CODEWITHSUNDEEP
ccontrol-flow
Togeri
Togeri

Reputation: 40

Control flow understanding

I'm having a lot of trouble understanding the following program flow: 

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

int main ()
{
 FILE *fp;
 int index = 0;
 char word[45];
 int words = 0;

 fp = fopen("names.txt","r");
 if(fp == NULL) 
 {
   perror("Error in opening file");
   return(-1);
 }


 for (int c = fgetc(fp); c != EOF; c = fgetc(fp))
 {
     // allow only alphabetical characters and apostrophes
     if (isalpha(c) || (c == '\'' && index > 0))
     {
         // append character to word
         word[index] = c;
         index++;

         // ignore alphabetical strings too long to be words
         if (index > 45)
         {
             // consume remainder of alphabetical string
             while ((c = fgetc(fp)) != EOF && isalpha(c));

             // prepare for new word
             index = 0;
         }
     }

     // ignore words with numbers (like MS Word can)
     else if (isdigit(c))
     {
         // consume remainder of alphanumeric string
         while ((c = fgetc(fp)) != EOF && isalnum(c));

         // prepare for new word
         index = 0;
     }

     // we must have found a whole word
      else if (index > 0)
     {
         // terminate current word
         word[index] = '\0';

         // update counter
         words++;

         //prepare for next word
         printf("%s\n", word);
         index = 0;
     }
     //printf("%s\n", word);
 }

 printf("%s\n", word);

 fclose(fp);
 return(0);
}

As you can see, it's just a plain program that stores characters from words into an array, back-to-back, from a file called 'names.txt'.

My problem resides in the else if(index > 0) condition. I've run a debugger and, obviously, the program works correctly.

Here's my question:

On the first for-loop iteration, index becomes 1. Otherwise, we wouldn't be able to store a whole word in an array.

If so, how is it possible that, when the program flow reaches the else if (index > 0) condition, it doesn't set word[1] to 0? (Or the subsequent values of index).

It just finishes the whole word and, once it has reached the end of the word, then it proceeds to give word[index] the value of 0 and proceed to the next word.

I've tried reading the documentation, running half of the program and asking with echo, and running a debugger. As it should be, everything runs perfectly, there's no problem with the code (as far as I know). I'm the problem. I just can't get how it works.

PS: sorry if this might be so trivial for some of you, I'm really starting to learn programming and I find sometimes really hard to understand apparently so simple concepts.

Thank you very much for you time guys.

Upvotes: 0

Views: 320

Answers (2)

Mike Nakis
Mike Nakis

Reputation: 61986

Note the else in the beginning of the else if (index > 0) condition.

This means that it will only execute if none of the previous if() and else if() executed.

The previous if() and else if() statements do keep executing if the character is alphanumeric, or a non-leading slash, so the last else if() only executes once a non-alphanumeric, or leading slash is encountered.

Upvotes: 1

lordingtar
lordingtar

Reputation: 1052

As soon as something is executed in the if...else block, it moves out of the block. So if it satisfies the first if condition, the else if condition is not even checked. So if index > 0 AND c=\ or c is an alphabet, it runs the if statement, and if even one of these conditions does not hold true, it will move to the else if portions of the block.

Upvotes: 1

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