Numeric Types
Use int when you need to store a whole number without decimals, like 35 or 1000, and float or double when you need a floating point number (with decimals), like 9.99 or 3.14515.
int
int myNum = 1000;
printf("%d", myNum);
float
float myNum = 5.75;
printf("%f", myNum);
double
double myNum = 19.99;
printf("%lf", myNum);
float vs. double
The precision of a floating point value indicates how many digits the value can have after the decimal point. The precision of float is six or seven decimal digits, while double variables have a precision of about 15 digits. Therefore, it is often safer to use double for most calculations – but note that it takes up twice as much memory as float (8 bytes vs. 4 bytes).
Scientific Numbers
A floating point number can also be a scientific number with an “e” to indicate the power of 10:
Example
float f1 = 35e3;
double d1 = 12E4;
printf("%f\n", f1);
printf("%lf", d1);