checking string if number or not

I've made a function that determines if character string is a number or not.Using only isdigit().

Here is my sample code

Code:

---

#include <string.h>
#include <stdio.h>
#include <ctype.h>

int isNumber(char str[])
{
        int withDecimal =0,isNegative=0 ,i=0;
        int len = strlen(str);

        for (i=0; i<len; i++)
        {
                if (!isdigit(str[i])) // if1
                {
                        if (str[i] == '.')
                        {
                                if(withDecimal){
                                        return 0;
                                }
                                withDecimal =1;
                        }
                        else if (str[i] == '-')
                        {
                                if(isNegative){
                                        return 0;
                                }

                                if(i==0){
                                        isNegative = 1;
                                }else{
                                        return 0;
                                }

                        }else{
                                return 0;
                        }
                } //end if1
        }// end for
        return 1;
}

int main(){
       
        //for testing purpose only
        char charNum[10][15] = {
                "100.23" ,
                "-200.456",
                "200..",
                "..200",
                "4.9.9.6",
                "--500",
                "500--",
                "59-52-6",
                "50.69-5",
                "5000"
        };

        int cntr = 0;

                for(cntr=0; cntr<10; cntr++){
                        printf("%s\t\t", charNum[cntr]);

                        if(isNumber(charNum[cntr])){
                                printf("is a number!\n");
                        }else{
                                printf("NOT a NUMBER!\n");
                        }
                }

}

---

and here is the output

Code:

---

100.23          is a number!
-200.456        is a number!
200..          NOT a NUMBER!
..200          NOT a NUMBER!
4.9.9.6        NOT a NUMBER!
--500          NOT a NUMBER!
500--          NOT a NUMBER!
59-52-6        NOT a NUMBER!
50.69-5        NOT a NUMBER!
5000            is a number!

---

as you can see. The function works fine.But to be on the safe side.I need to know your reaction on the function, whether it needs to improve on some aspects (memory utilization or something).

regards,
Jaro

Personally, I would have your function return 0 when the char is a digit and -1 on error

...but this is probablyon a personal thing

I think "-." also passes your tests

Quote:

---

Originally Posted by Salem

I think "-." also passes your tests

---

my lame excuse behind allowing "-." because some people input number this way.
i.e. normally we write some number in this format "-0.25" but some people will write "-.25" which mathematically speaking is the same.
well,I hope that this reasoning is good for your taste.

anyway, thanks for the quick feedback.and actually without your comment I would not found another two errors.Apparently the characters "-" and "." will be considered a number.

regards,
Jaro

I think Salem was saying if you inputted "-." only it would register as a number. An easy way out if simply to check the length of the input string. If it has a length of 1 and withDecimal or isNegative are set return 0, and if the length is 2 and withDecimal and isNegative are set return 0.

Quote:

---

Personally, I would have your function return 0 when the char is a digit and -1 on error

...but this is probablyon a personal thing

---

Consider the function name, isNumber(). Now all of the standard is*() functions return nonzero if the input meets the condition and 0 if it does not. So the way the function is written is in keeping with that. Also 0 means false while nonzero means true. So a commonily used shortcut would be something like:

Code:

---

if ( isNumber("2.7818") )
{
  // Blah
}

---

and

Code:

---

if ( !isNumber("3.1478") )
{
  // Bah its not a number!
}

---

Changing the return values to what you suggested would negate the logical use of the function.

Quote:

---

Originally Posted by jaro

I've made a function that determines if character string is a number or not.

---

Why not just learn how to use strtod? Consider also something like "+1.234E-05".

Quote:

---

Originally Posted by Thantos

I think Salem was saying if you inputted "-." only it would register as a number.

---

I just realized what Salem was really trying to convey after I post a reply.I was so comsumed in finding a fix in this error, that why I failed to understand it clearly.

anyway, with your suggestion I have made some changes in the code.

and here's the code

Code:

---

#include <string.h>
#include <stdio.h>
#include <ctype.h>

int isNumber(char str[])
{
        int withDecimal =0,isNegative=0 ,i=0;
        int len = strlen(str);

        for (i=0; i<len; i++)
        {
                if (!isdigit(str[i])) // if1
                {
                        if (str[i] == '.')
                        {
                                if(len!=1 && str[i+1] != '-')
                                {
                                        if(withDecimal){
                                                return 0;
                                        }
                                        withDecimal =1;
                                }else{
                                        return 0;
                                }
                        }
                        else if (str[i] == '-')
                        {
                                if(len!=1){
                                        if(str[i+1] == '.')
                                        {
                                                if(isdigit(str[i+2])){
                                                        if(isNegative){
                                                                return 0;
                                                        }

                                                        if(i==0){
                                                                isNegative = 1;
                                                        }else{
                                                                return 0;
                                                        }
                                                }else{
                                                        return 0;
                                                }
                                        }
                                }else{
                                        return 0;
                                }
                        }else{
                                return 0;
                        }
                } //end if1
        }// end for
        return 1;
}

int main(){
       
        //for testing purpose only
        char charNum[10][15] = {
                "100.23" ,
                "-200.456",
                "200..",
                "-..30",
                "4.9.9.6",
                "-.65",
                "-0.54",
                "-.",
                ".",
                "-"
        };

        int cntr = 0;

                for(cntr=0; cntr<10; cntr++){
                        printf("%s\t\t", charNum[cntr]);

                        if(isNumber(charNum[cntr])){
                                printf("is a number!\n");
                        }else{
                                printf("NOT a NUMBER!\n");
                        }
                }

}

---

OUTPUT

Code:

---

100.23          is a number!
-200.456        is a number!
200..          NOT a NUMBER!
-..30          NOT a NUMBER!
4.9.9.6        NOT a NUMBER!
-.65            is a number!
-0.54          is a number!
-.              NOT a NUMBER!
.              NOT a NUMBER!
-              NOT a NUMBER!

