Reversing a string in C using loop [duplicate]

Question

Beginner programmer here. I'm trying to take an input from user, reverse it and show the result. For some reason, it's printing blanks instead of the reversed string. I know that array[i] has the right information because if I use this loop on line for (int i=0; i<count; i++), it's printing the right characters. It's just not printing in reverse. What am I not getting here?

#include <stdio.h>
#include <cs50.h>
#include <string.h>

int main(void)
{
    printf("Please enter a word: ");
    char *word = get_string();

    int count = strlen(word);

    char array[count];

    for (int i=0; i< count; i++)
    {
        array[i] = word[i];
    }

    for (int i=count-1; i==0; i--)
    {
        printf("%c ", array[i]);
    }
    printf("\n");
}

Your condition is wrong. It should be: for (int i=count-1; i>=0; i--) — P.P
– P.P, Commented Feb 23, 2017 at 20:22
What's the point of copying the input verbatim someplace else? Why not just print it from where it is? — David Schwartz
– David Schwartz, Commented Feb 23, 2017 at 20:23
If you want to reverse the string your first for loop has to change. One array counts up the other counts down — Brian from state farm
– Brian from state farm, Commented Feb 23, 2017 at 20:26
@P.P. thank you, I don't know why it didn't come to my mind. — ahmed_imtiaz
– ahmed_imtiaz, Commented Feb 23, 2017 at 20:41
@DavidSchwartz I did all in one array at first, broke it down to debug it better. — ahmed_imtiaz
– ahmed_imtiaz, Commented Feb 23, 2017 at 20:42

StoryTeller - Unslander Monica · Accepted Answer · 2017-02-23 20:29:28Z

1

for (int i=0; i< count; i++)
{
    array[i] = word[i];
}

You go over the string and copy it, you do not reverse it.

There is also a subtle bug in-waiting in your declaration of array, since you do not leave space for the '\0' character terminator. Passing your buffer to printf as a C-string, as opposed to character by character will have undefined behavior.

So to fix those two particular errors:

char array[count + 1];
array[count] = '\0';

for (int i = 0; i< count; i++)
{
    array[i] = word[count - i];
}

As a side note, it may not mean much to use a VLA for this small exercise, but for larger inputs it could very well overflow the call stack. Beware.

edited Feb 23, 2017 at 20:29

answered Feb 23, 2017 at 20:21

StoryTeller - Unslander Monica

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9 Comments

Jean-François Fabre Over a year ago

and you forgot the null termination character (and the buffer is too short by 1)

StoryTeller - Unslander Monica Over a year ago

@Jean-FrançoisFabre - did I? If this is why you down-voted, you really are trigger happy

David Schwartz Over a year ago

@Jean-FrançoisFabre He's not using a termination character and his buffer is not too short. The buffer holds characters whose length is known, not a C-style string. There is no need to allocate an extra byte.

StoryTeller - Unslander Monica Over a year ago

@Jean-FrançoisFabre - Easy enough mistake to make, a downvote followed by a comment from you not 15 seconds later. Apologies.

StoryTeller - Unslander Monica Over a year ago

@Jean-FrançoisFabre - It's a bit of a chicken and egg thing. I usually improve in iterations until I'm satisfied. But I can't prove it's not the DV that made me do it. And regardless, the fact the DV remains strongly suggests it's not a factual DV.

|

cmdLP · Accepted Answer · 2017-02-23 20:46:31Z

0

// the header where strlen is
#include <string.h>

/**
 * \brief reverse the string pointed by str
**/
void reverseString(char* str) {
    int len = strlen(str);
    // the pointer for the left and right character
    char* pl = str;
    char* pr = str+len-1;
    // iterate to the middle of the string from left and right (len>>1 == len/2)
    for(int i = len>>1; i; --i, ++pl, --pr) {
        // swap the left and right character
        char l = *pl;
        *pl = *pr;
        *pr = l;
    };
};

And just call the function:

int main(void) {
    printf("Please enter a word: ");
    char *word = get_string();

    // Just call the function. Note: the memory is changed, if you want to have the original and the reversed just use a buffer and copy it with srcpy before the call
    reverseString(word)
    printf("%s\n", word);
};

And just change

char array[count];

for (int i=0; i< count; i++)
{
    array[i] = word[i];
}

to

// add an other byte for the null-terminating character!!!
char array[count+1];
strcpy(array, word);

edited Feb 23, 2017 at 20:46

answered Feb 23, 2017 at 20:28

cmdLP

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2 Comments

Jean-François Fabre Over a year ago

code without any explanation isn't gonna help a bit. You should post assembly code while you're at it.

Jean-François Fabre Over a year ago

okay but there are tons of code snippets which do that on SO (follow my duplicate link). The OP wants to know the problems in his code.

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