Generate random integer within a range (Infinite loop)

Question

I'm having problems with this code because when I run it i get an infinite loop with the number random generator. What I'm trying to do is to assign to an array , 99 numbers from 1 to 9 and then make some mathematical simple operations.

#include <stdio.h>
#include <stdlib.h>
#define SIZE 99
void mean(const int a[]);
void median( int a[]);
void mode(int freq[] , const int a[]);

int main (void) {
   int response[SIZE];
   int frequency [10];
   int i;
   srand(time(NULL));
   for (i = 0; i<= SIZE ; i++) {
      response[i] = (rand() % 6) +1 ;
      printf("%d", response[i]);
   }
   mean(response);
   median( response);
   mode(frequency , response);

return 0;
}


void mean(const int a[]){
   int j , total = 0;
   float mean;
   printf("********\n%6s\n********\n\nThe mean is the average value of the data\nitems.", "Mean");
   printf("The mean is equal to the total of\n all the data items");
   printf("divided by the number\n of data items (%d):", SIZE);
   for( j = 0 ; j <= SIZE ; j++){
      total += a[j];
   }
   mean = (float) total / SIZE;
   printf("The mean value for\nthis run is %d / %d = %f", total, SIZE, mean);
}

void median( int a[]){
   int i, j, n, median, hold;
   n=1;
   hold = 0;
   printf("********\n%7s\n********\n\nThe unsorted array of responses is\n", "Median");
   for (i=0;i<=SIZE;i++){
      if ((i/10) <= n){
         printf("%d", a[i]);
      }
      else{
         printf("\n");
         n++;
      }
   }
   printf("The sorted array is\n");
   for(i=0;i<=SIZE;i++){
      for(j=0;j<=SIZE-1;j++){
         if (a[j]>a[(j+1)]){
            hold = a[j];
            a[j] = a[ (j + 1)];
            a[ (j + 1)] = hold;
         }
      }
   if ((i/10) <= n){
         printf("%d", a[i]);
      }
      else{
         printf("\n");
         n++;
      }
   }
   median = a[SIZE/2];
   printf("The median is element %d of\nthe stored %d element array.\n", SIZE/2 , SIZE);
   printf("For this run the median is %d", median);
}

void mode ( int freq [] , const int a[]){
   int j, o, mode , i, rating;
   printf("********\n%6s\n********\n\n%10s%12s%12s", "Mode" ,"Response" ,"Frequency", "Histogram");
   for(j=0; j<= SIZE ; j++){
      ++freq[a[j]];
   }
   for (i=0 ; i <= 10 ; i++){
      printf("%10d%12d            ", i, freq[i]);
      for (o=0; o<=freq[i];o++){
         printf("*");
      }
      printf("\n");
      if (freq[i] > freq[i+1]){
         mode = freq[i];
         rating = i;
      }
   }
   printf("The mode is the most frequent value.\n");
   printf("For this run the mode is %d which occured %d times", rating ,mode);
}

Why rand() % 6 when you want a number between 1 and 9. It should be rand() % 9 + 1... — ShuklaSannidhya
– ShuklaSannidhya, Commented May 27, 2013 at 17:49
frequency array should be local to the mode function. You don't use it any where else. There's no point in declaring it in main and passing it to mode. Instead, declare it as a local to mode. — ShuklaSannidhya
– ShuklaSannidhya, Commented May 27, 2013 at 18:05

simonc · Accepted Answer · 2013-05-27 17:57:14Z

3

C arrays are zero-based so valid indexes for

int response[SIZE];

are [0..SIZE-1]. Your loop writes to response[SIZE] which is beyond the end of memory assigned for response. This results in undefined behaviour.

If you're getting an infinite loop, it sounds as if the address of response[SIZE] is the same as the address of the loop counter i. (rand() % 6) +1 will be in the range [1..6] so the final iteration of the loop before exit will always reset i to a lower value.

You can fix this by changing your loop to exit one iteration sooner. i.e. Change

for (i = 0; i<= SIZE ; i++) {

to

for (i = 0; i< SIZE ; i++) {

Note that your other functions all have similar bugs. All for loops should replace their <= exit conditions with <

edited May 27, 2013 at 17:57

answered May 27, 2013 at 17:50

simonc

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2 Comments

ShuklaSannidhya Over a year ago

So why does that result in infinite loop?

simonc Over a year ago

@ShuklaSannidhya I've updated my answer. Let me know if its clearer now.

Jens · Accepted Answer · 2013-05-27 17:57:30Z

1

You write past the end of your arrays when you access array[SIZE]. Any array declared

type_t array[SIZE];

doesn't have an element array[SIZE]. So all loops must be from 0 to < SIZE, not <= SIZE. This is known as an off-by-one error in computer literature. You're not the first and will not be the last, if it's any consolation :-)

Technically this invokes undefined behavior, of which an infinite loop is one way. But see my comment below for a wild-assed guess at what is really happening here.

edited May 27, 2013 at 17:57

answered May 27, 2013 at 17:51

Jens

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4 Comments

ShuklaSannidhya Over a year ago

Any reason for why does that result in infinite loop.?

Jens Over a year ago

Very likely because in some function (mean(), median(), mode()) where you have a[] as the last argument, the first declared local variable is a loop variable i or j. Writing past a[] is then clobbering the loop variable.

P.P Over a year ago

That's not the only problem. Even fixing off-by-one won't stop infinite looping. There are many problems in the code.

Jens Over a year ago

@KingsIndian It stops being infinite here, but prints 6406162 asterisks, so yes, there is at least one more bug.

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