How to print a word backwards in C

Question

I though setting up a veritable len with strlen() function to find the last char, print it, and then decrement it by 1. It is not working in my code:

#include <stdio.h>
#include <string.h>
#define SIZE 4
int main(void)

{
    int index;
    char wordToPrint[SIZE];
    printf("please enter a random word:\n");
    for (index = 0; index < SIZE; index++)
    {
        scanf("%c", &wordToPrint[index]);
    }

    int len = strlen(wordToPrint);
    for (index = 0; index < SIZE; index++)
    {
        printf("%c", wordToPrint[len]);
        --len;
    }

    return 0;
}

input is "nir"

output is:

??
r

What is wrong in the last block?

tnx

"it is not working" is the best error description you can come up with?! — Kerrek SB, Jan 24 '13 at 09:27

unwind · Answer 1 · 2013-01-24T10:16:32.277

C arrays index from zero.

Think about how this affects the relationship between the length of a string, and the index of the last character.

Also, your way of reading in the word is very strange, it should just be:

scanf("%s", wordToPrint);

or, better:

fgets(wordToPrint, sizeof wordToPrint, stdin);

There's no need to loop and read a character at a time. The above will give you different lengths depending on the amount of input.

The second suggestion will not stop at whitespace though, so if you go with that you should probably replace word with line for clarity.

score 2 · Answer 2 · edited May 23 '17 at 12:27

2

Reverse iteration needs to be offset by one:

Forward:

for (size_t i = 0; i != N; ++i) { putchar(word[i]); }

Backward:

for (size_t i = 0; i != N; ++i) { putchar(word[N - i - 1]); }
//                                                  ^^^^

This is because an array of length N has valid indices in the range [0, N).

edited May 23 '17 at 12:27

Community

1
1

answered Jan 24 '13 at 09:28

Kerrek SB

464,522
92
875
1,084

score 2 · Answer 3 · answered Jan 24 '13 at 09:35

In addition to what other posters are saying, you should probably consider giving and extra byte to the string:

char wordToPrint[SIZE + 1];

and then set

wordToPrint[4] = '\0';

In the case that someone inputs a 4 letter word (such as 'blue') your string will look like { 'b', 'l', 'u', 'e' } but with no room for a null character.

Strlen and other functions rely on finding a null value at the end.

score 1 · Answer 4 · answered Jan 24 '13 at 09:30

int len = strlen(wordToPrint);
for (index = 0; index < SIZE; index++)
{
    printf("%c", wordToPrint[len]);
    --len;
}

return 0;

suppose you have word otto so int len = strlen(wordToPrint); will set len to 4, but when you use it in printf("%c", wordToPrint[len]); your array is getting out of bound wordToPrint[4] where as the last index in array is wordToPrint[3] as it starts from 0 index

score 1 · Answer 5 · answered Jan 24 '13 at 10:36

write your code like this because at wordToPrint[len], len is the +1 then the last index of the wordToPrint[] array so its showing garbadge/null value

#include <stdio.h>
#include <string.h>
#define SIZE 4
int main(void)

{
    int index;
    char wordToPrint[SIZE];
    printf("please enter a random word:\n");
    for (index = 0; index < SIZE; index++)
    {
        scanf("%c", &wordToPrint[index]);
    }

    int len = strlen(wordToPrint);
    for (index = 0; index < SIZE; index++)
    {
        --len;
        printf("%c", wordToPrint[len-1]);
    }

    return 0;
}

score 1 · Answer 6 · answered Jan 24 '13 at 11:20

This is the whole program, for your question.

