Exercise 4-7 (ungets - Calculator)

Chapter_4     Exercise_4-6

Exercises_4-8,9

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Exercise 4-7     K&R, p. 79

Exercise 4-7. Write a routine ungets(s) that will push back an entire string onto the input. Should ungets() know about buf[] and bufp, or should it just use ungetch()?

ungets.c         download

#include <stdio.h> // for printf(), getchar(), EOF
#include <stdlib.h> // for atof()

#define MAXOP 100 // max size of operand or operator
#define NUMBER '0' // signal that a number has been found

int getop(char []); // get the next operand or operator
void push(double); // push operand on stack
double pop(void); // pop operand from stack

int main()
{
int type, i1, i2;
double op2; // second operand for -, /, %
char s[MAXOP]; // operand or operator

while((type = getop(s)) != EOF && type != '\0')
{ // nonempty line ending with EOF will not be executed
switch(type)
{
case NUMBER :
push(atof(s));
break;
case '+' :
push(pop() + pop());
break;
case '-' :
op2 = pop();
push(pop() - op2);
break;
case '*' :
push(pop() * pop());
break;
case '/' :
op2 = pop();
if (op2 != 0.0)
{push(pop() / op2);}
else {printf("Error: zero divisor\n");}
break;
case '%' :
i2 = pop(); // automatic conversions
i1 = pop(); // from double to int
if (i2 != 0.0)
{push(i1 % i2);} // auto conversion from int to double, arg of push()
else {printf("Error: zero divisor\n");}
break;
case '\n' :
printf("\t%.8g\n", pop()); // top of stack
break;
default :
printf("Unknown command: %s\n", s);
break;
}
}

return 0;
}

#define MAXVAL 100 // stack size
// stack pointer (0 is top of stack):
int sp = 0; // next free stack position
double val[MAXVAL]; // value stack

void push(double f) // push f on (top of) stack
{
if (sp < MAXVAL) {val[sp++] = f;}
else {printf("Error: stack full, can't push %g\n", f);}
}

double pop(void) // pop and return top value from stack
{
if (sp > 0) {return val[--sp];}
else
{
printf("Error: stack empty\n");
return 0.0;
}
}

#include <ctype.h> // for isdigit()

int getch(void);
void ungetch(int);

int getop(char s[]) // get next operator or numeric operand
{
int c, i;

while ((s[0] = c = getch()) == ' ' || c == '\t') // skip beginning ' ', '\t'
{} // last value read in s[0] is not ' ' or '\t', but could be '\n' or EOF
s[1] = '\0'; // end string

if (!isdigit(c) && c != '.' && c != '+' && c != '-')
{return c;} // not a number, probably an operator (or '\n' or '\0' or EOF)
// here c is digit or '.' or '+' or '-'

i = 0;

if (c == '+' || c == '-') // unary or binary sign
{
c = getch();
if (!isdigit(c) && c != '.')
{
ungetch(c);
return s[0]; // binary '+' or '-'
}
else // c is digit or '.'
{
if (s[i] == '-') {i++;} // s[0]
// if (s[i] == '+') {} // skip adding '+' to s[]
s[i] = c; // digit or '.'
}
}

if (isdigit(c)) // collect integer part
{ // s[i] contains c (digit)
while (isdigit(s[++i] = c = getch()))
{}
} // here s[i] is not a digit
if (c == '.') // collect fraction part
{ // s[i] contains c (s[i] == '.')
while (isdigit(s[++i] = c = getch()))
{}
} // here s[i] is not a digit
s[i] = '\0'; // end string containing number (operand)

if (c != EOF) {ungetch(c);} // cannot ungetch(EOF) as EOF is not a char
else {ungetch('\0');} // EOF or '\0' returned by getop() will end the program

return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE]; // buffer for ungetch()
// buf[] pointer (0 is top of buf):
int bufp = 0; // next free position in buf[]

int getch(void) // get a (possibly pushed-back) character
{
return (bufp > 0) ? buf[--bufp] : getchar();
}
// push character back on input (make it available for the next getch()):
void ungetch(int c)
{
if (bufp >= BUFSIZE)
{printf("ungetch(): too many characters\n");}
else {buf[bufp++] = c;}
}

#include <string.h> // for strlen()

void ungets1(char s[]) // less efficient
{
int len = strlen(s);
int i;

for (i = 0; i < len; i++)
{
ungetch(s[i]); // many function calls (inefficient)
}
}

void ungets2(char s[]) // more efficient
{
int len = strlen(s);
if (bufp + len >= BUFSIZE)
{
printf("ungetch(): too many characters\n");
return;
}

int i;
for (i = 0; i < len; i++)
{
buf[bufp++] = s[i]; // no function calls here (more efficient)
}
}
/*
gcc ungets.c -o ungets
./ungets
1 2 +
3
1 2 + 3 * 4 /
2.25
-1 2 +
1
-1 -2 +
-3
-1 2 + 3 *
3
2 0 /
Error: zero divisor
2 // pop() called by '\n' returns first operand, still on stack
1 2 /
0.5
2 3 %
2
4 2 %
0
4 1 %
0
4 3 %
1
4 0 %
Error: zero divisor
Error: stack empty // nothing pushed, '\n' calls pop():
0 // value returned by pop() for an empty stack
a b +
Unknown command: a
Unknown command: b
Error: stack empty // nothing pushed, pop() called by +
Error: stack empty // pop() called by '\n'
0 // value returned by pop() for an empty stack
1 2 9
9 // pop() called by '\n'
+
3 // 1 + 2
1 2 c
Unknown command: c
2 // pop() called by '\n'
3 +
4 // 1 + 3
CTRL^D (EOF) or CTRL^C in Linux, CTRL^Z + Enter in Windows

./ungets < compute.txt
3 // 1 2 +
0 // 1 2 * 3 + 5 -
1 // -1 2 +
1 // -1 -2 -
1 // 4 3 %
Error: zero divisor // 2 0 %
Error: stack empty // nothing pushed, '\n' calls pop():
0 // value returned by pop() for an empty stack
0.33333333 // 1 3 /
Error: zero divisor // 1 0 /
1 // pop() called by '\n' returns first operand, still on stack
*/

Note:  See Exercise_4-3 for the file compute.txt.

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