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Error in function returning string value.

I have this function which takes a double value and returns the engineering notation as a string. Two problems: 1) it can't be called multiple times in a printf, see run results below. Two, if I remove the static declaration I get a compile error (I wrote this code long ago and don't know why I made it static).

#define true 1
#define false 0
#define len 32

char *dtos_engr (double value, char *units, short precision)
/*******************************************************************************************
Returns the string equivalent to <value> in engineering units, i.e.,
<value> = 3.14e-9 and <units> = 'S' will return 3.14nS.
*******************************************************************************************/
{
double lvalue;
short neg=false;
static char str1[len],str2[len];

lvalue = value;
if (lvalue < 0.0) {
lvalue = -lvalue;
neg = true;
}

if (precision == 6) {
if (lvalue == 0.0)
strcpy (str1,"0");
else if (lvalue >= 1.0e9) {
sprintf (str1, "%-.6f",(lvalue/1.0e9));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"G");
} else if (lvalue >= 1.0e6) {
sprintf (str1, "%-.6f",(lvalue/1.0e6));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"M");
} else if (lvalue >= 1.0e3) {
sprintf (str1, "%-.6f",(lvalue/1.0e3));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"K");
} else if (lvalue >= 1.0) {
sprintf (str1, "%-.6f",lvalue);
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
} else if (lvalue >= 1.0e-3) {
sprintf (str1, "%-.6f",(lvalue/1.0e-3));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"m");
} else if (lvalue >= 1.0e-6) {
sprintf (str1, "%-.6f",(lvalue/1.0e-6));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"u");
} else if (lvalue >= 1.0e-9) {
sprintf (str1, "%-.6f",(lvalue/1.0e-9));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"n");
} else {
sprintf (str1, "%-.6f",(lvalue/1.0e-12));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"p");
}
} else if (precision == 3) {
if (lvalue == 0.0)
strcpy (str1,"0");
else if (lvalue >= 1.0e9) {
sprintf (str1, "%-.3f",(lvalue/1.0e9));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"G");
} else if (lvalue >= 1.0e6) {
sprintf (str1, "%-.3f",(lvalue/1.0e6));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"M");
} else if (lvalue >= 1.0e3) {
sprintf (str1, "%-.3f",(lvalue/1.0e3));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"K");
} else if (lvalue >= 1.0) {
sprintf (str1, "%-.3f",lvalue);
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
} else if (lvalue >= 1.0e-3) {
sprintf (str1, "%-.3f",(lvalue/1.0e-3));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"m");
} else if (lvalue >= 1.0e-6) {
sprintf (str1, "%-.3f",(lvalue/1.0e-6));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"u");
} else if (lvalue >= 1.0e-9) {
sprintf (str1, "%-.3f",(lvalue/1.0e-9));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"n");
} else {
sprintf (str1, "%-.3f",(lvalue/1.0e-12));
while (str1[strlen(str1)-1] == '0')
      str1[strlen(str1)-1] = '\0';
if (str1[strlen(str1)-1] == '.')
      strcat (str1,"0");
strcat (str1,"p");
}
} else {
if (lvalue == 0.0)
strcpy (str1,"0");
else if (lvalue >= 1.0e9)
sprintf (str1, "%-.1fG",(lvalue/1.0e9));
else if (lvalue >= 1.0e6)
sprintf (str1, "%-.1fM",(lvalue/1.0e6));
else if (lvalue >= 1.0e3)
sprintf (str1, "%-.1fK",(lvalue/1.0e3));
else if (lvalue >= 1.0)
sprintf (str1, "%-.1f",lvalue);
else if (lvalue >= 1.0e-3)
sprintf (str1, "%-.1fm",(lvalue/1.0e-3));
else if (lvalue >= 1.0e-6)
sprintf (str1, "%-.1fu",(lvalue/1.0e-6));
else if (lvalue >= 1.0e-9)
sprintf (str1, "%-.1fn",(lvalue/1.0e-9));
else
sprintf (str1, "%-.1fp",(lvalue/1.0e-12));
}
if (!strcmp(str1,"0") && !strcmp(units,"S")) {
strcat (str1,"n");
strcat (str1,units);
} else
strcat (str1,units);
if (neg) {
strcpy (str2,"-");
strcat (str2,str1);
strcpy (str1,str2);
}
return (str1);
} /* end of dtos_engr */

main (int argc, char *argv[])
{
double value[6];

value[0] = 1.6e-9;
value[1] = 1.6e-8;
value[2] = 1.6e-7;
value[3] = 1.6e-6;
value[4] = 1.6e-5;
value[5] = 1.6e-4;

printf ("Engr values = %s, %s, %s, %s, %s, %s\n",
       dtos_engr(value[0],"S",1),dtos_engr(value[1],"S",1),
       dtos_engr(value[2],"S",1),dtos_engr(value[3],"S",1),
       dtos_engr(value[4],"S",1),dtos_engr(value[5],"S",1));
}

Output:
Engr values = 160.0uS, 160.0uS, 160.0uS, 160.0uS, 160.0uS, 160.0uS

ddunlea

Hi jimdgar,

You are having trouble because of pointers and variable scoping. What you should do is call the funtion with a char** as an arg.

So in main, have

char str1[LEN];

And then call the function with str1 as an argument.

If you remove the static declaration, then when you return, the char* you are returning goes out of scope and no longer references memory. By having the str1 variable defined within main, it remains in scope and is not destroyed when the function exists. Static variables are persistent between calls, which is why the compile error went away, but that really is not what they are meant for.

ASKER CERTIFIED SOLUTION

imladris

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ddunlea

Not to be a stickler, but I am curious to know why the person who posted after me, with the same answer as me, was awared the points?

jimdgar

ASKER

ddunlea,

The two responses arrived almost simultaneously (yours was first). Even tho you pointed out the root problem your solution didn't work; passing str1 six times returned the same value. imladris' solution to define six separate vars worked right off the bat.

jimdgar

ddunlea

Fair enough. I suspect it was the implementation as opposed to my solution that didn't work though as it worked for me.