Thread: Really Basic code, help.

Originally Posted by Ryan Gregg

Hey i'm just practicing my knowledge of C so i've tried to make this crude calculator where you enter two numbers, type whether you want to multiply, add, subtract or divide and it returns the answer. My code is here:

However when I compile, it just comes up with the "Insert two positive values for x and y" message even after putting two numbers in. I get the feeling I need to assign memory for this, but im unsure as how to do this.

I'm also unsure whether validInput = (argc == 7); is correct either. Is 7 correct? I don't know what number to put here, in a previous example there were two values for the valid inputer and it had argc == 3.

Any help would be great thanks.

*EDIT* Also as well, should I be using if, else, if, else... etc? I've just got a line of if's... Got a feeling that isnt correct either.

Most of this can't be answered because we don't know how your program should be called. Some possibilities

Code:

$ calculator 3 + 4
$ calculator 3 4 -
$ calculator 3 4 addition=no subtraction=no multiplication=yes division=no
...

The last one looks weird, but makes of bit of sense if you think argc should be 7.

So first, study this: FAQ > Accessing command line parameters/arguments - Cprogramming.com. Get a basic feel for working with command line arguments. Note, argc is the number of things passed on the command line, so the correct value depends on how you want your program to be run.

Thus, after you understand how to use them in general, you will need to figure out how you want your program to be run.

As for the list of if statements, it depends. You use a sequence of if's when you have several conditions that may all be true: e.g. you want the user to be able to specify more than one operation, to see the sum, difference, product and quotient of 3 and 4 all in one run. You use an if-else if sequence if the conditions should be mutually exclusive: you just want one mathematical operation per run. You'll have to determine how you want your program to behave. Often times, I prefer switch/case statements instead of if-else.

Oh, and this line

Code:

if(x && y <= 0)

Doesn't do what you think. Consult a C operator precedence chart(C Operator Precedence Table). You'll notice that <= has higher precedence than &&. Thus, the above line is equivalent to

Code:

if(x && (y <= 0))

In plain English: If x is non-zero and y is less than or equal to zero.... You need to check these independently: if (x <= 0 && y <= 0)

Also, note that scanf (and fscanf and sscanf) return the number of this successfully scanned, or EOF on end of input or error. EOF is guaranteed to be negative (non-zero). If you ask for 1, a return value of 1 is successful, however if there were problems, you could get a 0 or EOF. Thus, your validInput && scanf... statements are not correct, they will report validInput as true if scanf returns EOF. Instead, try

Code:

validInput = validInput && (scanf(...) != 1);  // replace 1 with the correct number of things you are trying to scan with your format specifier

Also, you should consider checkint validInput after each step, so you can report specifically what the user did wrong: numbers must be positive, $ is not a valid operator, only one operator allowed, only two numbers allowed, etc.

Note, input validation is a giant pain in the butt to get correct. For basic programs like this that need some tolerance, but do not need to be as robust as commercial products, I recommend just reading a line with fgets (remember to remove the newline if there: FAQ > Get a line of text from the user/keyboard (C) - Cprogramming.com) and then use sscanf or strtol/strtod functions to parse out numbers.

Maybe other errors, but this should keep you plenty busy.

Thanks a lot for replying, it helped a lot. I'm not sure what this part means though:

Code:

$ calculator 3 + 4
$ calculator 3 4 -
$ calculator 3 4 addition=no subtraction=no multiplication=yes division=no
...

Where about would I put that in the code?