i am trying to print an integer property multiplied by another number as follows:
NSLog(#"ReceivedData: %i", (self.receivedData*10));
but Xcode highlights that line in red and says:invalid operands
I tried the following
NSInteger *newVal = #([self receivedData] * 10);
NSLog(#"ReceivedData: %i", (int) self.receivedData);
but I receive invalid operands error
receivedData is NSInteger*
how to solve it
You are not multiplying an NSInteger with a number, you are multiplying a pointer to an NSInteger with a number, which is not allowed, because multiplying a pointer by a number is nonsense.
99.99% chance that receivedData shouldn't be an NSInteger* but an NSInteger. NSInteger is not an object.
The line
NSInteger *newVal = #([self receivedData] * 10);
shows that you should go back to basics and actually learn the programming language that you are using.
Related
I have a query regarding floating value increment in loop.
I have following code
float add = 1.02f;
float counter = 0.0f;
for (int i = 0; i < 20; i++) {
counter += add;
NSLog(#"%f",counter);
}
While executing this loop I am getting following result
1.020000
2.040000
3.060000
4.080000
5.100000
6.120000
7.140000
8.160000
9.180000
10.200001
11.220001
12.240002
13.260002
14.280003
15.300003
16.320004
17.340004
18.360004
19.380005
20.400005
Here is expected result
1.020000
2.040000
3.060000
4.080000
5.100000
6.120000
7.140000
8.160000
9.180000
10.200000
11.220000
12.240000
13.260000
14.280000
15.300000
16.320000
17.340000
18.360000
19.380000
20.400000
Why i am getting some floating point in loop without adding it.
I need to loop more then 1000 times. And I want the value in float variable.
Thanks in advance.
This happens because float cannot represent the values that you have with exact precision. There are two simple ways of fixing this:
Represent the number as 100 times the target value, and use integers - 1.02 becomes 102, 2.04 becomes 204, and so on.
Use NSDecimalNumber to represent your numbers - Unlike float, NSDecimalNumber can represent all your values with full precision.
Here is how to implement the first approach:
int add = 102;
int counter = 0;
for (int i = 0; i < 20; i++) {
counter += add;
NSLog(#"%d.%d", counter/100, counter%100);
}
Here is how to implement the second approach:
NSDecimalNumber add = [NSDecimalNumber decimalNumberWithString:#"1.02"];
NSDecimalNumber counter = [NSDecimalNumber zero];
for (int i = 0; i < 20; i++) {
counter = [counter decimalNumberByAdding:add];
NSLog(#"%#", counter);
}
Why i am getting some floating point in loop without adding it.
Because float is a binary type that doesn't represent decimal values exactly. Rather than trying to explain completely and correctly, let me point you to the well-known paper What Every Computer Scientist Should Know About Floating Point Arithmetic.
Floating point number representations in computers are approximations, they are not exact. Sometimes you end up trying to display a number that can't be exactly represented in the computer's floating point number implementation, so it gives you an approximation. Also you get small arithmetic errors from repeated multiplications, additions, etc. of floating point numbers. The best you can do is to use doubles, which have more precision than floats do. In special circumstances, you could also represent your data in a different format and just change how you display it to the user to fit what they expect. For example, when working with dollars and cents, you could just store a total as a number of cents (which would be only an integer) and then format it to be shown as dollars and cents correctly for the user. There's no floating point rounding issues happening then.
Floating point numbers use four bytes = 32 bits.
1 bit for sign
8 bits for exponent
23 bits for mantissa
Precision: The number of decimal digits precision is calculated via number_of_mantissa_bits * Log10(2). Thus ~7.2 and ~15.9 for single and double precision respectively.
That's why you start to see rounding errors on the 7th digit
Source link.
in iOS Objetive-C I am trying to get the number typed by the user in a text field to set the upper bounder of a random number generation function in C.
- (IBAction)pushTheButton2:(id)sender {
u_int32_t upperBound = (u_int32_t) textField3.text;
textField4.text = [NSString stringWithFormat:#"%d", arc4random_uniform(upperBound)];
}
The output is a giant number that makes no sense. To test if function works, if I hardcode the actual upper bound in the arc4random_uniform function, such as arc4random_uniform(5), then it works!
I figured this could be some kind of literal conversion, so I tried to make this work with u_int32_t but still not outputting the right range.
Can someone help? Thanks
You are currently taking the memory reference pointer of the text and using that as the upper bound.
Try doing something like this instead...
NSInteger upperBound = [textfield.text intValue];
This will convert the string into an int that you can then use in the arc random function.
To parse string to integer you should do:
NSInteger upperBound = [textfield.text integerValue];
I have a problem with my float variable, i have to do some operation and then i have a final number what I saw is that some time the number is not correct, but just for a point for example when i have this number and i try to print it i don't give me back the same number:
float myNumber = 27589353.0f;
NSLog(#"My Number is %.2f", myNumber);
the result is: My Number is 27589352.00
I've tried to put the variable double but i have the same issue.
The problem is that floats do not have enough precision, as maddy said in the comments above.
And this code:
double myNumber = 27589353.0f;
NSLog(#"My Number is %.2f", myNumber);
Won't work either because the "f" qualifier on the constant forces the number to be a float, causes the loss of precision, then promotes the value to a double, once the damage is done.
This code however, will work correctly:
double myNumber = 27589353.0;
NSLog(#"My Number is %.2f", myNumber);
(Note that I'm assigning a floating point value with a decimal, but no final "f"
try like that it's work, you have to remove the 'f' as well
double myNumber = 27589353.0;
NSLog(#"My Number is %.2f", myNumber);
I am trying to calculate a long value divided by an integer to give me what I would expect to be a double, although the result I am getting is 0. The code I am using...
double daysByYear = daysSinceBirthdayToService/365;
NSLog(#"%d", daysByYear);
In this code, daysSinceBirthdayToService variable is a Long Double which can be NSLogged using the following code (long)daysSinceBirthdayToService
It is declaired in the header file as a property of
#property (nonatomic) NSInteger daysSinceBirthdayToService;
Can anybody help me out with this, thanks!
The issue is that / between two longs will do an integral division.
To force a floating point division at least one of the operands needs to be cast to double.
e.g.
double daysByYear = daysSinceBirthdayToService/(double)365;
or if you have a literal make that a double by adding a decimal point
double daysByYear = daysSinceBirthdayToService/365.0;
double daysByYear = daysSinceBirthdayToService/365.0;
Can it be that %d outputs decimal number not a double?
The UITableView data source method numberOfSectionsInTableView: has a return type of NSInteger. However, a UITableView cannot have a negative amount of rows; it has 0 or greater rows, so why is the return type of NSInteger? Doesn't that allow for crashes relating to a negative integer being returned?
You can't do the check (if var < 0) return; with an unsigned integer. That is the standard reason for preferring one. Really the only reason to use an unsigned integer is if you need the extra room for larger digits, and you can guarantee the input will never try to be less than zero.