Expression Tree - Operations with Characters - c

Question

The user enters a string with an operation such as 4*5+2/3 and the code is supposed to make an expression tree out of it and the calculate said expression tree. I am having a problem where the program is making the expression tree with the decimal values of the ascii table instead of the actual numbers.
For example instead of 4*5+2/3, the program is storing and using 52 42 53 43 50 47 51 for the calculations. My desired run screen would be:
1 //this is the number of strings
4*5+2/3 //this is the string itself
20 //this is the result
However what I am getting is:
1 //this is the number of strings
4*5+2/3 //this is the string itself
2756 //this is the result
That is because the code is doing 52*53+50/51(because it is using the ascii values) and not 4*5+2/3.
I believe the reason for this is because I am storing 4*5+2/3 in a string of char and not in an array of int. I do not know if this is the case and would like some help.
You will not be able to run the following code as it is not complete but the whole program is five files and I do not know if I should put all of it here. I am new to both trees and StackOverflow.
This is my Make Expression Tree function and my Calculate Expression Tree function:
BTreeNode* MakeExpTree(char* exp, int len)
{
Stack stack;
BTreeNode * node, *right_node, *left_node;
InitStack(&stack);
for(int i = 0; i < len; i++){
if('0' <= exp[i] && exp[i] <= '9'){
node = CreateNode(exp[i]);
}
else{
right_node = PeekNode(&stack), Pop(&stack);
left_node = PeekNode(&stack), Pop(&stack);
node = CreateNode(exp[i]);
CreateRightSubtree(node, right_node);
CreateLeftSubtree(node, left_node);
}
PushNode(&stack, node);
}
return PeekNode(&stack);
}
int CalculateExpTree(BTreeNode* root)
{
int ret, op1, op2;
if(root == NULL){
return 0;
}
if(root->left_child == NULL && root->right_child == NULL){
return root->item;
}
op1 = CalculateExpTree(root->left_child);
op2 = CalculateExpTree(root->right_child);
switch(root->item){
case '+':
ret = op1 + op2;
break;
case '-':
ret = op1 - op2;
break;
case '*':
ret = op1 * op2;
break;
case '/':
ret = op1 / op2;
break;
case '#':
ret = op1 * pow( 2, op2);
break;
case '#':
ret = op1 / pow( 2, op2);
break;
}
return ret;
}
This is how I store the string from stdin in main function:
int main()
{
int num_exp, result, len = 0;
char input[10];
char IDK[129];
fgets(input, 9, stdin); //user enters number of strings
int m = sscanf(input, "%d", &num_exp);
char string[100][129] = { 0 };
char postfix[100][129] = { 0 };
for(int i = 0; i < num_exp; i++){
fgets(IDK, 129, stdin); //user enters string
int mm = sscanf(IDK, "%s", string[i]); //is this where the problem lies?
} //should I not be storing it in a char string?
for(int x = 0; x < num_exp; x++){
InfixToPostfix(string[x], postfix[x]); //converts strings from infix to postfix
}
BTreeNode* tree;
for(int k = 0; k < 129; k++){ //calculates length of string
if(postfix[0][k] == '\0'){
break;
}
len++;
}
tree = MakeExpTree(postfix[0], len); //makes expression tree
result = CalculateExpTree(tree); //calculates expression tree
//or is the problem in this function?
printf("%d \n", result);
return 0;
}

Answer 1

I am not understanding clearly what you are doing and it would help my learning too if you shared from what source are you learning these (we can chat in chat room) but I also tried to do similar thing and in my style it works partially (that is, sometimes when I insert larger number the answer is wrong but for small numbers generally correct).So you might get some help form my style of this code that calculates given string (A little help for me also would be appreciated commenters!!). My code:
#include <stdio.h>
#include <string.h>
#include "stackforcalc.h"
int isOperand(char b){
if(b>='0' && b<='9'){
return 1;
}else{
return 0;
}
}
int isOperator(char b){
if(b=='+' || b=='-' || b=='*' || b=='/'){
return 1;
}
return 0;
}
int getwt(char b){
int g=-1;
switch (b)
{
case '+':
case '-':
g=1;
break;
case '/':
case '*':
g=28787868;
break;
}
return g;
}
int higherprecedence(char a,char b){
int c=getwt(a);
int d=getwt(b);
return (c>=d)?1:0;
}
int infToPost(char *b,char *str){
int j=0;
for(int i=0;i<strlen(b);i++){
if(b[i]== ' ' || b[i]== ',' ){
continue;
}
else if(isOperator(b[i])){
str[j]=' ';
j++;
while(!empty() && gettop() != '(' && higherprecedence(gettop(),b[i])){
str[j]=gettop();
j++;
pop();
}
push(b[i]);
}
else if(isOperand(b[i])){
str[j]=b[i];
j++;
}
else if(b[i]=='('){
push(b[i]);
}
else if(b[i] ==')'){
while(!empty() && gettop() != '('){
str[i]=gettop();
j++;
pop();
}
pop();
}
}
while(!empty()){
str[j]=gettop();
j++;
pop();
}
}
int Evaluate(int t,char y,int r){
int ty;
switch(y){
case '+':
ty=t+r;
break;
case '-':
ty=r-t; //I inverted these.
break;
case '*':
ty=r*t;
break;
case '/': //I inverted these because
ty=r/t; //even though I did t/r it performed r/t.
break; //may be somewhere before the numbers were swapped
default:
ty=-1;
break;
}
return ty;
}
int calculatepostfix(char *c){
for(int i=0;i<strlen(c);i++){
if(c[i]==' ' || c[i]==','){
continue;
}
else if(isOperator(c[i])){
int op1=gettop2();
pop2();
int op2=gettop2();
pop2();
int oper=Evaluate(op1,c[i],op2);
push2(oper);
}
else if(isOperand(c[i])){
int res=0;
while(i<strlen(c) && isOperand(c[i])){
res=(res*10)+(c[i]-'0');
i++;
}
i--;
push2(res);
}
}
return gettop2();
}
int main(){
char b[65];
printf("\n \n**-- Calculator --**\n");
printf("Enter expression: ");
fgets(b,sizeof(b),stdin);
char str[50];
infToPost(b,str);
int tt =calculatepostfix(str);
printf("Your answer is: %d",tt);
}
The code in "stackforcalc.h" is
#ifndef stacycalc
#define stacycalc
#define maxsize 50
char a[maxsize];
int top=-1;
int abc[maxsize];
int to=-1;
void push2(int re){ abc[++to]=re; }
void push(char b){ a[++top]=b; }
void pop2(){ to--; }
void pop(){ top--;}
int gettop2(){ return (to==-1)?-1:abc[to]; }
char gettop(){ return (top==-1)?0:a[top]; }
int empty(){ return (top==-1)?1:0; }
#endif

Answer 2

That is because the code is doing 52*53+50/51 (because it is using the ascii values) and not 4*5+2/3.
Yes.
I believe the reason for this is because I am storing 4*5+2/3 in a string of char and not in an array of int.
No.
In C, int is just a bigger char. There's nothing magical about them; they both hold numbers.
There are a variety of ways to derive the integer value from an ASCII character. If you have exactly one character and don't want to use a library, you can "mask off" the bits: since the ASCII range for digits is 0x30 - 0x39,
static char string[] = "4";
int value = string[0] & 0x0F;
does the trick. For more complex operations, my favorite is sscanf(3), but many use atoi(3) or various flavors of strtol(3).