Converting a Character String to a Float - c

Question

I'm trying to take the user input, which is a character string, and convert it to a float. In my case, gas keeps being printed as 55.000000 when the user enters 7 - I'd like it to be printed as 7.0.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
int main()
{
char gas_gallons;
float gas;
printf("Please enter the number of gallons of gasoline: ");
scanf("%c", &gas_gallons);
while (!isdigit(gas_gallons))
{
printf("\nYou need to enter a digit. Please enter the number of gallons of gasoline: ");
scanf("%c", &gas_gallons);
}
if (isdigit(gas_gallons))
{
printf("\nHello %c", gas_gallons);
gas = gas_gallons;
printf("\nHello f", gas);
}
return 0;
}

Answer 1

Why not do this? It's much simpler.
#include<stdio.h>
int main()
{
int gas;
printf("Please enter the number of gallons of gasoline.\n : ");
//use the %d specifier to get an integer. This is more direct.
//It will also allow the user to order more than 9 gallons of gas.
scanf("%d", &gas);
printf("\nHello %d", gas);//prints integer value of gas
//Using the .1f allows you to get one place beyond the decimal
//so you get the .0 after the integer entered.
printf("\nHello %.1f\n", (float) gas);//print floating point
return 0;
}

Answer 2

You said:
In my case, gas keeps being printed as 55.000000 when the user enters 7
When the user enters 7 as input the digit 7 is stored as the character in gas_gallons. The decimal value of the character 7 is 55 in ASCII encoding. You can see the decimal values of other characters in ASCII encoding at Wikipedia and many other places on the web.
When you use:
gas = gas_gallons;
the integer value of gas_gallons is, i.e. 55, is assigned to gas. That explains why you get 55.000000 as the output when you print gas.
You can fix the problem many ways. Here are a couple of suggestions.
Option 1
Convert the digit to a number by using:
gas = gas_gallons - '0';
Option 2
Discard the code to read the number of gallons of gasoline as a digit and converting the digit to a number. Using a digit is also limiting since you cannot have 10 or 12.5 as input.
Read the number of gallons of gasoline as a number directly. With this approach, your input can be a any floating point number that can be represented by a float.
#include <stdio.h>
int main()
{
float num_gallons;
while ( 1 )
{
printf("Please enter the number of gallons of gasoline: ");
// Read the number of gallons of gas.
// If reading is successful, break of the loop.
if (scanf("%f", &num_gallons) == 1 )
{
break;
}
// There was an error.
// Read and discard the rest of the line in the input
// stream.
scanf("%*[^\n]%*c");
printf("There was an error in reading the gallons of gasoline.\n");
}
printf("\nHello %f\n", num_gallons);
return 0;
}

Answer 3

The ASCII-characters '0'-'9' do not have the integer values 0-9. You can find the appropriate value by subtracting '0'.

Answer 4

To convert a string to float you can use atof (ASCII to float) function included in stdlib.h.
Here is the full declaration of this function: double atof(const char *str)
so you can do a simple cast
gas = (float) atof(gas_gallons);