Need help with binary search

Hello all, I am currently writing a program that deals with binary searching.To give you some background on the program, a file is opened, loaded into an array.From there, the string the user enters is passed to a function that searches to see if that word exists within the file. Now, I can't seem to get it to work. It's returning false everytime the function executes.here is the code I have

Code:

---


int first=0;
int last =0;
word findword;
last=dictionary.size-1;
int middle=0;
int position=-1;
 

findword.found=false;
 
  while(findword.found==false && first<=last)
 {
  middle=(first+last)/2;
  if (strcmp(s,dictionary.words[middle].letters)==0)
 
 
    {
    findword.found=true;
    position=middle;
    return true;
    }
  else if (dictionary.words[middle].letters>s)
  {
    last=middle-1;
  }
  else
  first=middle+1;
  }
  return false;
  }

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Provide all the relevant code, and post the real code, not some snippet you dredged out of your memory. Make sure it's properly indented and formatted, so we can actually read it.

sorry...that should be better...I hope

Your function lost me. Try it like this:

Code:

---

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

int binarySearch(char words[6][30], char target[6]);

int main(void) { 
  int i;
  char words[6][30] = {
  {"Alfa"},{"Bravo"},{"Charlie"},{"Delta"},{"Echo"},{"Foxtrot"}};
  char target[30];

  printf("Enter the word you are searching for: ");
  fflush(stdout);
  scanf("%s",target);
 
  i=binarySearch(words,target);

  if(i>-1)  //zero is a legitimate array index
      printf("Found it, index %d in the array\n",i);
  else
      printf("Target word: %s was not found in the list\n",target);
 
 

  return 0;
}
int binarySearch(char words[6][30], char target[30]) {
  int i, lo=0,hi=5,mid;
 
  while(lo<=hi) {
      mid = (lo + hi)/2;
      i=strcmp(words[mid],target);
              //helpful for debugging:
              //printf("i:%d  mid:%d  words[mid]: %s\n",i,mid,words[mid]); getchar();
      if(i>0) {
        hi=mid-1;
      }else if(i<0) {
        lo=mid+1;
      }else
        return mid; //target was found, this is it's index
  }
  return -1; //target was not found

}

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Adak has a nice example for you, that includes a fix to the problem you had, but just to be clear about why your code wasn't working:

Code:

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else if (dictionary.words[middle].letters>s)

---

That is not a valid comparison. You don't compare strings with ==, !=, <, <=, > or >=. You should be looking at the result of strcmp to see if one string is greater than the other. Save the result in a variable (to avoid calling it multiple times) and use that in your if/else if statements. Something like:

Code:

---

int comp_result = strcmp(s, dictionary.words[middle].letters);
if (comp_result == 0)
    // found it
else if (comp_result < 0)  //s is less than the dictionary word
    last = middle - 1;
else
    first = middle + 1;

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