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[QUOTE="matt.jaffe, post: 5088200"]
I'm trying to show how floating point numbers are represented internally.  I thought the easiest way would be to print the same floating point number once with %f and then again with %x, but the results surprised me.  I can do it with a union and bit fields, but why doesn't the simpler way work?   Here's the code:

#include<stdio.h>
int main(void)
{
 
union 
{
float aFloat;
int anInt;
 
struct 
 
unsigned int sig:23;   /* significand without the most significant 1 */
unsigned int expo:8;   /*  biased exponent                         */
unsigned int sign:1;
} fields;
 
} uEx;                     /*  abbreviation of unionExample            */
 
uEx.aFloat = 1.0;
 
printf("\n The union as a float: %f; as in integer in hex: %x; \n the sign bit is %x; the biased exponent is %x; the signifcand is %x \n", \
uEx.aFloat,                 uEx.anInt,   uEx.fields.sign,           uEx.fields.expo,       uEx.fields.sig);
 
printf("\n  1.0 printed with with %%f is %f and with %%x is 0x%x \n", 1..0, 1.0);
}

The size of both integers and floats on the machine is 32 bits.  Here's theresult:

The union as a float: 1.000000; as in integer in hex: 3f800000; 
the sign bit is 0; the biased exponent is 7f; the signifcand is 0

1.0 printed with with %f is 1.000000 and with %x is 0x0

The first two lines of output are what I was expecting per IEEE 754; but the 0x0 in the last line has me confused.  Why is it not printing as 0x3f800000 ?
[/QUOTE]

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