【C/C++】字符串打印(format, printf, sprintf)

format

A format specifier follows this prototype: 
%[flags][width][.precision][length]specifier

最常用

 printf("|%015d|\n",int_value);     //右对齐，15位长度，不够补0
 printf("|%15d|\n",int_value);  //右对齐，15位长度，不够补空格
 printf("|%-15d|\n",int_value);  //左对齐，15位长度，不够补空格
 printf("|%-15.2f|\n",float_value);  //左对齐，15位长度，带两位小数，不够补空格

熟能生巧

#include 
int main()
{
    //print char => Characters: a A
   printf ("Characters: %c %c \n", 'a', 65);
   // print int, long int => Decimals: 1977 650000
   printf ("Decimals: %d %ld\n", 1977, 650000L);
   // print int with preceding blanks or zeros => Preceding with blanks:       1977
   printf ("Preceding with blanks: %10d \n", 1977);
   // Preceding with zeros: 0000001977 
   printf ("Preceding with zeros: %010d \n", 1977);
   //print int in different radices => Some different radices: 100 64 144 0x64 0144 
   printf ("Some different radices: %d %x %o %#x %#o \n", 100, 100, 100, 100, 100);
   //print float => floats:       3.14 +3e+00 3.141600E+00 
   printf ("floats: %10.2f %+.0e %E \n", 3.1416, 3.1416, 3.1416);
   // print with width trick => Width trick:    10 
   printf ("Width trick: %*d \n", 5, 10);
   // print a string, the simpliest => A string 
   printf ("%s \n", "A string");
   return 0;
}

关于数据对齐

int main(int argc, char const *argv[])
{
    	//整型
	printf("#####\n");//5个#
	printf("%2d\n",666);
	printf("%5d\n",666);//右对齐
	printf("%-5d\n",666);//左对齐
    //字符串
	printf("#####\n");//5个#
	printf("%2s\n","abc");
	printf("%5s\n","abc");//右对齐
	printf("%-5s\n","abc");//左对齐
	//浮点数
	printf("##########\n");//10个#
	printf("%2f\n",6.66);
	printf("%10f\n",6.66);
	printf("%-10f\n",6.66);
    ///////////////////////////////////////////
	printf("#####\n");//5个#
	printf("%5.2d\n",666);
	printf("%5.5d\n",666);//右对齐
	printf("%5.8d\n",666);//左对齐
 
	printf("#####\n");//5个#
	printf("%5.2s\n","abc");
	printf("%5.5s\n","abc");//右对齐
	printf("%5.8s\n","abc");//左对齐
 
	printf("#####\n");//5个#
	printf("%5.1f\n",6.55);
	printf("%5.1f\n",6.551);
	printf("%5.1f\n",6.65);
	printf("%5.5f\n",6.66);

    return 0;
}

结果

#####
666
  666
666  
#####
abc
  abc
abc  
##########
6.660000
  6.660000
6.660000  
#####
  666
00666
00000666
#####
   ab
  abc
  abc
#####
  6.5
  6.6
  6.7
6.66000

sprintf

sprintf 常用来构造带变量的字符串, 比如根据参数生成的图像命名的时候最好把使用的参数值写进去.

int sprintf ( char * str, const char * format, ... );

• Write formatted data to string

• Composes a string with the same text that would be printed if format was used on printf, but instead of being printed, the content is stored as a C string in the buffer pointed by str.
• The size of the buffer should be large enough to contain the entire resulting string (see snprintf for a safer version).
• A terminating null character is automatically appended after the content.
• After the format parameter, the function expects at least as many additional arguments as needed for format.
• the return value : On success, the total number of characters written is returned. This count does not include the additional null-character automatically appended at the end of the string. On failure, a negative number is returned.

/* sprintf example */
#include 

int main ()
{
  char buffer [50];
  int n, a=5, b=3;
  n=sprintf (buffer, "%d plus %d is %d", a, b, a+b);
  printf ("[%s] is a string %d chars long\n",buffer,n);
  return 0;
}

Output:
[5 plus 3 is 8] is a string 13 chars long

13 = 12 个字符  + 1 个终止符 `\0`

但是sprintf 是 old C-style，不安全，比如在VS2015 中就会收到这样的警告

warning C4996: ‘sprintf’: This function or variable may be unsafe. Consider using sprintf_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.
1> c:\program files (x86)\windows kits\10\include\10.0.10240.0\ucrt\stdio.h(1769): note: 参见“sprintf”的声明

Ref

• printf()中的格式问题
• http://www.cplusplus.com/reference/cstdio/printf/ , 中译版
• http://www.cplusplus.com/reference/cstdio/sprintf/