Converting string to float

[DirtyRasa]

Here is what my professor told me to to.

Write a function that converts a string to a float and returns a float.
Test your function by entering f4 and with 4f. Both should say
Data entered was not a number. Try again.
and ask the user to type in another number.

Here's what I have so far.

Code:

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

#define MAX_SIZE 80

void readString(char prompt[], char str[]);// Exercise 1
float readNumber(char prompt[], char str[]);// Exercise 2

int main()
{
	char str[MAX_SIZE];
        printf("You entered: %f\n", readNumber("Please enter a number: ", str));
}

void readString(char prompt[], char str[]) //This asks the user for a string
{
	printf("%s", prompt);
	gets(str);
}//Exercise 1

float readNumber(char prompt[], char str[]) //This function will convert the string   in to a float using readString
{
	int flag = 0; //0 means no; 1 means yes
	int i;
	float number;

	readString(prompt, str);
	sscanf(str, "%f", &number);

	if(number == 0)
	{
		while(flag == 0)
		{
			printf("Data entered was not a number, Try again.\n");
			readString(prompt, str);
			sscanf(str, "%f", &number);
			if(number != 0)
				flag = 1;
		}
	}
	return number;
}//Exercise 2

My problem is using sscanf. I'm not exactly sure how to use it so that it will only return a number if and only if the user enters a number. I'm guessing my if statements are incorrect as well, but I don't know how to use sscanf at all so I don't know how to check if the string only contains floats.

[Shashi Sadasivan]

is the code returning any problems?
what you also need to consider is if the user enters 0 (zero) as input.
In that case your code will tell that the user did not enter a number.

Have a look at the return type of sscanf, your code of

if(number == 0)

will throw errors if the string entered by the user is not a number

[DirtyRasa]

No, its just that when I enter 4F it will return 4.000000. I want it to return: "Data entered was not a number. Try again."

So, somehow I need it to use the sscanf function and make sure that when it converts the string to a float that the string ONLY contains numbers and if it doesn't it will ask the user to enter a new value for the string until the user enters ONLY numbers including floats.
So if I enter 4.0 it will return 4.00000
or if I enter -2 it will return -2.0000
but if I enter 4f it should return "Data entered was not a number. Try again." and prompt me for another number.

Hopefully you understand and can help me.
I'm guessing my problem lies in the if statement. I have no idea on how to check if the string is made up of ONLY numbers.

EDIT
I changed my code around a bit

float readNumber(char prompt[], char str[])
{
	int flag = 0; //0 means no; 1 means yes
	int i;
	float number;

	readString(prompt, str);
	i = sscanf(str, "%f", &number);

	if(i != 1)
	{
		while(flag == 0)
		{
			printf("Data entered was not a number, Try again.\n");
			readString(prompt, str);
			i = sscanf(str, "%f", &number);
			if(i == 1)
				flag = 1;
		}
	}
	return number;
}//Exercise 2

I just changed the if statements. Now if the user enters 0 it will work. and if the user enters a character first than it will ask the user for another number. but if i have a character after the number it will just print out the number which should NOT happen.

[mattmao]

I'm guessing my problem lies in the if statement. I have no idea on how to check if the string is made up of ONLY numbers.

Hi.

There is a potentially useful function in the ctype.h lib for your consideration:

Function: int isdigit (int c)
Returns true if c is a decimal digit (`0' through `9').

If you take the user input as a string, then you can use a loop to check each character to validate it.

Also you can use another function: strtol

http://www.gnu.org/software/libc/manual/html_node/Parsing-of-Integers.html

BTW I took a look at your code, it is made up of exercise fragments, which is not a good idea. For keeping the top-down design, you should think the case to handle as a whole, then divide and conquer. The variable parameters in each of your functions don't make any sense up to now.

My suggestion is: try to finish the input validation part first, then think further...

[DirtyRasa]

Hi.

There is a potentially useful function in the ctype.h lib for your consideration:

Function: int isdigit (int c)
Returns true if c is a decimal digit (`0' through `9').

If you take the user input as a string, then you can use a loop to check each character to validate it.

Also you can use another function: strtol
http://www.gnu.org/software/libc/man...-Integers.html

Is there a way to do this using only the sscanf function? I think my prof would get mad if I did it any other way.
He didn't really tell us what we could or could not do though so I don't know. It's just when I started to use atof() he yelled at me so I don't know. So if there is a way to do this with sscanf mainly then any pointers in the right direction would be a great help. For now I'll just use isdigit.

[mattmao]

Is there a way to do this using only the sscanf function? I think my prof would get mad if I did it any other way.
He didn't really tell us what we could or could not do though so I don't know. It's just when I started to use atof() he yelled at me so I don't know. So if there is a way to do this with sscanf mainly then any pointers in the right direction would be a great help. For now I'll just use isdigit.

Emmm, that is pretty strange.

This is not a research at all, google the references online shouldn't be blamed for...

This is some references regarding sscanf, I am looking at it:

http://www.crasseux.com/books/ctutorial/sscanf.html

http://www.cplusplus.com/reference/clibrary/cstdio/sscanf.html

Yeah, you can use the return value of sscanf to check if input is valid or not.

if(sscanf (input, "%f", &number))
{
//do what ever you want to for the valid input.
}
else
{
//ask to give a new input.
}


BTW, the gcc compiler warns me that:

bash-3.2$ gcc test.c
/tmp/ccqdvPdm.o: In function `takeInput':
test.c:(.text+0x42): warning: the `gets' function is dangerous and should not be used.

[DirtyRasa]

I still get it where if I enter 4f it returns 4

float readNumber(char prompt[], char str[])
{
	int flag = 0; //0 means no; 1 means yes
	float number;

	readString(prompt, str);

	if(sscanf (str, "%f", &number))
		return number;
	else
	{
		while(flag == 0)
		{
			printf("Data entered was not a number, Try again.\n");
			readString(prompt, str);
			if(sscanf(str, "%f", &number))
				{
					flag = 1;
					return number;
				}
		}
	}

}//Exercise 2

I'm not sure what the if statement should be checking for... Thanks for your help though.

[mattmao]

I still get it where if I enter 4f it returns 4

float readNumber(char prompt[], char str[])
{
	int flag = 0; //0 means no; 1 means yes
	float number;

	readString(prompt, str);

	if(sscanf (str, "%f", &number))
		return number;
	else
	{
		while(flag == 0)
		{
			printf("Data entered was not a number, Try again.\n");
			readString(prompt, str);
			if(sscanf(str, "%f", &number))
				{
					flag = 1;
					return number;
				}
		}
	}

}//Exercise 2

I'm not sure what the if statement should be checking for... Thanks for your help though.

Hi.

Use this:

float num;
    char *temp;
    if(sscanf(str, "%f%*s", &num, temp))

This can solve that particular problem.