;
*b = tmp;
}

Answer 5

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.

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

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 8

This won't 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 9

#include<stdio.h>
void reverse(char s[])
{
int i=0,j,x=0,z;
printf("\nThe string is : ");
printf("%s",s);
printf("\nThe reverse string is : ");
while(s[i] != ' ')
{
while(s[i] != ' ')
i++;
z=i+1;
for(j=i-1;j>=x;j--)
printf("%c",s[j]);
printf(" ");
i=z;
x=z;
}
}
main()
{
char s[50];
int a;
for(a=0;a<50;a++)
s[a]=' ';
puts("\nEnter a sentence : ");
fgets(s,50,stdin);
reverse(s);
}