PROSAGA码农传奇-5G-用于Java的C兼容printf输出

0# 咿呀哟 | 2019-08-31 10-32

<div class =“post-text”itemprop =“text”>
  
    这段代码
  
   <pre>
 <code>
 #include <stdio.h>

int main() {

double v;
    char format[] = "%.5g\n";

v = 1234.0;
    printf(format, v);

v = 123.45678;
    printf(format, v);

v = 0.000123456;
    printf(format, v);

v = 0.000000000000123456;
    printf(format, v);

}

</code>
 </pre>
  
    给我
  
   <pre>
 <code>
 1234
123.46
0.00012346
1.2346e-13

</code>
 </pre>
  
    和这段代码
  
   <pre>
 <code>
 public class App13749802 {

/**
     * @param args
     */
    public static void main(String[] args) {

double v;
        String format = "%.5g";

v = 1234.0;
        System.out.println(String.format(format, v));

v = 123.45678;
        System.out.println(String.format(format, v));

v = 0.000123456;
        System.out.println(String.format(format, v));

v = 0.000000000000123456;
        System.out.println(String.format(format, v));
    }

}

</code>
 </pre>
  
    给我
  
   <pre>
 <code>
 1234,0
123,46
0,00012346
1,2346e-13

</code>
 </pre>
  
    逗号是因为我的区域设置。所以只有一个区别。
  
</DIV>

1# 离线请留言 | 2019-08-31 10-32

<div class =“post-text”itemprop =“text”>
  
    好吧，我结束了我自己的功能编码。在所有double范围内使用gcc和tcc进行测试，得到完全相同的输出（除非极少数非常小的值，小于1E-319）
  
  
    我发布它以防有人发现它有用。
  
  
    Java的：
  
   <pre>
 <code>
 /**
 * Returns a double with an adhoc formatting, compatible with its C counterpart
 * 
 * If the absolute value is not too small or too big (thresholdLow-thresholdHigh)
 * the floating format is used, elsewhere the scientific.
 * In addition 
 * - trailing zeros in fractional part are removed
 * - if the value (or mantisa) is integer, a trailing .0 is always included
 * - the exponent in sci notation is two or three digits
 * - positive and negative zero returns "0.0"
 * - special vals: "NaN" "Infinite" "-Infinite"
 * 
 * Remember to set Locale.setDefault(Locale.US) in your program.
 * 
 * @param v double
 * @param formatFloat floating point format, suggested: "%.5f"
 * @param formatSci scientific format, must use lowercase 'e' : "%.5e" 
 * @param thresholdLow 
 * @param thresholdHigh
 * @return formatted string
 */
 public static String sprintfDouble(double v, String formatFloat, String formatSci, double thresholdLow,
 double thresholdHigh) {
 if(v==0.0)
 return "0.0"; //dont care about negative zero 
 if(Double.isInfinite(v) || Double.isNaN(v))
 return String.format(formatFloat,v);

boolean neg = false;
 if (v < 0) {
 v = -v;
 neg = true;
 }
 String e = "";
 String res;
 if (v > thresholdLow && v < thresholdHigh) {
 res = String.format(formatFloat, v);
 } else {
 res = String.format(formatSci, v);
 int sp = res.indexOf('e');
 e = res.substring(sp);
 res = res.substring(0, sp);
 }
 if (res.indexOf('.') < 0)
 res += "."; // add decimal point if not present
 res = res.replaceAll("0+$", ""); // trim trailing zeros 
 if (res.endsWith("."))
 res += "0"; // add traiing zero if nec
 res += e;
 if (neg)
 res = "-" + res;
 return res;
 }

public static String sprintfDouble5(double v){
        return sprintfDouble(v, "%.5f","%.5e",0.01,1000000.0);
    }

</code>
 </pre>
  
    C：
  
   <pre>
 <code>
 char * sprintfDouble(char *buf, double v, const char *floatFormat, const char *sciFormat, double thresholdLow, double thresholdHigh) {
 char *p;
 char *pd; /* pointer to '.' */
 char *pe; /* pd=, pe=pointer to 'e' (or null terminator) */
 char *buforig;
 int trimmed;
 if(v != v) { /* nan */
 sprintf(buf,"NaN");
 return buf;
 }
 if(v == v && (v - v) != 0.0) { /* infinity */
 sprintf(buf, v < 0 ? "-Infinity" :"Infinity");
 return buf;
 } 
 if(v==0) { /* positive or negative zero, dont distinguish*/
 sprintf(buf, "0.0");
 return buf;
 }
 buforig = buf;
 if(v <0) {
 v = -v;
 buf[0] = '-';
 buf++;
 }
 if( v > thresholdLow && v < thresholdHigh ) {
 sprintf(buf,floatFormat, v);
 pe = buf+strlen(buf);
 pd = (char *) strchr(buf,'.');
 if(pd == NULL) { /* no decimal point? add it */
 pd = pe;
 *pe++ = '.';
 *pe++ = '0';
 *pe = 0;
 }
 } else {
 sprintf(buf,sciFormat, v);
 pe = (char *)strchr(buf,'e');
 pd = (char *)strchr(buf,'.');
 if(pd ==NULL) { /* no decimal point with scientific notation? rare but... */
 p= buf+ strlen(buf);
 while(p>=pe) {
 *p = *(p-2);
 p--;
 }
 pd = pe;
 *pe++ = '.';
 *pe++ = '0';
 *pe = 0;
 }
 /* three digits exponent with leading zero? trim it */
 if( (*(pe+2) == '0' ) && ( strlen(buf) - (pe-buf))==5) {
 *(pe+2)=*(pe+3);
 *(pe+3)=*(pe+4);
 *(pe+4)=*(pe+5);
 }
 } /* now trim trailing zeros */
 trimmed = 0;
 p=pe-1;
 while(*p =='0' ) {
 p--;
 trimmed++;
 }
 if(*p=='.') {
 trimmed--; // dont trim the zero after the decimal point
 p++;
 }
 if(trimmed>0) {
 p = pe;
 while(1) {
 *(p-trimmed) = *p;
 if(*p==0) break;
 p++;
 }
 }
 return buforig;
}

