CODEWITHSUNDEEP
cc89
Lucky Jain
Lucky Jain

Reputation: 43

Why this program loops endlessly sometimes and not always

I successfully solved a programming project from a book named "C Programming: A modern approach" by K.N. King.

The program works fine but goes into endless loop sometimes. I am not getting the reason behind this.

I used a random function which will give random values ranging from 0 to 3 and I have used sufficient conditions to handle all the four random values.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
    char walk[10][10];
    int direction,currenti=5,currentj=5;

    int i,j;
    for(i=0; i<10; i++)
    {
        for(j=0; j<10; j++)
        {
            walk[i][j]='.';
        }
    }
    srand((unsigned) time(NULL));
    for(i=0; i<26;)
    {
        direction=rand()%4;
        printf("%d ",direction);
        if(direction==0&&walk[currenti][currentj-1]=='.'&&(currentj-1)>0&&(currentj-1)<10)
        {
            currentj=currentj-1;
            walk[currenti][currentj]='A'+i;
            i++;
        }
        else if(direction==1&&walk[currenti][currentj+1]=='.'&&(currentj+1)>0&&(currentj+1)<10)
        {
            currentj=currentj+1;
            walk[currenti][currentj]='A'+i;
            i++;
        }
        else if(direction==2&&walk[currenti-1][currentj]=='.'&&(currenti-1)>0&&(currenti-1)<10)
        {
            currenti=currenti-1;
            walk[currenti][currentj]='A'+i;
            i++;
        }
        else if(direction==3&&walk[currenti+1][currentj]=='.'&&(currenti+1)>0&&(currenti+1)<10)
        {
            currenti=currenti+1;
            walk[currenti][currentj-1]='A'+i;
            i++;
        }
    }
    for(i=0; i<10; i++)
    {
        for(j=0; j<10; j++)
        {
            printf("%c ",walk[i][j]);
        }
        printf("\n");
    }
    return 0;
}

Upvotes: 0

Views: 246

Answers (3)

Thibault Martin-Lagardette
Thibault Martin-Lagardette

Reputation: 4733

Only having a quick look, my guess would be because in your main for loop, you only increment i on certain conditions, and it is likely that none of them or met, which means i++ would then not be called.

Best way to verify this would be to put an else clause where you handle the case properly, by printing a message, or putting an assert if you should never reach that case.

As a measure of precaution, when you have if/else if clauses, you should always have an else clause where you handle the "if all else fails" value, where you either handle the error properly, or assert if the code should never reach the else clause.

Also, as a side note, you should consider using spaces in order to make your code a bit easier to read, such as:

if (direction == 3 && walk[currenti + 1][currentj] == '.' && (currenti + 1) > 0 && (currenti + 1) < 10)

Upvotes: 3

dvhh
dvhh

Reputation: 4750

Because your walk is random you could enter condition when it is not possible to move further whatever the number and i would not get increased

Example of infinite loop ( your program walks from 1 to 9 )

1 2 3 . . . . . . .
8 9 4 . . . . . . .
7 6 5 . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .

At step 9 your program would get blocked whatever would be the next direction as it has blocked all its possible next move.

There are of course a lot of variant of this kind of situation, but it also mean that there are situation where your program blocked itself before performing its 26 steps.

You should detect this kind of condition to terminate your loop early.

Upvotes: 0

Serve Laurijssen
Serve Laurijssen

Reputation: 9753

When you enter a corner and the array around it contains only dots you will have an infinite loop. This happens only sometimes because of the random behaviour of course

Upvotes: 1

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