Can anyone explain the output

Question

If I try to print a float as an int, this code:

main () {                         
    float a = 6.8f;                      
    printf("%d", a);                      
}

prints 1073741824, while this code:

main () {            
    float a = 9.5f;           
    printf("%d", a);            
}

prints 0.

Is the output undefined? Also when is %f used with integer and %d used with double?

possible duplicate of [print the float value in integer in C language](http://stackoverflow.com/questions/1632080/print-the-float-value-in-integer-in-c-language) — Leo Chapiro, May 24 '13 at 09:59
Also calling a function that accepts a variable number of arguments (`printf()`) without a prototype in scope is UB. Use `#include ` for the proper prototype. — pmg, May 24 '13 at 10:06

score 2 · Accepted Answer · answered May 24 '13 at 09:59

2

Not only the output, but the entire program has undefined behavior, since type of the value you are passing to printf() does not match the type the format string expects.

answered May 24 '13 at 09:59

Even the act of *compiling* that source invokes UB :) – pmg May 24 '13 at 10:02

score 2 · Answer 2 · answered May 24 '13 at 10:00

From the C99 standard section 7.19.6.1 The fprintf function:

If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

%d expects an int, not a float, so the program has undefined behaviour (including the output).

score 1 · Answer 3 · answered May 24 '13 at 11:05

As described in previous answers if the print format doesn't match the type passed, it shows undefined behavior.

If you want to view integer as float u need to typecast it.

int j = 5;
printf("%f",(float)(j));

This will print output as 5.0 ie as a floating digit number

score 0 · Answer 4 · answered May 24 '13 at 10:00

The C Standard says that the printf format must match the type passed in. If it doesn't, the behavior is expressly undefined:

C99, 7.19.6.1 # 9 (fprintf)

If a conversion specification is invalid, the behavior is undefined.239) If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

Can anyone explain the output

4 Answers4