使用C从字符串中删除连续的重复字符

Question

I'm trying to remove consecutive repeated characters from a given string.

Example:

bssdffFdcrrrtttii ***#

output is supposed to be:

bsdfFdcrti *#

This code doesn't work and only prints the first char (b), I want to learn about my mistake. when I'm doing a printf test, it works but not for spaces. I think the problem might be with the new char array.

void Ex6() {
    char* string[80];
    scanf("%s", &string);
    puts(removeDup(string));
}

char* removeDup(char *string) {
    int i, c = 0;
    char* newString[80];
    for (i = 0; i < strlen(string); i++) {
        if (string[i] != string[i + 1]) {
            newString[c++] = string[i];
        }
    }
    return newString;
}

Answer 1

There are several problems with your program:

• The declaration of newString should be char newString[80] , ie, an array of characters and not an array of pointers-to-characters, and likewise for the declaration in Ex6 .
• The call to scanf should then be scanf("%s", string) , since string is already the address of an array of characters, but...
• Use fgets to read a string from the user to ensure that you read whitespace, if it's important, and that the buffer is not exceeded.
• newString is allocated on the stack and so should not be returned to the caller. It is better to do a char *newString = strdup(string) , or, slightly less sloppy, char *newString = malloc(strlen(string)+1) , which will call malloc for a block of memory sufficient to hold the original string, and thus the version without duplicates -- the comments rightly point out that this could be optimized. In principle, the caller, ie, Ex6 , must free the returned pointer to avoid a memory leak but it hardly matters in such a short program.
• The result needs a null terminator: newString[c] = '\\0' .

Otherwise, the removeDup function seems to work correctly.

So, putting all of that together:

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

char* removeDup(const char *string)
{
    size_t i, c = 0;
    size_t string_len = strlen(string);
    char *newString = malloc(string_len + 1);

    for (i = 0; i < string_len; i++) {
        if (string[i] != string[i + 1]) {
            newString[c++] = string[i];
        }
    }
    newString[c] = '\0';

    return newString;
}

#define MAX_STRING_LEN 80

void Ex6() {
    char string[MAX_STRING_LEN];
    char* result;

    if (fgets(string, MAX_STRING_LEN, stdin) != NULL) {
        result = removeDup(string);

        printf("%s", result);
        free(result);
    }
}

Finally, I agree with @tadman's comment. Since the input string must anyway be traversed to calculate the length, we may as well optimize the size of the result string:

char* removeDup(const char *string)
{
    size_t i, c = 0;
    char *newString;

    for (i = 0; string[i] != '\0'; i++)
        c += (string[i] != string[i + 1]);

    newString = malloc(c + 1);

    for (i = c = 0; string[i] != '\0'; i++) {
        if (string[i] != string[i + 1]) {
            newString[c++] = string[i];
        }
    }
    newString[c] = '\0';

    return newString;
}

Answer 2

There are quite a few issues in your program. It wouldn't even compile let alone run. Also, the most problematic issue is that you are returning a pointer to a local variable from a function that ceases its scope upon completion. A simplified version of your program is as follows:

void Ex6() 
{
   char string[80];
    scanf("%s", string);
        int i, c = 0;
    char newString[80];
    for (i = 0; i < strlen(string); i++) {
        if (string[i] != string[i + 1]) {
            newString[c++] = string[i];
        }
    }
    newString[c] = '\0';
    puts(newString);
}

Answer 3

You can do it with O(n) time and O(1) space, by modifying existing string:

#include <stdio.h>

char* removeDup(char* input) {
        char* newTail = input, *oldTail = input;
        while (*oldTail) {
            if (*newTail == *oldTail) {
                ++oldTail;
            } else {
                *++newTail = *oldTail++;
            }
        }
    return newTail;
}

int main() {
   char string[] = "bssdffFdcrrrtttii ***#";
   char* newEnd = removeDup(string);
   char* tmp = string;
   while (tmp != newEnd) {
       printf("%c", *tmp++);
   }
   //Print the last char if string had any duplicates
   if(*tmp) {
       printf("%c", *tmp++);
   }
   return 0;
}