How do you make a menu interface that accepts double digit integers or characters in C? - c

Question

I was fumbling with this program for the last couple of hours and I can't seem to find a way to get this program to work. I started out with a switch statement style menu but then I had an issue where the menu would fall through and exit and I couldn't figure that out so I just switched my code over to an if else based menu. The idea behind the program is as follows:
Write and test a C program that implements a stack based integer-based calculator. The program accepts input until q is entered. However my difficulties lie in getting the menu to accept numbers larger than 10.
I have every single function working properly in my program except when I enter a two digit integer it will store both digits individually. I know that this is because I have the menu setup to read and work with chars, but I wasn't able to figure out how to get an array of chars to work. I've never programmed in C before so the idea of dynamic memory allocation alludes me as I'm not entirely sure when it is necessary. Here is the source code I have for the program so far:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <ctype.h>
#define SIZE 6
int stack[SIZE]; //stack size
int top = 0; //top of stack
void pop();
void clear();
void display();
void top_element();
void add();
void multiply();
void subtract();
void division();
void power();
int main()
{
char input;
int flag = 1;
while(flag == 1)
{
printf(": ");
scanf(" %c",&input);
if(isdigit(input))
{
if(top < SIZE)
{
stack[top] = input - '0';
top++;
}
else
printf("Error: stack overflow\n");
}
else if(input=='p')
pop();
else if(input=='c')
clear();
else if(input=='d')
display();
else if(input=='=')
top_element();
else if(input=='+')
add();
else if(input=='*')
multiply();
else if(input=='-')
subtract();
else if(input=='/')
division();
else if(input=='^')
power();
else if(input=='q')
flag = 0;
else
printf("Error: invalid command\n");
}
printf("Goodbye!\n");
return 0;
}
void pop()
{
if(top==0)
printf("Error: stack is empty\n");
else
top--;
}
void clear()
{
top=0;
}
void display()
{
int i;
if(top == 0)
printf("Error: stack is empty\n");
else
{
for(i = top - 1; i >= 0; i--)
printf("%d\n",stack[i] );
}
}
void top_element()
{
printf("%d\n",stack[top-1] );
}
void add()
{
if(top<2)
printf("Error: not enough operands for the requested operation\n");
else
{
int ans=stack[top-1]+stack[top-2];
stack[top-2]=ans;
top--;
}
}
void multiply()
{
int ans=stack[top-1]*stack[top-2];
stack[top-2]=ans;
top--;
}
void subtract()
{
if(top < 2)
printf("Error: not enough operands for the requested operation\n");
else
{
int ans = (stack[top-2] - stack[top-1]);
stack[top-2]=ans;
top--;
}
}
void division()
{
if(top < 2)
printf("Error: not enough operands for the requested operation\n");
else
{
if(stack[top-1]==0)
printf("Error: attempt to divide by 0\n");
else
{
int ans = (stack[top-2]/stack[top-1]);
stack[top-2]=ans;
top--;
}
}
}
void power()
{
if(top < 2)
printf("Error: not enough operands for the requested operation\n");
else
{
int ans = pow(stack[top - 2], stack[top - 1]);
stack[top - 2] = ans;
top--;
}
}

Answer 1

I have a few things to note and don't want to turn this into a TLDR so I'll try to keep each issue to separate paragraphs. You can take this all with a grain of salt; it is just advice, after all.
The format directive you're looking for is %2[0123456789]. Pass a pointer to a location suitably sized to store three characters (i.e. char something[3]; the third byte for the null character) and check the return value. This directive needs to go in a call to scanf on its lonesome, or you'll likely have an aneurysm debugging an issue related to empty fields later on, so the "green light" return value indicating your program is successfully processing good input is that scanf("%2[0123456789]", ptr_into_array_of_char) will return 1. Any other return value means amber or red lights happened. Mind you, I'm interpreting your specs (which are incomplete) quite strictly here... in reality I'd just use %d and be happy that my users are halving their chances of developing arthritis by entering 1 instead of 01 (and you're also less likely to have aneurysms when not dealing with %[).
Our compilers usually issue error messages and abort compilation when we make some syntax error, but this requirement goes against that grain: "The program accepts input until q is entered." I hope your full spec explains what should happen when the user deviates from the expectations. I suppose you could issue an error, clear the stack, read to end of line and just operate as though the program restarted... something like scanf("%*[^\n]"); getchar(); puts("Error message here"); top = 0;? We typically use some key combination like CTRL+d (on Linux) or CTRL+Z (on Windows) to close stdin thus denoting termination of input.
"the idea of dynamic memory allocation alludes me" and so you'll be thankful to know that you probably shouldn't use dynamic memory allocation here, unless you want your stack to grow beyond the hardcoded 6 slots that you've set, perhaps...
I assume the title for this question is mixed up in the confusion; you're not designing a menu, but instead implementing a grammar. Look how gccs "menu" is designed for inspiration here. If you're ever tempted to design a menu around stdin, stop; perhaps what you really want is a GUI to point and click because that's not how Unix tends to work.
Declaring void fubar(void); followed by void fubar() { /* SNIP */ } is undefined behaviour due to some technical historical artefacts, and the same goes for int main()... This is why you might be best to choose a book which teaches C specifically, written by somebody reputable, to learn C. There are lots of subtle nuances that can trap you.
On the note of function prototypes and so forth, consider that a stack is a generic data structure. As an alternative thought experiment, consider what a pain strcpy would be to use if it only operated on arrays declared with file scope. It follows logically that all of its external data requirements should come from its arguments, rather than from a variable i.e. stack declared with file scope.
We're taught to use memory somewhat cautiously, and it seems to me as though using a variable solely as a controlling expression like this contravenes those lessons. Where constructs such as break, continue and goto exist, a cleaner alternative without variable declarations (and thus an extra free register to use for something else) is possible.

