Check for balanced parentheses in an expression in Swift - ios

Question

I am trying to write this in swift as I could not find any solution written in swift but seem to have hit a roadblock with this implementation.
On the pop and push function, I am not sure if Character is the right return/argument.
class BalancedParam {
struct stack {
var top = -1
var items = [Character]()
mutating func push(_ x: Character) {
items.append(x)
if top == 99 {
print("Stack full")
} else {
top += 1
items[top] = x
}
}
mutating func pop() -> Character {
if top == -1 {
print("Underflow error")
return "0"
} else {
let element = items[top]
top -= 1
return element
}
}
mutating func isEmpty() -> Bool {
return (top == -1) ? true : false
}
}
static func isMatchingPair(_ character1: Character, _ character2: Character) -> Bool {
if character1 == "(" && character2 == ")" {
return true
} else if character1 == "{" && character2 == "}" {
return true
} else if character1 == "[" && character2 == "]" {
return true
} else {
return false
}
}
static func areParenthesisBalanced(_ exp: [Character]) -> Bool {
// Declare an empty character stack
var st = stack()
for i in 0..<exp.count {
if exp[i] == "{" || exp[i] == "(" || exp[i] == "[" {
st.push(exp[i])
}
if exp[i] == "}" || exp[i] == ")" || exp[i] == "]" {
if st.isEmpty() {
return false
} else if !isMatchingPair(st.pop(), exp[i] ) {
return false
}
}
}
if st.isEmpty() {
return true //balanced
} else {
//not balanced
return false
}
}
}
let exp: [Character] = ["{", "(", ")", "}", "[", "]"]
if BalancedParam.areParenthesisBalanced(exp) {
print("Balanced ")
} else {
print("Not Balanced ")
}
I will appreciate any help in understanding what I am doing wrong. Thanks

Answer 1

You can not assign to an empty array, i.e items[top] = x, you can only use this syntax if the array is of size top+1 or larger
So this is not ok
var arr = [Int]()
arr[0] = 1
but this is
var arr = [Int]()
arr.append(0)
arr[0] = 1
Another option since you have a cap on your array size is to init an array of that size and then continue to use the original syntax
var arr = Array(repeating: 0, count: 99)
arr[0] = 1