Program to convert infix expression to prefix notation.

 
    /* infix to prefix */

  # include < stdio.h >
    # include < string.h >
    # define MAX 20
    void infixtoprefix(char infix[20],char prefix[20]);
    void reverse(char array[30]);
    char pop();
    void push(char symbol);
    int isOperator(char symbol);
    int prcd(symbol);
    int top=-1;
    char stack[MAX];
    main() {
    	char infix[20],prefix[20],temp;
    	printf("Enter infix operation: ");
    	gets(infix);
    	infixtoprefix(infix,prefix);
    	reverse(prefix);
    	puts((prefix));
    }
    //--------------------------------------------------------
    void infixtoprefix(char infix[20],char prefix[20]) {
    	int i,j=0;
    	char symbol;
    	stack[++top]='#';
    	reverse(infix);
    	for (i=0;i< strlen(infix);i++) {
    		symbol=infix[i];
    		if (isOperator(symbol)==0) {
    			prefix[j]=symbol;
    			j++;
    		} else {
    			if (symbol==')') {
    				push(symbol);
    			} else if(symbol == '(') {
    				while (stack[top]!=')') {
    					prefix[j]=pop();
    					j++;
    				}
    				pop();
    			} else {
    				if (prcd(stack[top])<=prcd(symbol)) {
    					push(symbol);
    				} else {
    					while(prcd(stack[top])>=prcd(symbol)) {
    						prefix[j]=pop();
    						j++;
    					}
    					push(symbol);
    				}
    				//end for else
    			}
    		}
    		//end for else
    	}
    	//end for for
    	while (stack[top]!='#') {
    		prefix[j]=pop();
    		j++;
    	}
    	prefix[j]='\0';
    }
    ////--------------------------------------------------------
    void reverse(char array[30]) // for reverse of the given expression {
    	int i,j;
    	char temp[100];
    	for (i=strlen(array)-1,j=0;i+1!=0;--i,++j) {
    		temp[j]=array[i];
    	}
    	temp[j]='\0';
    	strcpy(array,temp);
    	return array;
    }
    //--------------------------------
    char pop() {
    	char a;
    	a=stack[top];
    	top--;
    	return a;
    }
    //----------------------------------
    void push(char symbol) {
    	top++;
    	stack[top]=symbol;
    }
    //------------------------------------------
    int prcd(symbol) // returns the value that helps in the precedence {
    	switch(symbol) {
    		case '+':
    		        case '-':
    		        return 2;
    		break;
    		case '*':
    		        case '/':
    		        return 4;
    		break;
    		case '$':
    		        case '^':
    		        return 6;
    		break;
    		case '#':
    		        case '(':
    		        case ')':
    		        return 1;
    		break;
    	}
    }
    //-------------------------------------------------
    int isOperator(char symbol) {
    	switch(symbol) {
    		case '+':
    		        case '-':
    		        case '*':
    		        case '/':
    		        case '^':
    		        case '$':
    		        case '&':
    		        case '(':
    		        case ')':
    		        return 1;
    		break;
    		default:
    		        return 0;
    		// returns 0 if the symbol is other than given above
    	}
    }