char * sprintfDouble5(char *buf,double v) {
    return sprintfDouble(buf, v, "%.5f", "%.5e", 0.01, 1000000.0);
}

</code>
 </pre>
  
    测试代码。
  
  
    Java的
  
   <pre>
 <code>
 static void test() { 
 Locale.setDefault(Locale.US);
 double start = 1.0;
 double x=start;
 for(int i=0;i<367;i++) {
 System.out.println(sprintfDouble5(x));
 x*= -7.0;
 }
 x=start;
 for(int i=0;i<6;i++) {
 System.out.println(sprintfDouble5(x));
 x/= -5;
 }
 for(int i=0;i<200;i++) {
 System.out.println(sprintfDouble5(x));
 x/= -42.01;
 }
 x=Math.PI*0.0000001;
 for(int i=0;i<20;i++) {
 System.out.println(sprintfDouble5(x));
 x*=10;
 }
 System.out.println(sprintfDouble5(0.0));
 System.out.println(sprintfDouble5(-0.0));
 System.out.println(sprintfDouble5(0.0/0.0));
}

</code>
 </pre>
  
    C：
  
   <pre>
 <code>
 void test1() { 
 char buf[64];
 double start,x;
 int i;
 start = 1.0;
 x = start;
 for(i=0;i<367;i++) {
 printf("%s\n",sprintfDouble5(buf,x));
 x *= -7.0;
 }
 x = start;
 for(i=0;i<6;i++) {
 printf("%s\n",sprintfDouble5(buf,x));
 x /= -5;
 }
 for(i=0;i<200;i++) {
 printf("%s\n",sprintfDouble5(buf,x));
 x/= -42.01;
 }
 x = atan(1.0) * 4 * 0.0000001; /* PI */
 for(i=0;i<20;i++) {
 printf("%s\n",sprintfDouble5(buf,x));
 x *= 10;
 }
 printf("%s\n",sprintfDouble5(buf,0.0));
 printf("%s\n",sprintfDouble5(buf,-0.0));
 printf("%s\n",sprintfDouble5(buf,0.0/0.0));
}

</code>
 </pre>
</DIV>

2# ⑩Ⅵ嵗Ｄ夨憶 | 2019-08-31 10-32

<div class =“post-text”itemprop =“text”>
  
    如果你真的想要base-10浮点输出，那么为C编写一个JNI包装器可能最简单
     <code>
 printf
 </code>
     这里。 Java人员决定他们需要这样做
     <code>
 printf
 </code>
     他们自己。除了你已经注意到的
     <code>
 %g
 </code>
    ，他们决定改变舍入行为并以奇怪的方式截断输出。以机智：
  
   <pre>
 <code>
 System.out.printf("%.5g\n", 1.03125);
System.out.printf("%.5g\n", 1.09375);
1.0313
1.0938

</code>
 </pre>
  
     <code>
 gcc
 </code>
     正确地舍入到偶数：
  
   <pre>
 <code>
 printf("%.5g\n", 1.03125);
printf("%.5g\n", 1.09375);
1.0312
1.0938

</code>
 </pre>
  
    请注意，自1/32 = 0.3125以来，1.03125和1.09375可以完全表示为双精度。
  
  
    Java的printf％g格式错误地截断了它的输出：
  
   <pre>
 <code>
 double d = 1;
for (int i = 0; i < 1035; i++) d /= 2;
System.out.printf("%.20g\n%.20a\n", d, d);
2.7161546124360000000e-312
0x0.00080000000000000000p-1022

</code>
 </pre>
  
    这是正确的答案：
  
   <pre>
 <code>
 double d = 1;
for (int i = 0; i < 1035; i++) d /= 2;
printf("%.20g\n%.20a\n", d, d);
2.7161546124355485633e-312
0x0.00080000000000000000p-1022

</code>
 </pre>
  
     <code>
 1.0e-200
 </code>
     是正常但不完全可代表。 Java假装不注意：
  
   <pre>
 <code>
 System.out.printf("%.20g\n%.20a\n", 1.0e-200, 1.0e-200);
1.0000000000000000000e-200
0x1.87e92154ef7ac0000000p-665

</code>
 </pre>
  
    这是正确的答案：
  
   <pre>
 <code>
 printf("%.20g\n%.20a\n", 1.0e-200, 1.0e-200);
9.999999999999999821e-201
0x1.87e92154ef7ac0000000p-665

</code>
 </pre>
  
    所以你要么在你的printf中接受奇怪的舍入行为，要么你背着
     <code>
 gcc
 </code>
     和
     <code>
 glibc
 </code>
    的工作。我不建议您自己尝试打印浮点数。或者你可以使用
     <code>
 %a
 </code>
    ，AFAIK在Java中完美运行。
  
</DIV>