Answer 2

The problem is not in scanf() this time but in the way you parse the input.
It is not wrong to parse the input character by character, to the contrary it make things much easier, at least in almost all of the cases. But parsing character by character means also that you parse every positive number larger than nine also character by character or better digit by digit--you have to build the complete number out of the single digits. You parse from "the left to the right", so just multiply by ten and add the digit. Rinse and repeat until you have no digits left and put the result on the stack.
Example:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <ctype.h>
#define SIZE 6
/* you should look up what "static" means and when and how to use */
static int stack[SIZE]; //stack size
static int top = 0; //top of stack
/*
* You need to expicitely add "void" to the argumet list.
* It defaults to "int" otherwise.
* Please do yourself a favor and switch all warnings on.
*/
void pop(void);
void clear(void);
void display(void);
void top_element(void);
void add(void);
void multiply(void);
void subtract(void);
void division(void);
void power(void);
/* Most checks and balances omitted! */
int main(void)
{
/* "int" to make things easier */
int input;
int flag = 1, anumber;
while (flag == 1) {
printf(": ");
/* get a(n ASCII) character */
input = fgetc(stdin);
if (isdigit(input)) {
anumber = 0;
/*
* We have a digit. Parse input for more digits until
* no further digits appear and add all digits to "anumber".
* We assume a decimal representation here.
*/
/* TODO: check for overflow! */
for (;;) {
anumber *= 10;
anumber += input - '0';
input = fgetc(stdin);
if (!isdigit(input)) {
break;
}
}
/* Push number on the stack */
if (top < SIZE) {
stack[top] = anumber;
top++;
} else {
printf("Error: stack overflow\n");
}
}
/* "input" from fgetc() is an integer, we can use a switch */
switch (input) {
case 'p':
pop();
break;
case 'c':
clear();
break;
case 'd':
display();
break;
case '=':
top_element();
break;
case '+':
add();
break;
case '^':
power();
break;
case 'q':
flag = 0;
break;
default:
printf("Error: invalid command\n");
break;
}
}
printf("Goodbye!\n");
return 0;
}
void pop(void)
{
if (top == 0)
printf("Error: stack is empty\n");
else
top--;
}
void clear(void)
{
top = 0;
}
void display(void)
{
int i;
if (top == 0)
printf("Error: stack is empty\n");
else {
for (i = top - 1; i >= 0; i--)
printf("%d\n", stack[i]);
}
}
void top_element(void)
{
printf("%d\n", stack[top - 1]);
}
void add(void)
{
if (top < 2)
printf("Error: not enough operands for the requested operation\n");
else {
int ans = stack[top - 1] + stack[top - 2];
stack[top - 2] = ans;
top--;
}
}
/* Using pow() from math.h is not a good idea beause it uses floating point */
/* TODO check for overflows! */
static int integer_pow(int x, int n)
{
int r;
r = 1;
while (n != 0) {
if (n & 1) {
r *= x;
}
x *= x;
n >>= 1;
}
return r;
}
void power(void)
{
if (top < 2)
printf("Error: not enough operands for the requested operation\n");
else {
int ans = integer_pow(stack[top - 2], stack[top - 1]);
stack[top - 2] = ans;
top--;
}
}
Test:
$ ./stackbcalc
: 123+23=
Error: not enough operands for the requested operation
: 23
: Error: invalid command
: q
Goodbye!
Does not work. Why? The function add() expects two operands on the stack. You need to put the + also on the stack (it is an integer) and once you are at the end with = you can evaluate the stack. You might need to learn something about infix/postfix/prefix notation to succesfully do so.
Hint: I would also ignore whitespace (space and tab, maybe even return) in the switch.