Write a program to check given input string have balance brackets - c

Question

Given a string of parentheses, write a program to find whether its valid or not.
Examples-
input : {{{}}}
output: Valid
input : }{}{}{}}
output: Invalid
I wrote the following code in C and tested that the output were coming correct.
#include <stdio.h>
#include <stdlib.h>
int main()
{
char str[20];
int i=0;
printf("Enter String: ");
gets(str);
int count = 0;
while (str[i] != '\0')
{
if (str[i] == '}')
count--;
if (str[i] == '{')
count++;
if (count < 0)
{
printf("\nInvalid");
break;
}
i++;
}
if (count == 0)
printf("\nValid");
return 0;
}
This program doesn't work for the case where input is {{{}}, what condition(s) am I missing?

Answer 1

Code should state if the final result is not 0 as in the case of "{"
if (count == 0) {
printf("Valid\n");
} else {
printf("Invalid\n");
}
return 0;
Also simple break out of loop.
if (count < 0) {
// printf("\nInvalid");
break;
}
gets() has been depreciated since C99 and eliminated from C (C11), use fgets().
char str[20];
fgets(str, sizeof str, stdin);
There is no need to read the entire string in. Code could use 1 char ar a time.
int ch;
while ((ch = fgetc(stdin)) != '\n' && ch != EOF) {
if (str[i] == '}')
count--;
if (count < 0) {
break;
}
else if (str[i] == '{')
count++;
}
}

Answer 2

You don't really need to input the whole string at once since you're only every sequentially processing the characters. Hence you can avoid using unsafe methods like gets() and even safe-but-complicating methods like fgets().
Instead, just use getchar() to read and process each individual character - that should greatly simplify what you need to do.
As to the logic, you basically have it right. Maintain the bracket level, a value initially set to zero. Then read each character and action it as follows:
If it's {, just add one to the level.
If it's }, subtract one from the level, then check to ensure the level is non-negative. If not, then you've had too many closing brackets and you can exit.
If it's end of line or end of file, stop processing characters. Check to make sure the final level is zero. If not, you haven't closed off all the brackets so it's invalid. If the level is zero, everything is balanced.
Any other character can be considered an error.
See below for one example on how to implement this:
#include <stdio.h>
int main (void) {
int debug = 0; // for debugging purposes.
int ch, level = 0; // character and current level.
// Output prompt, read characters while valid.
printf("Enter string: ");
while (((ch = getchar()) == '{') && (ch == '}')) {
// Select based on '{' or '}'.
if (ch == '{') {
// Open bracket, just add one.
++level;
if (debug) printf("DEBUG: {:%d\n",level);
} else {
// Close bracket, subtract one and check.
if (--level < 0) {
puts ("Level has gone below zero.");
return 1;
}
if (debug) printf("DEbug: }:%d ",level);
}
}
// If not endline/endfile, we have invalid character.
if ((ch != '\n') && (ch != EOF)) {
puts ("Invalid character in input.");
return 1;
}
// Level should be zero.
if (level != 0) {
puts ("Level still positive at end of line.");
return 1;
}
// All checks now passed okay.
puts ("Input was fine.");
return 0;
}

Answer 3

You should never use gets(), the gcc compiler even warns about it being dangerous because there is no way to prevent a buffer overflow, for example
char str[6];
gets(str);
with the following input
iharob
is a problem, because there is no room for the '\0' terminator or the '\n', instead
fgets(str, sizeof(str), stdin);
would be safe with any input, although the input string would be trimmed to fit the buffer, but no buffer overflow will occur.

Answer 4

Previous answers have covered avoiding buffer overflows and potential cases where it will not work - to improve performance I would modify the while loop to avoid checking conditions which we know will always be false. e.g. no point in checking if count is less than 0 unless we just decreased the count; no point in checking for an open bracket if the character was a close bracket:
while (str[i] != '\0')
{
if (str[i] == '}')
{
count--;
if (count < 0)
{
printf("\nInvalid");
break;
}
}
else if (str[i] == '{')
count++;
i++;
}

Answer 5

I hope you find this useful and simple ^-^
#include<iostream>
#include<string.h>
using namespace std;
{
string mathEx ;
cout<<"Please Enter math Expression contain ')' , '(' to
check balance \n"<<"MathExpression = ";
cin>>mathEx ;
int i =0 , count = 0 ;
while (mathEx [i] != '\0'){
if(mathEx[i]=='('){
count++;
}
if(mathEx[i]==')'){
count--;
}
if(count<0){
break ;
}
i++;
}
if(count==0){
cout<<"True !";
}
else {
cout<<"Invalid !"<<endl;
}
return 0;
}