二进制运算符-不能应用于字符串类型的操作数！ 和诠释

Question

I am trying to write the following line of code in swift:

 objManager.userObject.free_credit_counter = [NSString stringWithFormat:@"%ld", [objManager.userObject.free_credit_counter integerValue]-1];

i tried :

    objManager.userObject.free_credit_counter = objManager.userObject.free_credit_counter - 1

but I'm getting this error :

Binary operator - cannot be applied to operands of type String! and Int

Answer 1

Your objManager.userObject.free_credit_counter output value is String and you have to convert it to Int before use - operator

if let count = Int(objManager.userObject.free_credit_counter) {
    objManager.userObject.free_credit_counter = String(count - 1)
}

Answer 2

the error says that you are trying to do subtraction by 1 from a string value , basically which is not possible.

So, first make the bellow string to int

objManager.userObject.free_credit_counter

and then subtract by 1

Answer 3

To have exactly the same code written in Swift you should do:

objManager.userObject.free_credit_counter = NSString(format: "%ld", objManager.userObject.free_credit_counter.integerValue - 1)

In your original code you have this bit:

[objManager.userObject.free_credit_counter integerValue]

It converts the NSString value of free_credit_counter to an integer, which is then decremented by 1, and then the result is converted back to NSString using a format mask.

In the Swift code that you have posted in the question, the conversion to integer is missing. That's what the error tells you about