Print a floating point number as a decimal number with printf()

Asked Oct 25 '19 at 08:35

Active Oct 25 '19 at 08:57

Viewed 46 times

I found a very good answer to this question on this thread

I want to understand a little bit more about why printf() can't print a floating point number as a decimal (with %d).

The program is a simple one converting Fahrenheit to Celsius degree.

I understand that %.f or %.0f is doing what i want. But when i try to do the same thing with %d, the output is unpredictable.

I searched for more detailed pieces of information cplusplus, but i don't see where it overflows or why i get this result. For example, if you use an uninitialized variable, you will get some random (or not so random) value that is in that place of memory where your variable's name is "pointing" towards. Here, what is the reason ?

float fahr, celsius;
int lower, upper, step;

lower = 0;    upper = 300;    step = 20;

fahr = lower;
while(fahr <= upper){
    celsius = (5 / 9.) * (fahr - 32);

    printf("%d\t%d\n",fahr,celsius);

    fahr+=step;
}

i was expecting : 0 20 40 60 .... 300 (first column) -17, -6, -4, ... (second column)

instead i got 0 0 0 0 0 0 0 ,,, 0 (first column) 0, 1077149696, 1078198272, ... (second column)

edited Oct 25 '19 at 08:57

asked Oct 25 '19 at 08:35

Dragos Makovei

%d is for int data types. Whilst the value stored in a float may be an integer value, it is still stored in a float data type. – John Sheridan Oct 25 '19 at 08:40
Should't there be any implicit coersion? – Dragos Makovei Oct 25 '19 at 08:41
If you want to convert use a cast (int). – John Sheridan Oct 25 '19 at 08:42
So, if _lower_ is **int** and _fahr_ is **float**, then _lower_ variable will be "upgraded " to float (an implicit coersion). This isn't valid for **printf()** specifiers? – Dragos Makovei Oct 25 '19 at 08:45
There is a great answer to your question here ... https://stackoverflow.com/questions/7480097/what-happens-to-a-float-variable-when-d-is-used-in-a-printf – John Sheridan Oct 25 '19 at 08:49
Thank you very much. Shall i remove this post? I didn't see that post... And it is what i needed – Dragos Makovei Oct 25 '19 at 08:52
No. You should leave your post. – John Sheridan Oct 25 '19 at 08:52

Print a floating point number as a decimal number with printf()

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