casting data losing C

Question

I try to understand when casting causes data losing and how it works. so for the following examples i try to understand if there is data loss and if yes why: (i - int(4),f - float(4),d-double(8))

i == (int)(float) i; // sizeof(int)==sizeof(float) <- no loss
i == (int)(double) i; // sizeof(int)!=sizeof(double) <- possible loss
f == (float)(double) f;// sizeof(double)!=sizeof(float) <- possible loss
d == (float) d;// sizeof(double)!=sizeof(float) <- possible loss

Is it sufficient to base the answer only on type sizes?(+ round )

You should specify `sizeof(int)` - you're assuming 4 (32 bits) I expect ? — Paul R, Nov 08 '13 at 06:16
After `i == (int)(double) i` says `sizeof(int)!=sizeof(float)`. Are you sure it's not mistyped? — Y.Z, Nov 08 '13 at 06:17

score 3 · Accepted Answer · answered Nov 08 '13 at 06:18

3

Assuming 32 bit ints and normal 4 and 8 byte IEEE-754 floats/doubles it would be:

i == (int)(float) i;     // possible loss (32 -> 23 -> 32 bits)
i == (int)(double) i;    // no loss (32 -> 52 -> 32 bits)
f == (float)(double) f;  // no loss (23 -> 52 -> 23 bits)
d == (float) d;          // possible loss (52 -> 23 -> 52 bits)

Note that int has 32 bits of precision, float has 23 bits, double has 52.

answered Nov 08 '13 at 06:18

Paul R

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why do you base the answer only on precision,not exponent? – YAKOVM Nov 08 '13 at 06:57
Because the exponent does not affect the precision (or resolution) - think of it as a scaling factor. – Paul R Nov 08 '13 at 08:14

score 1 · Answer 2 · answered Nov 08 '13 at 06:21

1

The memory allocated to store variable of a type is not the only fact for you to consider loss of data. Generally, way to roundoff and how CPU processes numeric data in case of overflow would be other aspects you might want to look into.

answered Nov 08 '13 at 06:21

Y.Z

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score 0 · Answer 3 · answered Nov 08 '13 at 06:18

Because the sizeof reports the same size in memory does not mean there is data loss.

Consider 0.5.

Can store that in a float but cannot store it in an integer.

Therefore data loss.

I.e. I want 0.5 of that cake. Cannot represent that as an integer. Either get nothing or lots of cakes. Yum

score 0 · Answer 4 · answered Nov 08 '13 at 06:23

0

Why integer? because you may need only integer numbers for example an ID_num

Why float? because you may need to work on real numbers example % calculations

Why double? when you have real numbers that cannot fit into float size

answered Nov 08 '13 at 06:23

niko

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casting data losing C

4 Answers4