Why is this reversing sentence algorithm not working? - c

Question

There are some comments in the code for human-readable code:
#include <stdio.h>
#include <string.h>
#define SIZE 100 //size of the input array and output array
#define ACCUM_CHAR_SIZE 25 //size of the temp array
int main(){
char i[SIZE];
char acc[ACCUM_CHAR_SIZE];
char o[SIZE];
int it_l = 0, it_a = 0, it_r = 0;
//it_l is the iterator to the input sentence,
//it_a is the iterator to the temp array
//it_r is the iterator to the output sentence
printf("Enter a sentence:");
gets(i);
int len = strlen(i) - 1;
while(it_l <= len){
if(i[len - it_l] != ' '){
acc[it_a] = i[len - it_l]; //add letters to acc until space
it_a++;
}
else{
it_a -= 1;
//acc is reversed, I reversed it again to the output sentence
while(it_a >= 0){
o[it_r] = acc[it_a];
it_r++;
it_a--;
}
it_r += 1;
o[it_r] = 32; //put a space
it_a = 0; //reset the temp array
strcpy(acc, ""); //clear the acc
}
it_l++;
}
printf("%s", o);
}
The program theoretically looks fine, but when it is executed, it sometimes print garbage values, only some words, or sentence which has only reversed half with garbage value instead of spaces.
The program above is to save each word to the temp, and reverse temp (temp is reversed when storing the word) back to the output. However, it fails.
Thank you for your help.

Answer 1

#include <stdio.h>
#include <string.h>
#define SIZE 100 //The size of input array is prefered to be equal to ouput array.
int main(){
char input[SIZE];
char output[SIZE];
int i = 0, j = 0;
//i is the iterator to the input sentence,
//j is the iterator to the output sentence
printf("Enter a sentence:");
gets(input);
int len = strlen(input) - 1; //Total length.
j = len;
while(input[i]!= NULL){
output[j] = input[i];
i++;
j--;
}
output[len+1]= NULL;
printf("%s", output);
printf("Finished");
}

Answer 2

There are at least three problems.
The first problem is that you never terminate the string o To do that change:
printf("%s", o);
into
o[it_r] = '\0';
printf("%s", o);
The second problem is that you increment it_r incorrectly. Change
it_r += 1;
o[it_r] = 32; //put a space
into
o[it_r] = ' '; // Use ' ' instead of 32
it_r += 1;
The third problem is that you don't handle the first word of the input (because there is no space in front). I'll leave that problem to you as an exercise.
BTW: Don't use gets for reading input. Use fgets instead.

Answer 3

Try the modified code given below, the changed sections has been commented(all other comments were removed for readability)
#include <stdio.h>
#include <string.h>
#define SIZE 100
#define ACCUM_CHAR_SIZE 25
int main(){
char i[SIZE];
char acc[ACCUM_CHAR_SIZE];
char o[SIZE];
int it_l = 0, it_a = 0, it_r = 0;
printf("Enter a sentence:");
gets(i);
int len = strlen(i) - 1;
while(it_l <= len){
if(i[len - it_l] != ' '){
acc[it_a] = i[len - it_l];
it_a++;
}
else{
it_a -= 1;
while(it_a >= 0){
o[it_r] = acc[it_a];
it_r++;
it_a--;
}
/*it_r += 1; There is no need to increment it_r here as
it is already incremented in the above loop*/
o[it_r] = 32;
it_r += 1; /* The above increment statement was moved here(only
incremented when a new value is loaded in to output array) */
it_a = 0;
strcpy(acc, "");
}
it_l++;
}
/* The below section reverses and stores the first word of the
input which was not getting operated in the above loop */
it_a -= 1;
while(it_a >= 0){
o[it_r] = acc[it_a];
it_r++;
it_a--;
}
o[it_r] = '\0'; // Terminating output array
printf("%s", o);
}
The above code will work as expected but has some small issues(given below)
Using gets():- gets is not recommended for inputting strings. See this link for more info.
The Size of temporary array acc :- If a word greater than 25 characters is inputted the program may output garbage value.

Answer 4

Version without temporary storage.
read the input string from start (0)
copy any word from input into output, starting from the end of output - words are not reversed
copy any space in output, from position in output, starting from the end
Code
#define SIZE 100 //size of the input array and output array
int main(){
char i[SIZE];
char o[SIZE];
printf("Enter a sentence: ");
fgets(i, SIZE, stdin); // fgets instead of gets
int j,len = strlen(i) - 1; // assuming you eat the EOL character
o[len] = '\0'; // start by marking end of output string
for(j=0 ; j<len ; j++) { // navigating the input string
if (i[j] == ' ') {
o[len-j-1] = i[j]; // if space, copy (from end) in output
}
else {
int k=j;
do {
k++; // count(+1) the word, non-space, characters
} while (k<len && i[k] != ' ');
strncpy(&o[len-k], &i[j], k-j); // copy the word
j = k-1;
}
}
printf("%s\n", o);
}