The program below is used in various topics to demonstrate debugging tasks.
This program is a simple calculator that reads its input from a character buffer. If integers are read, they are pushed on a stack. If one of the operators (+ - * ⁄) is read, the top two elements are popped off the stack, the operation is performed on them, and the result is pushed on the stack. The = operator writes out the value of the top element of the stack to a buffer.
/*----- FILE CALC.H --------------------------------------------------*/
/* */
/* Header file for CALC.C PUSHPOP.C READTOKN.C */
/* a simple calculator */
/*--------------------------------------------------------------------*/
typedef enum toks {
T_INTEGER,
T_PLUS,
T_TIMES,
T_MINUS,
T_DIVIDE,
T_EQUALS,
T_STOP
} Token;
Token read_token(char buf[]);
typedef struct int_link {
struct int_link * next;
int i;
} IntLink;
typedef struct int_stack {
IntLink * top;
} IntStack;
extern void push(IntStack *, int);
extern int pop(IntStack *);/*----- FILE CALC.C --------------------------------------------------*/
/* */
/* A simple calculator that does operations on integers that */
/* are pushed and popped on a stack */
/*--------------------------------------------------------------------*/
#include <stdio.h>
#include <stdlib.h>
#include "calc.h"
IntStack stack = { 0 };
main()
{
Token tok;
char word[100];
char buf_out[100];
int num, num2;
for(;;)
{
tok=read_token(word);
switch(tok)
{
case T_STOP:
break;
case T_INTEGER:
num = atoi(word);
push(&stack,num); ⁄* CALC1 statement *⁄
break;
case T_PLUS:
push(&stack, pop(&stack)+pop(&stack) );
break;
case T_MINUS:
num = pop(&stack);
push(&stack, num-pop(&stack));
break;
case T_TIMES:
push(&stack, pop(&stack)*pop(&stack));
break;
case T_DIVIDE:
num2 = pop(&stack);
num = pop(&stack);
push(&stack, num/num2); ⁄* CALC2 statement *⁄
break;
case T_EQUALS:
num = pop(&stack);
sprintf(buf_out,"= %d ",num);
push(&stack,num);
break;
}
if (tok==T_STOP)
break;
}
return 0;
}/*----- FILE PUSHPOP.C -----------------------------------------------*/
/* */
/* A push and pop function for a stack of integers */
/*--------------------------------------------------------------------*/
#include <stdlib.h>
#include "calc.h"
/*--------------------------------------------------------------------*/
/* input: stk - stack of integers */
/* num - value to push on the stack */
/* action: get a link to hold the pushed value, push link on stack */
/* */
extern void push(IntStack * stk, int num)
{
IntLink * ptr;
ptr = (IntLink *) malloc( sizeof(IntLink)); /* PUSHPOP1 */
ptr–>i = num; /* PUSHPOP2 statement */
ptr–>next = stk–>top;
stk–>top = ptr;
}
/*--------------------------------------------------------------------*/
/* return: int value popped from stack */
/* action: pops top element from stack and gets return value from it */
/*--------------------------------------------------------------------*/
extern int pop(IntStack * stk)
{
IntLink * ptr;
int num;
ptr = stk–>top;
num = ptr–>i;
stk–>top = ptr–>next;
free(ptr);
return num;
}/*----- FILE READTOKN.C ----------------------------------------------*/
/* */
/* A function to read input and tokenize it for a simple calculator */
/*--------------------------------------------------------------------*/
#include <ctype.h>
#include <stdio.h>
#include "calc.h"
/*--------------------------------------------------------------------*/
/* action: get next input char, update index for next call */
/* return: next input char */
/*--------------------------------------------------------------------*/
static char nextchar(void)
{
/*--------------------------------------------------------------------*/
/* input action: */
/* 2 push 2 on stack */
/* 18 push 18 */
/* + pop 2, pop 18, add, push result (20) */
/* = output value on the top of the stack (20) */
/* 5 push 5 */
/* ⁄ pop 5, pop 20, divide, push result (4) */
/* = output value on the top of the stack (4) */
/*--------------------------------------------------------------------*/
char * buf_in = "2 18 + = 5 ⁄ = ";
static int index; ⁄* starts at 0 *⁄
char ret;
ret = buf_in[index];
++index;
return ret;
}
/*--------------------------------------------------------------------*/
/* output: buf - null terminated token */
/* return: token type */
/* action: reads chars through nextchar() and tokenizes them */
/*--------------------------------------------------------------------*/
Token read_token(char buf[])
{
int i;
char c;
⁄* skip leading white space *⁄
for( c=nextchar();
isspace(c);
c=nextchar())
;
buf[0] = c; ⁄* get ready to return single char e.g."+" *⁄
buf[1] = 0;
switch(c)
{
case '+' : return T_PLUS;
case '-' : return T_MINUS;
case '*' : return T_TIMES;
case '⁄' : return T_DIVIDE;
case '=' : return T_EQUALS;
default:
i = 0;
while (isdigit(c)) {
buf[i++] = c;
c = nextchar();
}
buf[i] = 0;
if (i==0)
return T_STOP;
else
return T_INTEGER;
}
}Refer to the following topics for more information related to the material discussed in this topic.