C loop won't exit - c

Question

Good-morning one and all!
This is going to end up being one of those blindingly-easy questions in hindsight, but for the life of me I'm stumped. I'm going through some of the exercises in The C Programming Language, and I've managed to write some code to initialize a loop. After some Googling, I found better ways of initializing a loop to 0, but I don't understand why the loop that I wrote to do it doesn't finish. I've used the debugger to find out that it's because the 'c' variable never reaches 50, it gets to 49 and then rolls over to 0, but I can't figure out why it's rolling over. The code is attached below, does anyone know what's going on here?
#include <stdio.h>
#define IN 1
#define OUT 0
/* Write a program to print a histogram of the lengths of words in
itsinput. */
main()
{
int c=0;
int histogram[50]={0}
int current_length=0;
int state=OUT;
//Here we borrow C so we don't have to use i
printf("Initializing...\n");
while(c<51){
histogram[c] =0;
c=c+1;
}
c=0;
printf("Done\n");
while( (c=getchar()) != EOF){
if( (c==32 || c==10) && state==IN ){
//End of word
state=OUT;
histogram[current_length++];
}else if( (c>=33 && c<=126) && state==OUT ){
//Start of word
state=IN;
current_length=0;
}else if( (c>=33 && c<=126) && state==IN ){
//In a word
current_length++;
} else {
//Not in a word
//Example, " " or " \n "
;
}
}
//Print the histogram
//Recycle current_length to hold the length of the longest word
//Find longest word
for( c=0; c<50; c++){
if( c>histogram[c] )
current_length=histogram[c];
}
for( c=current_length; c>=0; c--){
for( state=0; state<=50; state++){
if( histogram[c]>=current_length )
printf("_");
else
printf(" ");
}
}
}

Answer 1

It's because histogram[c] = 0 writes past the histogram memory when c = 50. So essentially histogram[50] overwrites c and makes it 0.
This happens because arrays start from 0 in C. So the last valid index in a 50-element array is 49.
Technically, while interesting and exploitable you can't rely on this. It's a manifestation of undefined behavior. The memory could easily have another layout causing things to "just work" or do something funnier.

Answer 2

histogram has 50 elements: from index 0 to index 49.
You attempt to write to index 50. ALL BETS ARE OFF
do
while (c < 50)
or, to avoid magic constants
while (c < sizeof histogram / sizeof *histogram)

Answer 3

You are accessing elements 0 to 50 in histogram, which only contains elements 0 to 49 (C/C++ use zero-indexing, so the maximum element of an array will always be size-1).
To avoid errors like this, you could define the histogram size as a constant, and use that for all operations relating to the histogram array:
#define HISTOGRAM_SIZE 50
Or (only works for C99 or C++, see below comment):
const int HISTOGRAM_SIZE = 50;
Then:
int histogram[HISTOGRAM_SIZE];
And:
while(c<HISTOGRAM_SIZE)
'#define' is a C-preprocessor statement, and will be processed before compilation. To the compiler, it will just look as if you've written 50 everywhere where HISTOGRAM_SIZE is used, so you wont get any extra overhead.
'const int' gives you a similar solution, which in many cases will give the same result as with the define (I'm not 100% certain under which circumstances though, others are free to elaborate), but will also give you the added bonus of type-checking.