I am looking for a way to dynamically set the size of an integer array depending on the passed parameter. For example this in pseudocode:
int MyFunction(int number)
{
int myarr[amount of digits in number];
}
So when the input is 13456 then the int array[] size should be 5.
Whats the quickest way to do this in C++, when I don't know the constant for the size?
You cannot create an array with run-time size, it must be known at compile time. I would recommend a std::vector instead.
One solution would be to count the characters after converting to a string
#include <string>
int MyFunction(int number)
{
std::vector<int> myarr(std::to_string(number).size());
}
Mathematically, you can take the log (base 10) to find the number of digits in a number.
#include <cmath>
int MyFunction(int number)
{
int numDigits = static_cast<int>(std::log10(number)) + 1;
std::vector<int> myarr(numDigits);
}
An additional option you could do is to avoid using an array altogether by accessing the digits of the number directly:
unsigned int getDigit(unsigned int number, unsigned int index) {
// See http://stackoverflow.com/questions/4410629/finding-a-specific-digit-of-a-number
}
unsigned int setDigit(unsigned int number, unsigned int index, unsigned int newVal) {
// intPower is from the question linked to above.
return number - get(number, index)*intPower(10, index) + newVal*intPower(10, index);
}
unsigned int size(unsigned int number) {
// See http://stackoverflow.com/questions/1306727/way-to-get-number-of-digits-in-an-int
}
unsigned int push_back(unsigned int number, unsigned int newDigit) {
// Assuming no overflow
return 10*number + newDigit;
}
unsigned int pop(unsigned int number) {
// Assume number != 0
return number / 10;
}
This lets you treat your number as an array without actually initializing the array. You can even turn this into a class and use operator overloading to get actual array semantics.
You may use vector instead - actually I think the best option in this case. Create a vector with some initial size. Then you may dynamically increase it -
int initialSize = 5;
vector<int> myvector(initialSize, 0); //hold "initialSize" int's
// and all initialized to zero
myvector[0] = 567; // assign values like a c++ array
With my gcc version 4.6.3 the following is possible:
int MyFunction(int number)
{
int myarr[int(ceil(log(number)))];
....
return 0;
}
Edit: C99 this is valid see: Array size at run time without dynamic allocation is allowed?
int myarr[runtime_value]is a VLA extension, prefer to usestd::vectoranyway.log10may help, or simplestd::vector::push_backdigit by digit.sizeof, something as simple asint myarr[11];would be enough in most situations (for 4 byte signed integers). This certainly is the quickest way!