---

regards,
Jaro

Quote:

---

Originally Posted by Dave_Sinkula

Why not just learn how to use strtod? Consider also something like "+1.234E-05".

---

just one quick question. can I use strtod in C?

edit:
just found out that I can use strtod in C. Well it does make works lot more easier :)

Yes.

Quote:

---

Originally Posted by Dave_Sinkula

Why not just learn how to use strtod? Consider also something like "+1.234E-05".

---

well strtod has made my number checking easy.and with less line of codes :) .

I have added an extra function (checkNumber) in my code which basically do the same as the previous isNumber function .

Code:

---

#include <string.h>
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

char errorNum[20]="";
char tempVal[100]="";

int isNumber(char str[])
{
        int withDecimal =0,isNegative=0 ,i=0;
        int len = strlen(str);

        for (i=0; i<len; i++)
        {
                if (!isdigit(str[i])) // if1
                {
                        if (str[i] == '.')
                        {
                                if(len!=1 && str[i+1] != '-')
                                {
                                        if(withDecimal){
                                                return 0;
                                        }
                                        withDecimal =1;
                                }else{
                                        return 0;
                                }
                        }
                        else if (str[i] == '-')
                        {
                                if(len!=1){
                                        if(str[i+1] == '.')
                                        {
                                                if(isdigit(str[i+2])){
                                                        if(isNegative){
                                                                return 0;
                                                        }

                                                        if(i==0){
                                                                isNegative = 1;
                                                        }else{
                                                                return 0;
                                                        }
                                                }else{
                                                        return 0;
                                                }
                                        }
                                }else{
                                        return 0;
                                }
                        }else{
                                return 0;
                        }
                } //end if1
        }// end for
        return 1;
}


int checkNumber(char str[]){

        char *ok;
        ok = str;
        if(strlen(ok)==0){
                return 1;
        }else{
                return 0;
        }

}


int main(){
       
        //for testing purpose only
        char charNum[10][15] = {
                "100.23" ,
                "-200.456",
                "0.0",
                "-..30",
                ".-",
                "-.65",
                "-0.54",
                "0",
                ".",
                "+1.234E-05"
        };
       
        int cntr = 0;
       

                for(cntr=0; cntr<10; cntr++){
                        printf("%s\t\t", charNum[cntr]);

                        if(isNumber(charNum[cntr])){
                                printf("is a number!\n");
                        }else{
                                printf("NOT a NUMBER!\n");
                        }
                }
               
                printf("\n=================================\n\n");

                for(cntr=0; cntr<10; cntr++){

                        printf("%s\t\t", charNum[cntr]);
                        if(checkNumber(charNum[cntr])){
                                printf("is a number!\n");
                        }else{
                                printf("NOT a NUMBER!\n");
                        }
                }

}

---

OUTPUT

Code:

---


100.23          is a number!
-200.456        is a number!
0.0            is a number!
-..30          NOT a NUMBER!
.-              NOT a NUMBER!
-.65            is a number!
-0.54          is a number!
0              is a number!
.              NOT a NUMBER!
+1.234E-05      NOT a NUMBER!

=================================

100.23          is a number!
-200.456        is a number!
0.0            is a number!
-..30          NOT a NUMBER!
.-              NOT a NUMBER!
-.65            is a number!
-0.54          is a number!
0              is a number!
.              NOT a NUMBER!
+1.234E-05      is a number!

---

I haven't add any error checking in strtod() but as you can see, the function that uses strtod is clearly accurate.

I'm not really familiar with the memory comsumption and portability by these function.
So the question now is which of the two, isdigit() or strtod() is portable?
and also between the two function checkNumber() and isNumber(), which is best to use, in terms of memory consumption.

regards,
jaro

Quote:

---

Originally Posted by wintellect

Personally, I would have your function return 0 when the char is a digit and -1 on error

...but this is probablyon a personal thing

---

Why would you do something like that? It's contrary to every other istype function (and C/C++ standards). Any idea what FALSE equates to?

My version was like this.

Code:

---

int my_isNumber(const char str[])
{
  char *end;
  double result = strtod(str, &end);
  return !isspace(*str) && end != str && *end == '\0';
}

---

I don't know whether you want to handle leading and trailing whitespace, however.

Quote:

---

Originally Posted by jaro

So the question now is which of the two, isdigit() or strtod() is portable?

---

Both are standard C; that's about as portable as it gets.

Quote:

---

Originally Posted by jaro

and also between the two function checkNumber() and isNumber(), which is best to use, in terms of memory consumption.

---

Code correctly first. If you have later proven it to be a troublesome bottleneck by profiling, then you worry about that. "Premature optimization is the root of all evil."