reverse a word using recursion - c

Question

I am trying to reverse a word using recursion in c and i got upto this.
#include<stdio.h>
void reverse(char *a){
if(*a){
reverse(a+1);
printf("%c",*a);
}
}
int main() {
char a[] = "i like this program very much";
reverse(a); // i ekil siht margorp yrev
return 0;
}
Suppose the input string is i like this program very much. The function should change the string to much very program this like i
Algorithm:
1) Reverse the individual words, we get the below string.
"i ekil siht margorp yrev hcum"
2) Reverse the whole string from start to end and you get the desired output.
"much very program this like i"
I have successfully completed up-to step 1 and i am not sure how to proceed further. Please help.

Answer 1

To help you out without giving too much away, I think your algorithm is too complicated. Instead of reversing individual words and then reversing the whole string, consider splitting the string into an array of strings with space as a delimiter. Then simply reverse the array.
The way to do this in C is a little bit odd; since you already use char arrays as strings, you actually need to make an array of char arrays. For example:
char a[] = "i like this program very much";
actually means
a = ['i', ' ', 'l', 'i', 'k', 'e', ' ', ... , 'c', 'h']
So, you want to create an array of char arrays so it now looks like this:
new_array = [['i'], ['l', 'i', 'k', 'e'], ... ['m', 'u', 'c', 'h']]
Then all you need to do is print the new array backwards, and you've got the output you want!

Answer 2

#include <stdio.h>
#include <ctype.h>
void swap(char *a, char *b){
char wk = *a;
*a = *b;
*b = wk;
}
void strpartrev(char *top, char *tail){
while(top < tail)
swap(top++, tail--);
}
void step1(char *a){
while(isspace(*a))
++a;
if(!*a)
return;
char *top =a;
while(a[1] && !isspace(a[1]))
++a;
char *tail = a;
strpartrev(top, tail);
step1(a+1);
}
int step2(char *str, int pos){
char ch = str[pos];
return (ch == '\0')? 0 : ((str[pos=step2(str, ++pos)]=ch), ++pos);
}
void reverse(char *a){
step1(a);
step2(a, 0);
}
int main() {
char a[] = "i like this program very much";
reverse(a);
puts(a);//much very program this like i
return 0;
}

Answer 3

The program for outputting the words in reverse has a very similar structure to the program you already have. Instead of making a recursive call at each individual character, though, make a recursive call at each word. Then instead of printing the character after your recursive call, you print the word. The recursion stops when you have reached the end of the string, indicating you have found the last word.
In pseudo-code:
reverse_words(string a)
start_of_word = a;
a = end_of_word(a);
if (a is not at end of string)
reverse_words(a)
print start_of_word up to a

Answer 4

#include <stdio.h>
#include <string.h>
// copy substring of str from [start] to [first occurance of ' '] into buf
void substr(char *str, int start, char* buf){
int end, strLength = strlen(str);
end = start;
do{
if(str[end-1] == ' ' || end == strLength){
strncpy(buf, str+(start*sizeof(char)), end-start);
buf[end-start] = '\0';
return;
}
end++;
}while(1);
}
void reverse(char *a){
if(strlen(a)==0){
return;
}
if(*a){
reverse(a+1);
printf("%c",*a);
}
}
void rec(char *str, int start){
if(start >= strlen(str)){
return;
}
char buf[32];
substr(str, start, buf);
rec(str, start + strlen(buf)+1);
reverse(buf);
}
int main(int argc, char* argv[]) {
char *str = "asd123 qwe kutu chu-chu";
rec(str, 0);
printf("\n");
return 0;
}