Use Format I/O in C/C++

Yorafa

2022-03-09

C/C++

The "I/O" standards input and output, and they usually refer to system stdin and stdout .

Format

For convenience reason, we want to quick read/write value from/to stdout / stdin with variable. We have idea of format.

For example printf("%d\n", var); print int type var

"%d": define the variable type, if the type of var is different will raise error, but char can present numbers by ASCII, vice versa. (ps: ASICII only from 0 to 255.)
"\n": mean new line
var: the respective variable of format

That's is, the format is the string, we can print variable in the given format or scan value write into variable in this way.

For example, we can have some format like "These %d apples belong to %s\n" follow two variable to print the result we want, or read from stdin we want. Since we define "%" to read/write variable, we need know about what value followed to correctly present variable.

Notice for format that read, for example "These %d apples belong to %s\n" we should input These 10 apples belong to me to correctly read 10 and me into variable by function like scanf.

Reference Table

Input	Meaning	Type
c	a single character	char
s	a string	char *
d	integer	int
f	float number	float
e/E	in scientist notation	double
l/ld/li	usually 32 bits integer	long
ll/lld	64 bits integer	long long
lf	long float	double
p	pointer	void *

Furthermore, we can add extra suffix to print more specific value.

For int, we can use 0n to extend the length of number by add 0 at head. For example printf("%04d", 1); will have a output 0001. That's 0n means extend to n length when n larger than the length of the variable.

Similarly, for float/double, we can use .n to specific the decimal place, for example printf("%.2f", 1.3459834); will have a output 1.35. But notice, round will apply for this.