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# i want to generate such number of random numbers as i want, but seems like it dont work properly

Posted on 2004-08-20
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here is the sample of my program, the M is the number of random numbers i want
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define X 100
#define M 10

main()
{
int m[X][X];
int i, j, s = 0, ran;

srand(time(NULL));

for(i = 0; i < X + 1; i++)
{
for(j = 0; j < X + 1; j++)
{
printf("m[%d][%d]\n", i, j);
while(s < M)
{
ran = rand() % 10 + 1;
if(ran == 1)
{
m[i][j] = 1;
s = s + 1;
printf("m[%d][%d] = %d\n", i, j, m[i][j]);
}
}
}
}

}
0
Question by:quinnyoom
• 4
• 2
• 2

LVL 22

Expert Comment

You need to initialize the array m[] to whatever value you want for untouched cells.

Your for loops are going one step too far I think.

for each cell, you're looping until you get 10 times a random number == 1, then setting that cell to 1.   Doesnt make much sense.

What do you really want to do?

0

LVL 16

Expert Comment

>>    for(i = 0; i < X + 1; i++)
>>     {
>>          for(j = 0; j < X + 1; j++)

Your loop limit is too high! You should use:

for(i = 0; i < X; i++)
{
for(j = 0; j < X; j++)

You have forgotten to intitialise m[i][j] so I would also suggest:

for(i = 0; i < X; i++)
{
for(j = 0; j < X; j++)
{
m[i][j] = 0;  // *** Add this! (as grg99 suggested).

Otherwise, this looks OK.

m[i][j] will contain a '1' if any of the M random numbers (between 1 and 10) generated was equal to '1'.

I hope this isnt homework or coursework because I am not allowed to give you code in this case.

Paul
0

LVL 16

Expert Comment

Err!!! I just noticed another one! You need to reset 's' to zero each time around or use a for loop.

Change:

>>               while(s < M)

to

for(s = 0; s < M; s++)

and remove the:

>>                       s = s + 1;

Thinking about it, this doesnt seem like a terribly useful process, I'm thinking more and more that this is homework. Sorry sunny, my mistake.

Paul
0

LVL 16

Expert Comment

If you change:

>>                         m[i][j] = 1;

to

m[i][j] += 1;

you get a much more meaningful array.

Paul
0

LVL 22

Expert Comment

The inner while loop is definitely "funny", the logic right now has it loop indefinitely, until the random number generator has given out ten "1"'s.

If you move the s = s + 1 to outside the if(), then you get a somewhat more useful loop, it sets each element to "1" if and only if yo get a "1" in 10 tries.   Still not terribly interesting, but better than before.

0

Author Comment

its not a homework ^^ just i m thinking, normal
for(i = 0; i < X; i++)
{
for(j = 0; j < X; j++)
{
ran = rand() % 10 + 1;
if(ran == 1)
{
m[i][j] = 1;
printf("m[%d][%d] = %d\n", i, j, m[i][j]);
}
}
}
}
we cant figure out how much m[x][y] = 1 will the program generate
and now i want that i can set M m[x][y] = 1(= i want to generate M points that mark with "1", but must place random within the 100 x 100) as i want, like below?

for(i = 0; i < X; i++)
{
for(j = 0; j < X; j++)
{
while(s < M)
{
ran = rand() % 10 + 1;
if(ran == 1)
{
m[i][j] = 1;
s = s + 1;
printf("m[%d][%d] = %d\n", i, j, m[i][j]);
}
}
}
}
0

LVL 16

Accepted Solution

PaulCaswell earned 500 total points
>>we cant figure out how much m[x][y] = 1 will the program generate

rand() % 10 + 1 should generate numbers from 1 to 10 at random so one in ten should be set to '1' the rest will be left alone. Unfortunately, because you are not clearing the array first, all the rest may be random values. If you use:

if(ran == 1)
{
m[i][j] = 1;
s = s + 1;
printf("m[%d][%d] = %d\n", i, j, m[i][j]);
}
else
{
m[i][j] = 0;
}

you will get a more reliable result.

>>and now i want that i can set M m[x][y] = 1(= i want to generate M points that mark with "1", but must place random within the 100 x 100) as i want, like below?

If this means what I think it means then perhaps:

>>               while(s < M)

should become

if(s < M)

but this would only fill the first ten that would normall be set.

If you really want ten random entries set to '1' then use something like:

// Clear the array.
for(i = 0; i < X; i++)
for(j = 0; j < X; j++)
m[i][j] = 0;
// None set.
s = 0;
// Set ten random ones to 1.
while ( s < M )
{
// Pick a random location.
i = rand()%X;
j = rand()%X;
// Has it already been set?
if ( m[i][j] == 0 )
{
// No! Set it.
m[i][j] = 1;
// And remember to count it.
s += 1;
}
}

Paul
0

Author Comment

thx for the advice, i can figure out how the random work well now
0

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