How can I deal with operation between Int and CGFloat?

In Swift you cannot multiply two numbers of different types (NSNumber, Int, Double, etc.) directly. The width of a CGRect is of floating point type and the array count is of integer type. Here's a working example:

let myArray: Int[] = [1,2,3]
let rect: CGRect = CGRect(x:0,y:0,width:100,height:100)
let total: Double = rect.size.width * Double(myArray.count)

Swift does not allow operations between two numbers of different types. Therefore, before to multiply your array.count (Int) by your width (CGFloat), you'll have to cast it to CGFloat.

Fortunately, Swift provides a simple CGFloat initializer init(_:) that creates a new CGFloat from an Int. This initializer has the following declaration:

init<Source>(_ value: Source) where Source : BinaryInteger

Creates a new value, rounded to the closest possible representation.

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The Swift 5 Playground sample code below shows how to perform your calculation by using CGFloat's initializer:

import UIKit
import CoreGraphics

// Set array and view
let array = Array(1...3)
let rect = CGRect(x: 0, y: 0, width: 100, height: 100)
let view = UIView(frame: rect)

// Perform operation
let width = view.bounds.width
let total = width * CGFloat(array.count)
print(total) // prints: 300.0

First, let's create some demo data

let array: NSArray = ["a", "b", "c"] //you could use a Swift array, too
let view = UIView() //just some view

Now, everything else works almost the same way as in Obj-C

let width: CGFloat = CGRectGetWidth(view.bounds)

or simply

let width = CGRectGetWidth(rect) //type of the variable is inferred

and total:

let total = width * CGFloat(array.count)

Note that we have to add a CGFloat cast for array.count. Obj-C would implicitly cast NSUInteger to CGFloat but Swift has no implicit casts, so we have to add an explicit one.