How do I ignore spaces in a string input? - c

Question

I am creating a program that evaluates a postfix expression contained in a single line of a text file. I'm having some trouble dealing with blank spaces in the scanned file. What I've done so far is scan the single line from the file into a buffer, and then process the string one character at a time. How do I ignore blank spaces once I've read the line into a string? For example:
2 4 3 * +
Here is the full program:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
int stack[1000];
int top = -1;
void push(int x);
int pop();
bool isOperator(char ch);
int performOperation(int op1, int op2, char op);
int main()
{
char exp[1000], buffer[15];
int i, num, op1, op2, len, j, x;
int stack[1000];
char fileName[20];
FILE *inFile;
char *e;
printf("Please enter text file:");
scanf("%s", fileName);
inFile = fopen(fileName, "r");
if (inFile == NULL) {
printf("Error\n");
return -1;
}
int N = 0; i = 0, temp;
while (!feof(inFile)) {
fgets(buffer, 15, inFile);
N++;
}
printf("Postfix expression:\n");
printf("%s", buffer);
e = buffer;
while (*e != '\0') {
if (isdigit(*e)) {
num = *e - 48;
push(num);
} else {
op1 = pop();
op2 = pop();
if (isOperator(*e)) {
int ans;
ans = performOperation(op1, op2, *e);
}
push(ans);
}
e++;
}
printf(" The value of the expression is %d\n", ans);
}
void push(int x)
{
stack[++top] = x;
}
int pop()
{
return stack[top--];
}
bool isOperator(char ch) {
if (ch == '+' || ch == '-' || ch == '*'|| ch == '/')
return true;
else
return false;
}
int performOperation(int op1, int op2, char op) {
int ans;
switch (op) {
case '+':
ans = op2 + op1;
break;
case '-':
ans = op2 - op1;
break;
case '*':
ans = op2 * op1;
break;
case '/':
ans = op2 / op1;
break;
}
return ans;
}
Any help is appreciated.
I forgot to mention that we are only dealing with single-digit numbers.

Answer 1

You can remove the spaces so you don't have to deal with them
void remove_spaces(char* s) {
const char* d = s;
do {
while (*d == ' ') {
++d;
}
} while (*s++ = *d++);
}

Answer 2

If I have understood correctly the problem is processing embedded white spaces.
To resolve the problem you can rewrite the while loop at least the following way
int ans = 0;
for ( e = buffer; *e != '\0'; ++e ) {
if ( !isspace( ( unsigned char )*e ) )
{
if ( isdigit( ( unsigned char )*e ) ) {
num = *e - '0';
push(num);
} else if ( isOperator(*e) ) {
op1 = pop();
op2 = pop();
ans = performOperation(op1, op2, *e);
push(ans);
}
}
}
ans = pop();
printf(" The value of the expression is %d\n", ans);
Pay attention to this statement before the printf call
ans = pop();
You have to pop the answer from the stack before printing it. Also you should process invalid characters and check whether the stack is empty.
Also this loop
int N = 0; i = 0, temp;
while (!feof(inFile)) {
fgets(buffer, 15, inFile);
N++;
}
seams does not make sense and the condition of the loop is incorrect. For example for an empty file the variable buffer will not contain a valid data.

Answer 3

There are multiple problems in your code:
The loop while (!feof(inFile)) is incorrect. You should instead use:
while (fgets(buffer, sizeof buffer, inFile)) {
/* handle the string expression in buffer */
you should not push the number immediately when encountering a digit, you should parse the number that may have more than one digit.
once you parse numbers correctly, you can discard any white space encountered in the parser.
Here is a modified version:
#include <stdio.h>
bool isOperator(char ch) {
if (ch == '+' || ch == '-' || ch == '*'|| ch == '/')
return true;
else
return false;
}
int performOperation(int op1, int op2, char op) {
int ans;
switch(op) {
case '+':
ans = op2 + op1;
break;
case '-':
ans = op2 - op1;
break;
case '*':
ans = op2 * op1;
break;
case '/':
ans = op2 / op1;
break;
}
return ans;
}
int main() {
char fileName[100];
char buffer[100];
FILE *inFile;
printf("Please enter text file:");
if (scanf("%99s", fileName) != 1) {
printf("No input\n");
return 1;
}
inFile = fopen(fileName, "r");
if (inFile == NULL) {
printf("Error\n");
return -1;
}
while (fgets(buffer, sizeof buffer, inFile);
printf("Postfix expression:\n");
printf("%s", buffer);
char *e = buffer;
while (*e != '\0') {
if (isdigit((unsigned char)*e)) {
int num = 0;
while (isdigit((unsigned char)*e)) {
num = num * 10 + *e++ - '0';
}
push(num);
} else
if (isspace((unsigned char)*e) {
e++; // ignore white space
} else
if (isOperator(*e)) {
int op1 = pop();
int op2 = pop();
int ans = performOperation(op1, op2, *e++);
push(ans);
} else {
printf("Invalid character in expression: %c\n", *e++);
}
}
int ans = pop();
printf(" The value of the expression is %d\n", ans);
}
fclose(inFile);
return 0;
}