K & R C Programs Exercise 4-6.

K and R C, Solution to Exercise 4-6:
K and R C Programs Exercises provides the solution to all the exercises in the C Programming Language (2nd Edition). You can learn and solve K&R C Programs Exercise.
C Program to Add commands for handling variables.(It’s easy to provide twenty-six variables with single-letter names.). Add a variable for the most recently printed value.

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#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <math.h>
#include <string.h>

#define MAXOP 100
#define NUMBER 0
#define IDENTIFIER 1
#define ENDSTRING 2
#define TRUE 1
#define FALSE 0
#define MAX_ID_LEN 32
#define MAXVARS 30


struct varType{
char name[MAX_ID_LEN];
double val;
};


int Getop(char s[]);
void push(double val);
double pop(void);
void showTop(void);
void duplicate(void);
void swapItems(void);
void clearStacks(struct varType var[]);
void dealWithName(char s[], struct varType var[]);
void dealWithVar(char s[], struct varType var[]);

int pos = 0;
struct varType last;

int main(void)
{
int type;
double op2;
char s[MAXOP];
struct varType var[MAXVARS];

clearStacks(var);

while((type = Getop(s)) != EOF)
{
switch(type)
{
case NUMBER:
push(atof(s));
break;
case IDENTIFIER:
dealWithName(s, var);
break;
case '+':
push(pop() + pop());
break;
case '*':
push(pop() * pop());
break;
case '-':
op2 = pop();
push(pop()- op2);
break;
case '/':
op2 = pop();
if(op2)
push(pop() / op2);
else
printf("nError: division by zero!");
break;
case '%':
op2 = pop();
if(op2)
push(fmod(pop(), op2));
else
printf("nError: division by zero!");
break;
case '?':
showTop();
break;
case '#':
duplicate();
break;
case '~':
swapItems();
break;
case '!':
clearStacks(var);
break;
case 'n':
printf("nt%.8gn", pop());
break;
case ENDSTRING:
break;
case '=':
pop();
var[pos].val = pop();
last.val = var[pos].val;
push(last.val);
break;
case '<':
printf("The last variable used was: %s (value == %g)n",
last.name, last.val);
break;

default:
printf("nError: unknown command %s.n", s);
break;
}
}
return EXIT_SUCCESS;
}

#define MAXVAL 100

int sp = 0;
double val[MAXVAL];

/* push: push f onto stack. */
void push(double f)
{
if(sp < MAXVAL)
val[sp++] = f;
else
printf("nError: stack full can't push %gn", f);
}

/*pop: pop and return top value from stack.*/
double pop(void)
{
if(sp > 0)
{
return val[--sp];
}
else
{
printf("nError: stack emptyn");
return 0.0;
}
}

void showTop(void)
{
if(sp > 0)
printf("Top of stack contains: %8gn", val[sp-1]);
else
printf("The stack is empty!n");
}


void duplicate(void)
{
double temp = pop();

push(temp);
push(temp);
}

void swapItems(void)
{
double item1 = pop();
double item2 = pop();

push(item1);
push(item2);
}


void clearStacks(struct varType var[])
{
int i;
sp = 0;

for( i = 0; i < MAXVARS; ++i)
{
var[i].name[0] = '';
var[i].val = 0.0;
}
}

void dealWithName(char s[], struct varType var[])
{
double op2;

if(!strcmp(s, "sin"))
push(sin(pop()));
else if(!strcmp(s, "cos"))
push(cos(pop()));
else if (!strcmp(s, "exp"))
push(exp(pop()));
else if(!strcmp(s, "pow"))
{
op2 = pop();
push(pow(pop(), op2));
}

else
{
dealWithVar(s, var);
}
}


void dealWithVar(char s[], struct varType var[])
{
int i = 0;

while(var[i].name[0] != '' && i < MAXVARS-1)
{
if(!strcmp(s, var[i].name))
{
strcpy(last.name, s);
last.val = var[i].val;
push(var[i].val);
pos = i;
return;
}
i++;
}

/* variable name not found so add it */
strcpy(var[i].name, s);
/* And save it to the last variable */
strcpy(last.name, s);
push(var[i].val);
pos = i;
}

int getch(void);
void unGetch(int);

/* Getop: get next operator or numeric operand. */
int Getop(char s[])
{
int i = 0;
int c;
int next;

/* Skip whitespace */
while((s[0] = c = getch()) == ' ' || c == 't')
{
;
}
s[1] = '';

if(isalpha(c))
{
i = 0;
while(isalpha(s[i++] = c ))
{
c = getch();
}
s[i - 1] = '';
if(c != EOF)
unGetch(c);
return IDENTIFIER;
}

/* Not a number but may contain a unary minus. */
if(!isdigit(c) && c != '.' && c != '-')
{
/* 4-6 Deal with assigning a variable. */
if('=' == c && 'n' == (next = getch()))
{
unGetch('');
return c;
}
if('' == c)
return ENDSTRING;

return c;
}

if(c == '-')
{
next = getch();
if(!isdigit(next) && next != '.')
{
return c;
}
c = next;
}
else
{
c = getch();
}

while(isdigit(s[++i] = c))
{
c = getch();
}
if(c == '.') /* Collect fraction part. */
{
while(isdigit(s[++i] = c = getch()))
;
}
s[i] = '';
if(c != EOF)
unGetch(c);
return NUMBER;
}

#define BUFSIZE 100

int buf[BUFSIZE];
int bufp = 0;

/* Getch: get a ( possibly pushed back) character. */
int getch(void)
{
return (bufp > 0) ? buf[--bufp]: getchar();
}

/* unGetch: push character back on input. */
void unGetch(int c)
{
if(bufp >= BUFSIZE)
printf("nUnGetch: too many charactersn");
else
buf[bufp++] = c;
}



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