Creating an array with variable size in c++

Question

I am new to c++ and I am trying to create a function to return a value from an array. Here are the instructions for the assignment:

In this exercise, you will create a function to return a value from an array. If the index is out of range, return 0.

Function Name: read01

Parameters: (data,size,index)

1) data: An array of constant int's

2) size: An int, the number of slots in data

3) index: An int, the desired position in data

Return Value: An int, the value of position index in data, or 0.

Here is what I tried so far:

int read01(int data[], int size, int index){
    return data[index];
}

I know how to do the "if" statement so that's not an issue, but where I am confused is how to define the size parameter as the size of data[]. I know you cannot put "int size" into the data parameter like so, "int data[int size]", but I cannot figure out another way to do it. Any help would be appreciated!

You should format the assignment as a blockquote, not as code (since it is not code). — Cody Gray
– Cody Gray ♦, Commented Sep 5, 2017 at 0:43
@M.M then what's the point of having a size parameter if it's not being used to define the size of data? — Brock Morrison
– Brock Morrison, Commented Sep 5, 2017 at 0:48
Are you asking how to set the length of the array via the size? Something like, size = 4, so The array size should be of length 4? — Sailanarmo
– Sailanarmo, Commented Sep 5, 2017 at 0:49
@BrockMorrison the "user" defining the size is not your function. The array is created outside your function and your function receives a view on it, and the length of the view is the size parameter. Your function does not create or resize any arrays, it reads a value out of an array that already exists — M.M
– M.M, Commented Sep 5, 2017 at 1:14

Anatoly · Accepted Answer · 2020-12-20 18:29:59Z

4

We can define size of array like:

int Data[5]; //here 5 is size of array or

int Data[] = {1, 2, 3, 4, 5}; //In this number of elements describe the size.

To get size of array use(standard c way: to get size of static array):

sizeof(Data) / sizeof(Data[0]);

you can also try Distance method to get size:

distance(begin(Data),end(Data));

Edit: You cant make array user defined because size of array needs to compile at compile time. So options are you can use vectors or pointers.

But in case of c++ use std::vector instead of arrays:

std::vector<int> vc;
vc.push_back(1);
vc.push_back(2);
vc.push_back(3);
std::cout << vc.size() << std::endl; // it will print 3(size of vector)

If you wanna define size dynamically then try something like:

int size;   
std::cin >> size;
int *A = new int[size];//dynamically assign memory to pointer A 
/*your code here*/
delete[] A;

Hope it will help you..

edited Dec 20, 2020 at 18:29

Anatoly

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answered Sep 5, 2017 at 1:02

Preet

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5 Comments

user4581301 Over a year ago

I don't see how any of this answers the OP's question.

Preet Over a year ago

@user4581301 if i get the question he wanna define size of array or i am missing something

Brock Morrison Over a year ago

@Preet I am asking how to set the length of the array via the size parameter... so when the the size parameter is defined as size = 4, the array size should be 4. As I stated in the comments above, this is being entered into a unit test where I have no idea what is being used as parameters in my code.

user4581301 Over a year ago

@BrockMorrison you cannot change the size of an array, nor does the assignment appear to be asking you to do so. My suspicion is you are expected to test if index is less than size ( and return the value in data at index) or return 0 if index is not less than size. Also watch out for "data: An array of constant int's" because you have not specified data to be constant in int read01(int data[], int size, int index). Read up on the const keyword.

Preet Over a year ago

sorry to miss the concept, Actually array size needs to define at compile time so you cant change it as @user4581301 said. if you wanna do this you can use std::vector

Mychsmit · Accepted Answer · 2017-09-06 17:42:35Z

1

I believe the answer would be

#include <iostream>
int read01(const int data[], int size, int index) {
    if ( index >= size ) {
        return 0;
    } else {
        return data[ index ];
    }
}

edited Sep 6, 2017 at 17:42

answered Sep 6, 2017 at 16:35

Mychsmit

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1 Comment

Brock Morrison Over a year ago

This worked perfectly! I had it returning 0 outside of the if statements and it kept throwing me an error. Thank you @mychsmit!

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