Question

I’m fairly new to C programming but trying my best to understand it. I have two dynamic strings that are populated from two plain text files. One being a form of a dictionary, and the other one just a user input. What I want to get is binary search each user input word in the dictionary and find out if it is present (sort of a spell checker I guess).

I’m stuck on my binary search function:

char **dictElem;
int dictSize;
char **inputElem;

int binsearch(const char *val){
  int pos;
  int beg=0;
  int end=dictSize-1;
  int cond=0;

  while (beg<=end){
    pos=(beg+end)/2; //Jump in the middle
    if ((cond=strcmp(dictElem[pos],val)) == 0)
      return pos;
    else if (cond<0)
      beg=pos+1;
    else
      end=pos-1;
  }
  return 0;
}

Both dictElem and inputElem were already read by other methods and (let’s say) both [0] elements are equal strings "aa".

However after I run the binsearch(inputElem[0] it always returns 0. I tried just strcmp(dictElem[0],inputElem[0]) and it returns 1.

Where am I going wrong? Is it comparing char** and char*?

UPD:
Function that’s loading the dictElem

void readd(FILE *file){
  int i=0,size=0; /* local size */
  char line[1024]; /* Local array for a single word read */
  printf("Loadingn dict...\n");
  while ((fgets(line,sizeof(line),file))!=NULL){
    dictElem=(char**)realloc(dictElem,(size+1)*sizeof(char *));
    dictElem[size++]=strdup(line);
  }
  printf("Total elements loaded: %d\n",size);
}

Function that reads a user file is very similar, just a little different format.

Answer 1

The problem with your code is in this line if ((cond=strcmp(dictElem[pos],val) == 0)). This line of code assigns the result of expression strcmp(dictElem[pos], val) == 0 to the variable cond, and then checks whether cond is zero or not.
I guess your original intent was to store in cond the result of strcmp, so you should move the closing parenthesis before ==. The correct line is if ((cond = strcmp(dictElem[pos], val) == 0).

There are some other problems with your code:

1. 0 is used as special not-found value, but in the same time 0 can be
   returned when element is found at index 0.
2. Using char *val, when
   it is better to use const char *val, because contents of this
   string aren’t going to be modified. It is always better to write const-correct code.