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PCD: Comparing two floating point numbers

 
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cuma_polat



Joined: 24 Nov 2008
Posts: 3

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PCD: Comparing two floating point numbers
PostPosted: Sun Jan 18, 2009 2:26 pm     Reply with quote
Hi friends,
I have a very simple problem. I want to compare two float value with "if". example program is here;
Code:
 #include <30F4013.h>
#device ADC=10
#use delay(clock=10000000)
#FUSES HS
#use rs232(UART2,baud=9600,xmit=PIN_F3,rcv=PIN_F2,parity=N,bits=8)
#include <math.h>
#include <flex_lcd.c>
#use fast_io(B)
#use fast_io(C)
#use fast_io(D)
#use fast_io(F)
float n=0.1,nce=0.2,c=0;
float positive (float ee){
      float nu_p;
    if (n<0.0) nu_p=1;
    else nu_p=2;
     return ((float)nu_p);
}
void main(){
lcd_init();  // Always call this first.
while(true){
c=positive(1.0);
 printf(lcd_putc,"\fc=%f ",c);
 delay_ms(500);
}
}


As you see from the code, I try to compare n with 0.0 in subprogram "positive" . It is always true because n=0.1. so nu_p must be 1.0 but LCD shows that c=2.0
On the other hand I compare n with nce in another program but it didn't give the true result although the condition was true.
Can I compare two float value with CCSC by using if-statement?
Please Help!
ckielstra



Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Sun Jan 18, 2009 4:29 pm     Reply with quote
Quote:
It is always true because n=0.1.
This is incorrect. With n == 0.1 the comparison becomes:
Code:
    if (0.1 < 0.0) nu_p=1;
    else nu_p=2;
The IF-statement is always FALSE and the returned value of 2 is correct.
cuma_polat



Joined: 24 Nov 2008
Posts: 3

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PostPosted: Sun Jun 28, 2009 6:21 am     Reply with quote
Yes you are right,

I wrote the wrong code, this is the right one;

Code:

 #include <30F4013.h>
#device ADC=10
#use delay(clock=10000000)
#FUSES HS
#use rs232(UART2,baud=9600,xmit=PIN_F3,rcv=PIN_F2,parity=N,bits=8)
#include <math.h>
#include <flex_lcd.c>
#use fast_io(B)
#use fast_io(C)
#use fast_io(D)
#use fast_io(F)
float n=0.1,c=0.0;
float positive (float ee){
      float nu_p;
    if (n<1.0) nu_p=1;
    else nu_p=2;
     return ((float)nu_p);
}
void main(){
lcd_init();  // Always call this first.
while(true){
c=positive(1.0);
 printf(lcd_putc,"\fc=%f ",c);
 delay_ms(500);
}
}


I compare n with 1.0, n is smaller than 1.0 but the LCD shows c=2.00..
FvM



Joined: 27 Aug 2008
Posts: 2337
Location: Germany

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PostPosted: Sun Jun 28, 2009 10:55 am     Reply with quote
It's possibly a bug of an old PCD version. I tested the code, compiled with PCD V4.093 in MPLAB SIM (directing the output to UART instead of LCD) and it showed c=1.00.
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