& non lvalue error

[fperri]

I'm just learning C, and I'm trying to get this simple program to validate the user's entry making sure that they entered a number.

I compile and I get this:

Miracle C Compiler (r3.2), written by bts.
Compiling C:\Users\Franccesca\Documents\Code\userInput.c
main

C:\Users\Franccesca\Documents\Code\userInput.c
line 13: & non lvalue
'scanf("%s", &buff)'
aborting compile

Here is my code, and I can't figure out why it isn't working. Does anyone have any ideas?

#include <stdio.h>
 
 
main () {
char buff[10];
int validInput; //1 = Good , 2 = Bad
int i;
 
 
//GET PURCHASE AMOUNT FROM USER
do {
    printf("\nPlease enter purchase amount:", stdout);
    scanf("%s", &buff);
    validInput = 1; //SET TO VALID TO START
 
    // IF NOTHING ENTERED
        if(lstrlen(buff) == 0) validInput = 2;
        for(i = 0; i < lstrlen(buff); i++){
                if(buff[i] != 46){ // IF NOT A DECIMAL POINT
                   if((buff[i] < 48) || (buff[i] > 57)){   //IF NOT 0 - 9
                      validInput = 2;
                   }
                }
 
            }
        }
    while (validInput == 2);
}

[Banfa]

The & on line 13 is not required buff is alread a pointer to char (char *) which is what is required.

Using %s in scanf is extremely dangerous as there is no buffer overrun protection.

[JosAH]

    printf("\nPlease enter purchase amount:", stdout);

This is so very wrong while harmless in this particular case ...
Check your C standard library and check Banfa's reply for the cause of your error.

kind regards,

Jos

[fperri]

The & on line 13 is not required buff is alread a pointer to char (char *) which is what is required.

Using %s in scanf is extremely dangerous as there is no buffer overrun protection.

What would you suggest using?

[fperri]

This is so very wrong while harmless in this particular case ...
Check your C standard library and check Banfa's reply for the cause of your error.

kind regards,

Jos

Why is it so very wrong?

[weaknessforcats]

What would you suggest using?

Use a getchar(). Then you can call IsDigit() on the character and if it is a digit, you can add it to a buffer. When the numbers are entered, add a \0 the buffer and then use atoi() to get your int.

[fperri]

Use a getchar(). Then you can call IsDigit() on the character and if it is a digit, you can add it to a buffer. When the numbers are entered, add a \0 the buffer and then use atoi() to get your int.

Doesn't the getchar() only grab one character? I need to be able to grab a number like 125.25 if that's what they enter.

[fperri]

Doesn't the getchar() only grab one character? I need to be able to grab a number like 125.25 if that's what they enter.

Why is it so hard to find information on validating user input in C? I just want to be able to only take in a number and have the program keep prompting for the number until a valid number is entered. I cannot find a decent example anywhere online. I can find all the functions for input, but no real examples using it how I want to. Is it really this difficult in C or is it just me?

[Banfa]

I would use fgets. This reads from a stream but you can use the identifier stdin to read from standard in.

The advantage of this function is that it takes the buffer size so you can eliminate the possibility of buffer overrun errors.

Having read a string into the buffer I would then call strtol (or strtoul), these function convert a string of digits to a long value, however the advantage of these over others is that they return a point to the place the conversion stop so you can see what caused the conversion to halt thus allowing you to fairly easily validate your input as a number.


Don't worry you are not the only person having trouble with this sort of thing, we see it all the time on the forum.

[weaknessforcats]

Why is it so hard to find information on validating user input in C? I just want to be able to only take in a number and have the program keep prompting for the number until a valid number is entered. I cannot find a decent example anywhere online. I can find all the functions for input, but no real examples using it how I want to. Is it really this difficult in C or is it just me?

Both C and C++ avoid making assumptions. In this case, you are expected to write a function that prompts for data and returns your number. Once you have got that working, you put it in it's own source file, you write a header file for it and you use it for the rest of your life.

Input acquisition and validation are not simple. Requirements vary from program to program so it's really not possible to provide a solution for a specific case. There may not be a keyboard. The number may be in a different number system. The prompt may not apply to all programs, etc.