// printfdemo.c-- printf()函数用法示例
#include <stdio.h>

char *m1 = "Binary";
char *m2 = "Decimal";
char *m3 = "Octal";
char *m4 = "Hexadecimal";

int main(void)
{
float d1 = 10000.123;
int n;

puts("Outputting a number with different field widths.\n");
printf("%5f\n", d1);
printf("%10f\n", d1);
printf("%15f\n", d1);
printf("%20f\n", d1);
printf("%25f\n", d1);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nUse the * field width specifier to obtain field width");
puts("from a variable in the argument list.\n");

for (n = 5; n <= 25; n += 5)
printf("%f %*f\n", d1, n, d1);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nInclude leading zeros.\n");

printf("%05f\n", d1);
printf("%010f\n", d1);
printf("%015f\n", d1);
printf("%020f\n", d1);
printf("%025f\n", d1);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nDisplay in octal, decimal, and hexadecimal.");
puts("Use # to precede octal and hex output with 0 and 0X.");
puts("Use - to left-justify each value in its field.");
puts("First display column labels.\n");

printf("%-15s%-15s%-15s", m2, m3, m4);

for (n = 1; n < 20; n++)
printf("\n%-15d%-#15o%-#15X", n, n, n);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nUse the %n conversion command to count characters.\n");
printf("%s%s%s%s%n", m1, m2, m3, m4, &n);
printf("\n\nThe last printf() output %d characters .\n", n);

return 0;
} 

可以把这个程序复制粘贴到编译器中……原本的10000.123后面怎么多了047？
（这回我真的没有抄错程序……）

如果浮点数能精确的表示所有的实数，
那么全世界的硬盘都可以销毁了，
首先，将全世界的所有数据拼接起来，组成一个长长的整数，然后取这个整数的倒数，存入float类型即可。
4字节的float就可以存储无限多的实数，那还要硬盘干什么，直接一张白纸就行了，比如 0x437A0000，能写下8个字符的小纸条就够过了。

#include <stdio.h>

char *m1 = "Binary";
char *m2 = "Decimal";
char *m3 = "Octal";
char *m4 = "Hexadecimal";

int main(void)
{
float d1 = 10000.123;
int n;

puts("Outputting a number with different field widths.\n");
printf("%5.3f\n", d1);
printf("%10.3f\n", d1);
printf("%15.3f\n", d1);
printf("%20.3f\n", d1);
printf("%25.3f\n", d1);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nUse the * field width specifier to obtain field width");
puts("from a variable in the argument list.\n");

for (n = 5; n <= 25; n += 5)
printf("%.3f %*.3f\n", d1, n, d1);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nInclude leading zeros.\n");

printf("%05.3f\n", d1);
printf("%010.3f\n", d1);
printf("%015.3f\n", d1);
printf("%020.3f\n", d1);
printf("%025.3f\n", d1);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nDisplay in octal, decimal, and hexadecimal.");
puts("Use # to precede octal and hex output with 0 and 0X.");
puts("Use - to left-justify each value in its field.");
puts("First display column labels.\n");

printf("%-15s%-15s%-15s", m2, m3, m4);

for (n = 1; n < 20; n++)
printf("\n%-15d%-#15o%-#15X", n, n, n);

puts("\n Press Enter to continue...");
fflush(stdin);
getchar();

puts("\nUse the %n conversion command to count characters.\n");
printf("%s%s%s%s%n", m1, m2, m3, m4, &n);
printf("\n\nThe last printf() output %d characters .\n", n);

return 0;
}

注意，这样只是不打印出来了，不代表后面没有数字

[此贴子已经被作者于2016-8-12 12:22编辑过]

说明这个问题涉及到float的存储，不是一两句能说清楚的，就不提了。
你只要记住浮点数有效数字超过7位改用double就行了。