Replace multiple spaces by single space in C

I would do something like this:

void replace_multi_space_with_single_space(char *str)
{
    char *dest = str;  /* Destination to copy to */

    /* While we're not at the end of the string, loop... */
    while (*str != '\0')
    {
        /* Loop while the current character is a space, AND the next
         * character is a space
         */
        while (*str == ' ' && *(str + 1) == ' ')
            str++;  /* Just skip to next character */

       /* Copy from the "source" string to the "destination" string,
        * while advancing to the next character in both
        */
       *dest++ = *str++;
    }

    /* Make sure the string is properly terminated */    
    *dest = '\0';
}

Of course, the above function requires you to properly terminate the string, which you currently do not.

What the function above does, is basically copy the string over itself. The exception is when there is a space, when multiple spaces are simply discarded.

Since the function modifies the source string, it can not be used on string literals.

if(input[i]==' ' && (input[i+1]==' ' || input[i-1]==' '))

case " 1 3" : when i == 0 accses input[i-1] Out-of-Bounds

scanf("%d",&n);

remain newline, (input[0] <-- '\n')

fix to

scanf("%d%*c",&n);

#include <stdio.h>

char* uniq_spc(char* str){
    char *from, *to;
    int spc=0;
    to=from=str;
    while(1){
        if(spc && *from == ' ' && to[-1] == ' ')
            ++from;
        else {
            spc = (*from==' ')? 1 : 0;
            *to++ = *from++;
            if(!to[-1])break;
        }
    }
    return str;
}

int main(){
    char input[]= "  abc   de  f  ";

    printf("\"%s\"\n", uniq_spc(input));//output:" abc de f "
    return 0;
}

Why make it more complicated than it needs to be? You can use strtok to check for single whitespaces and just ignore those. Then you can use strcat to concatenate the string into a full sentence and then you're done.

This is how I did it:

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

int main(void) {
    char *pch;
    char sentence[1000];
    char without[1000];

    printf("Sentence: ");
    fgets(sentence,1000, stdin);
    strtok(sentence, "\n"); // remove any newlines
    pch = strtok(sentence, " ");
    while(pch != NULL) {
      strcat(without, pch);
      strcat(without, " \0");
      pch = strtok(NULL, " ");
    }
    without[strlen(without)-1] = '\0'; // remove extra whitespace at the end
    printf("|%s|\n",without);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
void remove_blanks(char* s);

int main()
{
    char const s[] = {'1',' ',' ','2',' ',' ','3'};
    remove_blanks(s);
    printf("%s",s);
    return 0;
}

void remove_blanks(char* s){
    int i=0, delta=0, cnt=0;

    for (i=0;s[i];++i){
        if (s[i]==' ') cnt++;
        if (cnt>1){
            delta+=1;
            cnt=0;
        }
        s[i-delta]=s[i];
        if(delta>0) s[i]='\0';

    }
}

You cant try this simple code:

#include <stdio.h>

#define IN 1
#define OUT 0

int main() {
  int c, spaces, state;

  spaces = 0;
  state = OUT;  
  while ((c = getchar()) != EOF) {

      if ( c == ' ') {
           ++spaces;
           state = OUT;
       }
      else if (state == OUT) {
          state = IN;
          spaces = 0;
      }
      if (c == ' ' && spaces > 1 && state == OUT)
          c = 0;            
      putchar(c);
  }
  return 0;
}

You have to fix the following for loop. the limit of your for loop should be z and not n

for(j=i;j<n;j++)
input[j]=input[j+1];

by

for(j=i;j<z;j++)
input[j]=input[j+1];

BTW: the fist charachter get by your scanf() (which read charachters) is newline (\n). this newline come from the first scanf() of decimal(%d)

#include<stdio.h>
#include<string.h>
int main(void)
    {
        char input[1000];
        int i=0;
        gets(input); 
        for(i=0;input[i]!='\0';i++)
        {
            if(input[i]!=' ' || input[i+1]!=' ')
                printf("%c",input[i]);
        }
        return 0;
    }

Simplest way, using two methods from string.h library:

void strRemoveBlanks(char* _s){
    char* ptr = NULL;
    while ((ptr = strstr(_s, "  ")) != NULL)
        memmove(ptr, ptr + 1, strlen(ptr));
};