Bizarre output in random word generator - c

Question

I am facing some problems with the output of the following program. The program randomly generate words, first a random consonant then a random vowel, and then the same again for the maximum number of letters you want.
When I print the word generated right after it is generated it gives me a list of words, as intended. When I am out of the word generating and stacking loop, and print an output of the words again, I get all the words mashed up in one long chain, it is really bizarre. What is wrong?
#include <stdio.h>
#define MAXWORDS 10
#define MAXLETTERS 6
int lcg(int Xcur) /*linear congruential generator*/
{
int A = 445, C = 700001, M = 2097152;
int Xnext = (A * Xcur + C) % M;
return Xnext;
}
int main()
{
int x;
printf("input seed for linear congruential generator: ");
scanf("%d",&x);
char consonants[]="bcdfghjklmnprstvwxz";
char vowels[]="aeiou";
int i,j;
int turn;
char words[MAXWORDS][MAXLETTERS];
for(i=0;i<MAXWORDS;i++)
{
turn=1;
for(j=0;j<MAXLETTERS;j++,turn++)
{
x=lcg(x); /* random number generated */
if(turn%2) /* one consonant, one vowel, in turn */
words[i][j]=consonants[x%19];
else
words[i][j]=vowels[x%5];
}
words[i][j]='\0';
/* print each word generated */
printf("word %d: %s\n", i+1, words[i]);
}
/* print the first word again */
printf("\n\nthe 1st word again: %s\n",words[0]);
return 0;
}
output:
input seed for linear congruential generator: 23
word 1: wuduca
word 2: navozo
word 3: depiza
word 4: jukiti
word 5: raliwi
word 6: danila
word 7: cexewi
word 8: bamohu
word 9: jiruzi
word 10: temomo
the 1st word again: wuducanavozodepizajukitiraliwidanilacexewibamohujiruzitemomo

Answer 1

It is because is is getting stored in continuous manner without any null char.
Method 1:
If you just want to print it properly you can do it by writing below line:
printf("\n\nthe 1st word again: %s\n",words[0]);
as
printf("\n\nthe 1st word again: %.*s\n",MAXLETTERS, words[0]);
Method 2:
Declare words as
char words[MAXWORDS][MAXLETTERS+1];
and write the inside for loop as
for(j=0;j<MAXLETTERS;j++,turn++)
{
x=lcg(x); /* random number generated */
if(turn%2) /* one consonant, one vowel, in turn */
words[i][j]=consonants[x%19];
else
words[i][j]=vowels[x%5];
}
words[i][MAXLETTERS] = '\0';

Answer 2

There is no room in the array for the string terminator. Consequently writing the '\0' terminator is undefined behaviour. In practice (since a 2D array is contiguous), the terminator was written to the next string, which was then overwritten by its data.
Try changing
char words[MAXWORDS][MAXLETTERS];
to
char words[MAXWORDS][MAXLETTERS+1];

Answer 3

Also,
Watch out for int overflow.
Code does not insure 0 <= Xcur <= (INT_MAX - C)/A, so A * Xcur + C results can then exceed INT_MAX...
int A = 445, C = 700001, M = 2097152;
int Xnext = (A * Xcur + C) % M;
... resulting in UB: likely negative values can then return from the function.
x=lcg(x);
words[i][j]=consonants[x%19];
x%19 results then in -18 ... 18 leading to bad indexing.
Simple fix, yet does not address the root of the issue.
//Use %19u so the result is never < 0, unlike %19
words[i][j]=consonants[x%19u];
I would also use unsigned math to avoid UB of overflow and not generate negative results.
int lcg(int Xcur) /*linear congruential generator*/
{
unsigned A = 445, C = 700001, M = 2097152;
int Xnext = (A * Xcur + C) % M;
return Xnext;
}
Code also has issue with 16-bit int.