Function to remove spaces from string/char array in C

Question

The question asked here is very similar to what I am having a problem with. The difference is that I must pass an argument to a function that removes the spaces and returns the resulting string/char array. I got the code working to remove the spaces but for some reason I am left with trailing characters left over from the original array. I even tried strncpy but I was having lots of errors.

Here is what I have so far:

#include <stdio.h>
#include <string.h>
#define STRINGMAX 1000                                                      /*Maximium input size is 1000 characters*/

char* deblank(char* input)                                                  /* deblank accepts a char[] argument and returns a char[] */
{
    char *output=input;
    for (int i = 0, j = 0; i<strlen(input); i++,j++)                        /* Evaluate each character in the input */
    {
        if (input[i]!=' ')                                                  /* If the character is not a space */
            output[j]=input[i];                                             /* Copy that character to the output char[] */
        else
            j--;                                                            /* If it is a space then do not increment the output index (j), the next non-space will be entered at the current index */
    }
    return output;                                                          /* Return output char[]. Should have no spaces*/
}
int main(void) {
    char input[STRINGMAX];
    char terminate[] = "END\n";                                             /* Sentinal value to exit program */

    printf("STRING DE-BLANKER\n");
    printf("Please enter a string up to 1000 characters.\n> ");
    fgets(input, STRINGMAX, stdin);                                         /* Read up to 1000 characters from stdin */

    while (strcmp(input, terminate) != 0)                                   /* Check for que to exit! */
    {
        input[strlen(input) - 1] = '\0';
        printf("You typed: \"%s\"\n",input);                                /* Prints the original input */
        printf("Your new string is: %s\n", deblank(input));                 /* Prints the output from deblank(input) should have no spaces... DE-BLANKED!!! */

        printf("Please enter a string up to 1000 characters.\n> ");
        fgets(input, STRINGMAX, stdin);                                     /* Read up to another 1000 characters from stdin... will continue until 'END' is entered*/
    }
}

Answer 1

After removing the white spaces from the input you have not terminated it with nul-terminator ( \\0 ) because the new length is less than or equal to the original string.

Just nul-terminate it at the of end your for loop:

char* deblank(char* input)                                         
{
    int i,j;
    char *output=input;
    for (i = 0, j = 0; i<strlen(input); i++,j++)          
    {
        if (input[i]!=' ')                           
            output[j]=input[i];                     
        else
            j--;                                     
    }
    output[j]=0;
    return output;
}

Answer 2

You're not terminating the output, and since it might have shrunk, you're leaving the old tail in there.

Also, I would suggest that the treatment of j , which is always incremented in the loop and then has to be manually decremented if the current character is not copied, to be somewhat sub-optimal. It's not very clear, and it's doing pointless work (incrementing j ) which even has to be undone when it's not desired. Quite confusing.

It's easier written as:

char * deblank(char *str)
{
  char *out = str, *put = str;

  for(; *str != '\0'; ++str)
  {
    if(*str != ' ')
      *put++ = *str;
  }
  *put = '\0';

  return out;
}

Answer 3

As others mentioned, same string is used for both source and destination, and a end of string is not maintained.

You could do in the following way also.

char* deblank(char* input)                                                  /* deblank accepts a char[] argument and returns a char[] */
{
    char *output;
    output = malloc(strlen(input)+1);

     int i=0, j=0;
    for (i = 0, j = 0; i<strlen(input); i++,j++)                        /* Evaluate each character in the input */
    {
        if (input[i]!=' ')                                                  /* If the character is not a space */
            output[j]=input[i];                                             /* Copy that character to the output char[] */
        else
            j--;                                                            /* If it is a space then do not increment the output index (j), the next non-space will be entered at the current index */
    }

    output[j] ='\0';
    return output;                                                          /* Return output char[]. Should have no spaces*/
}

Answer 4

You have to return the string after adding the null(\\0) terminator after the for loop block

char* deblank(char* input)                                                  
{
char *output=input;
for (int i = 0, j = 0; i<strlen(input); i++,j++)                        
{
    if (input[i]!=' ')                                                  
        output[j]=input[i];                                             
    else`enter code here`
        j--;                                                            
}
output[j]='\0';
return output;                                                          
}

Answer 5

If you need to filter more than one character at a time, you might find something like:

char *FilterChars(char *String,char *Filter){
  int a=0,i=0;
  char *Filtered=(char *)malloc(strlen(String)*sizeof(char));
  for(a=0;String[a];a++)
    if(!strchr(Filter,String[a]))
      Filtered[i++]=String[a];
  Filtered[i]=0;
  return Filtered;
}

Useful; just provide a list of characters in *Filter you wish to strip out. For example "\\t\\n ", for tabs, newlines and spaces.

Answer 6

This code works with time complexity of O(n).

char str[]={"my name    is Om"};
int c=0,j=0;
while(str[c]!='\0'){
    if(str[c]!=' '){
        str[j++]=str[c];
    }
    c++;
}
str[j]='\0';
printf("%s",str);