问题
I have the following code:
double dtemp = (some value)
printf("\n***Hand washing: cost per kg/item: %.2f, cost: %.2f.\n", 5, dtemp);
It prints out:
***Hand washing: cost per kg/item: 0.00, cost: 0.00.
And when I change the constant 5 into a double variable holding 5, it prints (according to the input):
***Hand washing: cost per kg/item: 5.00, cost: 20.00.
Why would the constant 5 affect the evaluation of the dtemp? I'm using gcc 4.6.2 (MinGW) and tested it also in TCC.
回答1:
My compiler's (gcc 4.4.3) warning message explains this:
format ‘%.2f’ expects type ‘double’, but argument 2 has type ‘int’
Since you are passing a different type of value (int) than specified (double) in the format string, the behavior is undefined due to this mismatch
As you observed, once you adjust this to be consistent you get the output you expect. I.e.,
/* provide a double value */
printf("\n***Hand washing: cost per kg/item: %.2f, cost: %.2f.\n", 5.0, dtemp);
output:
***Hand washing: cost per kg/item: 5.00, cost: 3.14.
or
/* specify an integer value in the format string */
printf("\n***Hand washing: cost per kg/item: %d, cost: %.2f.\n", 5, dtemp);
output:
***Hand washing: cost per kg/item: 5, cost: 3.14.
Always a good idea to crank up the warning levels on the compiler and then follow up on all warnings and make deliberate decisions about what can be ignored and what can't.
回答2:
f conversion specifier requires a double when used with printf functions and your are passing an int. Passing an int is undefined behavior which means anything can happen.
回答3:
e the printf function first parameter is string, it check every %d,then move the point,for example: %d,move 4 len; %lld move 8. int64_t a = 1;int b = 2; printf("%d, %d\n", a, b); the answer is 1,0. because the two %d just get the a lower 4 byte and hight 4 byte.
printf("\n***Hand washing: cost per kg/item: %.2f, cost: %.2f.\n", 5, dtemp); sizeof(doubel) = 8 ,so the printf function just get 0x0000000000000005.
I hope it would help you.
来源：https://stackoverflow.com/questions/11382694/printf-parameters-affecting-each-other