Numbers positive and negative

Question

What am i doing wrong? I want to have a random numbers. Negative and positive integers

#include <iostream>
#include <cstdlib>
int main(){
   int a;
   cin<<a;
   int t[a];
   for(int i=0;i<a;i+=1)
      t[a]=rand();
   for(int i=0;i<a;i++)
      cout>>t[i];
}

Im very beginner and I wanted to have random numbers but i have only one very big. Can anybody help?

Sorry, but you see problems which i haven't. Now i know i should only write >>endl

EDIT: I corrected my code and it works now

#include <cstdlib>
#include <time.h>
int main(){
   int a;
   cin>>a;
   for(int i=0;i<a;i++)
      cout<<(rand()-rand())<<endl;//i asked only for this "endl"
}

Also, not even trying to help by posting the errors you get. — chris, Apr 27 '13 at 22:20
There are too many errors in your code. Please post a self-contained compilable example reproducing your problem. — juanchopanza, Apr 27 '13 at 22:39

jrok · Answer 1 · 2013-04-27T22:28:23.470

2

You haven't seeded the random number generator. Call srand() at the start of your program.

The usual way is to call it with whatever current time is:

std::srand(std::time(0))

You'll need to include <ctime>, too.

Note that rand() doesn't return negative values, that logic you'll need to implement by yourself.

edited Apr 27 '13 at 22:28

answered Apr 27 '13 at 22:21

jrok

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score 1 · Accepted Answer · answered Apr 27 '13 at 22:16

1

cin<<a;

Here, you're using the wrong operator. Think of the angle brackets as showing the flow of data. You want:

cin>>a;

Similarly with cout:

cout<<t[i];

You are also indexing t with a, rather than with i, in the first for loop:

t[i]=rand();
//^ This is i, not a

Another problem is that you are using a compiler extension to create an array of run-time size. That is, with standard C++ you cannot use a as the size of your array since it is not known at compile time.

answered Apr 27 '13 at 22:16

Joseph Mansfield

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Ok i did it. But that wasnt the problem. I just write it here wrong. Cout too – Antua Apr 27 '13 at 22:17
@Antua What? Did that fix it? – Joseph Mansfield Apr 27 '13 at 22:19
Yes, it does. I still have big number – Antua Apr 27 '13 at 22:20
@Antua Well `rand` can give a big number... Have you looked up what [`rand`](http://en.cppreference.com/w/cpp/numeric/random/rand) does? You need to seed it if you want different values each time. – Joseph Mansfield Apr 27 '13 at 22:21
@Antua Have you tried putting a newline between each outputted number? As in `cout << t[i] << '\n';`? – Joseph Mansfield Apr 27 '13 at 22:24

score 1 · Answer 3 · answered Apr 27 '13 at 22:19

1

t[i]=rand();

not

t[a]=rand();

or did you write that wrong too?

answered Apr 27 '13 at 22:19

john

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If you want negative numbers then you just got to subtract something from the value returned by rand(). rand() only returns positive numbers. – john Apr 27 '13 at 22:25

score 1 · Answer 4 · answered Apr 27 '13 at 22:35

I'm not sure at 100% but IMO, rand() can create only positive numbers, from 0 to 65535, depending on the time that the executable was compiled: this mean that once you had compiled the .exe file, it will give the same numbers over and over again!

In order to make him produce different numbers you have to use srand().

Another thing: In C++ you can't allocate a dynamic array with type array[n] [it isn't in the standard!], even if the compiler allows you to do it! Actually the compiler translate your code in array = new type[n] that you have to deallocate with the delete[] keyword.

Numbers positive and negative

4 Answers4