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

char *strrev(char *); // Prototype

/* The following function is by me, to reverse a string, because strrev is not available in GCC */

char *strrev(char *str)
{
      char *p1, *p2;

      if (! str || ! *str)
            return str;
      for (p1 = str, p2 = str + strlen(str) - 1; p2 > p1; ++p1, --p2)
      {
            *p1 ^= *p2;
            *p2 ^= *p1;
            *p1 ^= *p2;
      }
      return str;
}

/*================================================================Begin Main======================================================================================*/

int main()
{
    char sentence[100], rev_sentence[100], c;

    int j = 0, i = 0, m = 0;

    sentence[i] = ' ';                 // The first char in the sentence should be a space to reverse this first word
    i++;
    printf("Enter a sentence : ");
    while((c = getchar()) != '\n')
    {
        sentence[i] = c;
        i++;
    }
    sentence[i] = '\0';


    printf("Reversed word is: ");


    for(i = strlen(sentence) - 1 ; i >= 0; i = i - 1)
    {
         if(sentence[i] == ' ')
        {
            rev_sentence[j] = '\0'; // because strrev fun reverses string ntil it encounters a first \0 character
            strrev(rev_sentence);
            printf("%s ", rev_sentence);
            for(m = 0; m < 100; m++)
            rev_sentence[m] = 0;
            j = 0;
            continue;
        }
        rev_sentence[j] = sentence[i];
        j++;
    }

    rev_sentence[j] = '\0';

}

This program reverses entire sentence, or a word if u enter just 1 word and press enter, hope u like it and understand it

score 0 · Answer 7 · answered Jan 24 '13 at 09:28

This code is wrong:

int len = strlen(wordToPrint);
for (index = 0; index < SIZE; index++)
{
    printf("%c", wordToPrint[len]);
    --len;
}

Its printing chars from 4 to 1 but you want to print chars from 3 to 0.

You should do it this way:

for (index = len - 1; index >= 0; index--)
    printf("%c", wordToPrint[index ]);

score 0 · Answer 8 · answered Jan 24 '13 at 09:31

0

In addition to the other answers, why use strlen() when you know that the word has SIZE chars?

answered Jan 24 '13 at 09:31

anumi

3,869
2
22
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I'm guessing his point was to give a max length, so "tar" is still a valid input – h4lc0n Jan 24 '13 at 09:38
Yes, but then the input loop has also a problem. And there is no null character at the end of the string to apply strlen(). – anumi Jan 24 '13 at 09:41

score 0 · Answer 9 · answered Jan 24 '13 at 10:14

Here is a working code based on the posted one:

#include <stdio.h>
#include <string.h>
#define SIZE 4
int main(void)

{
int index;
char wordToPrint[SIZE];
printf("please enter a random word:\n");
//Why you need a loop to read a word?
//for (index = 0; index < SIZE; index++)
//{
//    scanf("%c", &wordToPrint[index]);
//}

scanf("%s",wordToPrint);

int len = strlen(wordToPrint);

//Modify the loop like this:
for (index = len; index >= 0 ; index--)
{
    printf("%c", wordToPrint[len]);  
}
printf("\n");//Add this line for clearing the stdout buffer and a more readable output.
return 0;
}

Note: its always a good practice to memset() or bzero() the array before using it.

score 0 · Answer 10 · answered Feb 15 '18 at 23:06

Lil' hack for fun :)

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

#define LENGTH 32

int main(void)
{
  // allocate a zero-filled buffer of LENGTH bytes
  // we add LENGTH so that we start writing right *after* the buffer
  char *word = LENGTH + calloc(LENGTH, 1);

  // 0xFF will limit our buffer so that we do not write out of bounds
  *(word - LENGTH) = 0xFF;

  // order is important: we are decrementing the pointer and **then**
  // reading a character from the standard input, we compare it to the
  // line feed character so that we stop reading characters when return
  // key is pressed
  while ((*--word = fgetc(stdin)) != '\n' && *word != 0xFF);
  printf("The word is: %s", word);

  // tidy up!
  free(word);
  return 0;
}

Of course, this won't work as expected in "modern" systems, that implement line-buffered consoles rather than allowing the user to manually read one character at a time (albeit it can be implemented using platform-specific APIs.)

How to print a word backwards in C

10 Answers10