Fast abs function
How to write absolute value in c - Stack Overflow
microcontroller - Implementing an absolute value function in C - Electrical Engineering Stack Exchange
Write a function that computes the absolute value of an integer.
Videos
So, I was thinking of making fast abs function, which would be necesary to improve performance, and I had an idea to do it something like this
int abs(int input){
return input & -0;
}Essentially, I am trying to make a simple function that removes the sign bit. The problem is, I heard that alot of compliers would ignore this because its a zero. How could I do it that compliers wouldnt ignore it, and it would work for the intended purpose?
Edit: Thanks for all the answers the issue has been resolved!
The shortest solution in your first piece of code is to change the printf statement as follows:
printf("absValue = %u\n", (unsigned)((u<0)?-u:u));
This will print the absolute value of u. The type conversion (unsigned) ensures that the data type is as expected by printf. The statement (u<0)?-u:u uses the conditional operator to select the value -u if the condition (u<0) is true and u if the condition is false (i.e. u>=0).
The problem in your code is that u is a signed integer which means its value is stored using the Two's complement representation in 4 bytes(*) and printf is not intelligent. When you tell printf to display an unsigned integer, then printf will take the 4 bytes holding u and interpret them as an unsigned integer. Since negative numbers in Two's complement are stored as large positive integers, that is the result you see.
(*) The use of Two's complement and the int size of 4 is machine-dependent, but common.
As an alternative, you can also use the standard C function abs() (or one of its related functions):
7.22.6.1 The abs, labs and llabs functions
Synopsis
#include <stdlib.h> int abs(int j); long int labs(long int j); long long int llabs(long long int j);Description
The
abs,labs, andllabsfunctions compute the absolute value of an integerj. If the result cannot be represented, the behavior is undefined.Returns
The
abs,labs, andllabs, functions return the absolute value.Footnotes
The absolute value of the most negative number cannot be represented in two's complement.
Note the footnote "The absolute value of the most negative number cannot be represented in two's complement." and "If the result cannot be represented, the behavior is undefined." Strictly speaking, you'd likely need to use long long int and llabs() to avoid undefined behavior in converting INT_MIN to a positive value, assuming a 32-bit int value, and long is often 32-bits, even on 64-bit Windows.
However, since double values are likely implemented in IEEE format with 53 bits of precision, a 32-bit int value can be converted to double with no loss of precision, so you can use the fabs() function to get the absolute value of a 32-bit int value in one call:
7.12.7.2 The fabs functions
Synopsis
#include <math.h> double fabs(double x); float fabsf(float x); long double fabsl(long double x);The
fabsfunctions compute the absolute value of a floating-point numberx.
So your code would be:
#include <stdio.h>
#include <math.h>
int main (int argc, char *argv[]) {
int u;
scanf("%d", &u);
printf("absValue = %u\n", (unsigned) fabs((double) u));
return 0;
}
Note that in (unsigned) fabs((double) u), casting u to double is not strictly necessary, as the int value will be implicitly converted to a double because of the double fabs(double) function prototype from stdlib.h. But the cast back to unsigned is exremely necessary to pass the unsigned int value you want to pass to printf().
You could also do this:
#include <stdio.h>
#include <math.h>
int main (int argc, char *argv[]) {
int u;
scanf("%d", &u);
unsigned int absValue = fabs(u);
printf("absValue = %u\n", absValue);
return 0;
}
That works because unsigned int absValue is explicitly an unsigned int.
Also, on modern CPUs, conversion between int and double is usually done by a single relatively fast instruction.
The standard C library is providing the optimized solutions for many problems with considerations based on the architecture, compiler in use and others. The abs() function defined in stdlib.h is one of these, and it is used for your purpose exactly. To emphasize the point, here is ARM compiler result when using abs vs a version of a homebrew abs: https://arm.godbolt.org/z/aO7t1n
Paste:
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
srand(111);
int x = rand() - 200;
printf("%d\n", abs(x));
}
results in
main:
push {r4, lr}
mov r0, #111
bl srand
bl rand
sub r1, r0, #200
cmp r1, #0
rsblt r1, r1, #0
ldr r0, .L4
bl printf
mov r0, #0
pop {r4, pc}
.L4:
.word .LC0
.LC0:
.ascii "%d\012\000"
And
#include <stdio.h>
#include <stdlib.h>
int my_abs(int x)
{
return x < 0 ? -x : x;
}
int main(void)
{
srand(111);
int x = rand() - 200;
printf("%d\n", my_abs(x));
}
results in
my_abs:
cmp r0, #0
rsblt r0, r0, #0
bx lr
main:
push {r4, lr}
mov r0, #111
bl srand
bl rand
sub r1, r0, #200
cmp r1, #0
rsblt r1, r1, #0
ldr r0, .L5
bl printf
mov r0, #0
pop {r4, pc}
.L5:
.word .LC0
.LC0:
.ascii "%d\012\000"
Notice that the main code is identical (only a label name is different) in both programs as my_abs got inlined, and its implementation is the same as the standard one.
The speed of a given solution will depend greatly on the architecture, but in C I would say
return (n > 0 ? n : -n);
and let the compiler figure out the best solution.
EDIT: @jonk points out correctly that this will fail for the most-negative possible value of n, assuming that two's-complement arithmetic is used.
Yes, my solution has a conditional branch, but yours has an arithmetic operator and two bitwise operators. Can your microcontroller shift 15 places in a single clock?