CODEWITHSUNDEEP
c++arrays
user1234567800
user1234567800

Reputation: 13

How to define size of an array by passing a variable in [] in C++?

My code is given below. I want to declare an array of size n.

FILE *fp;
fp=fopen("myfile.b", "rb");
if(fp==NULL){ fputs("file error", stderr); exit(1); }
int* a;
fread(a, 0x1, 0x4, fp);
int n=*a;
int array[n];  // here is an error

How can I declare an array of size n in this code?

Upvotes: 0

Views: 143

Answers (5)

Ross Wang
Ross Wang

Reputation: 27

Array only accept const object or expressions, the value can be decided by the compiler during compiling, vector from c++ is more suitable from this case, otherwise we need to dynamically allocate memory for it.

Upvotes: 0

AndersK
AndersK

Reputation: 36082

Since your code looks more like C...

FILE *fp = fopen("myfile.b", "rb");

if(fp==NULL)
{ 
  fputs("file error", stderr); 
  exit(1); 
}

//fseek( fp, 0, SEEK_END ); // position at end
//long filesize = ftell(fp);// get size of file
//fseek( fp, 0, SEEK_SET ); // pos at start

int numberOfInts = 0;
fread(&numberOfInts, 1, 4, fp); // you read 4 bytes sizeof(int)=4?
int* array = malloc( numberOfInts*sizeof(int) );

Upvotes: 0

Ashalynd
Ashalynd

Reputation: 12563

You cannot declare an array of variable size in C++, but you can allocate memory once you know how much you need:

int* a = new int[n];

//do something with your array...

//after you are done:

delete [] a;

Upvotes: 0

CodeJuan
CodeJuan

Reputation: 36

int *array = (int*)malloc( n * sizeof(int) );
//..
//your code
//..
//..
free(array);

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409136

That is a declaration of a variable-length array and it's not in C++ yet.

Instead I suggest you use std::vector instead:

std::vector<int> array(n);

You also have other problems, like declaring a pointer but not initializing it, and then using that pointer. When you declare a local variable (like a) then its initial value is undefined, so using that pointer (except to assign to it) leads to undefined behavior. In this case what will probably happen is that your program will crash.

Upvotes: 4

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