What is the difference between int8(0x98), uint8(0x98), typecast(uint8(0x98), 'int8')

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Henry
Henry 2023-6-6
编辑： Dyuman Joshi 2023-6-7
>> A=int8(0x98)
A =
int8
127
>> B=uint8(0x98)
B =
uint8
152
>> C=typecast(uint8(0x98), 'int8')
C =
int8
-104
I want 0x98 to be handled as signed Integer. Expectation is a negative values (-104) - two's complement.
Why is the calculation A=int8(0x98) leading to 127 and not to -104.
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Henry
Henry 2023-6-6
All of above is understood.
The ind8 value range is specified to be [-2^7,2^7-1].
How do I get the negative values?
Dyuman Joshi
Dyuman Joshi 2023-6-6
Ran in:
I don't understand, is using typecast() not satisfactory?
Or am I misunderstanding what you want to obtain? In that case, please specify.
A=int8(0x98)
A = int8 127
B=uint8(0x98)
B = uint8 152
C=typecast(B, 'int8')
C = int8 -104

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回答（2 个）

Steven Lord
Steven Lord 2023-6-6
Ran in:
If you use the following syntax, you create a uint8 value and then cast that value to int8. Since the uint8 value is larger than intmax for int8, it saturates at intmax.
y1 = 0x98
y1 = uint8 152
z = int8(y1)
z = int8 127
Why uint8 not int8? From that documentation page I linked above: "By default, MATLAB stores the number as the smallest unsigned integer type that can accommodate it."
Instead tell MATLAB you want an int8 (a signed 8-bit integer) value from the start. To do this add the "s8" suffix to the hex value.
y2 = 0x98s8
y2 = int8 -104
The typecast function works because it converts not the value stored in the input but the bit pattern of the input to the new type.
format hex % Show the hex digits of the bit pattern
y1
y1 = uint8
98
y2
y2 = int8
98
Swastik Sarkar
Swastik Sarkar 2023-6-6
In MATLAB, the number 0x98 will be represented as 127 in int8 due to overflow.
It will map to the nearest endpoint of the range. This is how MATLAB handles the overflow instead of setting the sign bit, like you would see in C.
However while typcasting a uint to int, it does not truncate and keeps the last bit. Hence you get -104
Read the description in this page to understand it better: int8
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Henry
Henry 2023-6-7
my takeaaway:
y=int8(x) is not implemented as one might understand from the documentation int8: signed integer with number range [-2^7 .. 2^7-1]
  • The number range is limited to [0 .. 2^7-1]
  • The defined number range [-2^7 .. -1] cannot be achieved: value saturates at 2^7-1 instead
  • negative sign is not provided
Several workarounds are present to generate a signed interger among which:
  • y=typecast(uint8(x), 'int8')
  • "s8"-suffix to HEX-number
  • ...
I consider this topic closed.
Steven Lord
Steven Lord 2023-6-7
Ran in:
The number range is limited to [0 .. 2^7-1]
The range of values that can be represented in the int8 data type is:
[intmin('int8'), intmax('int8')]
ans = 1×2
-128 127
where 2^7-1 is:
2^7-1
ans = 127
So your statement is incorrect.
The defined number range [-2^7 .. -1] cannot be achieved: value saturates at 2^7-1 instead
Incorrect. If you try to cast a uint8 value to int8, anything greater than intmax of int8 becomes intmax of int8.
y = intmax('uint8')
y = uint8 255
z1 = int8(y)
z1 = int8 127
If you try to typecast a uint8 bit pattern to int8 then you can easily obtain a negative int8 value.
z2 = typecast(y, 'int8')
z2 = int8 -1
But you can certainly create negative int8 values directly
y = int8(-1)
y = int8 -1
negative sign is not provided
Incorrect, as shown above.

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