Thread: Homework help! C program value of math expression using strings

Hi, I really need some help with this code. I am really stuck on this and have no clue where to go, any help would be much appreciated.

here is my objective:

Write a c program thatcalculates the value of a mathematical expression comprised ofpositive numbers and the operations ‘+’ and ‘-’.Specifically, first prompt the user to input an expression, read itin as a string, and then print the value of the expression. You mayassume that the expression does not contain spaces, maximumsize of the expression (including digits and operators) can be 20,and that all numbers are single digit numbers.


Hints: Note that, thedigits would be read in as characters; you will need to translatethem to numbers (recall the ASCII table).


Implementation Requirements

1. Write a function called “evaluate” that takes as input a mathematical expression (as a string) and returns the value of the expression. The prototype of the function is—“int evaluate(char expr[]);”

Sample Output:


Input: 4+2-1+7
Output:12

here is the code:

Code:

int main()
{
   char expr[21];
   int a,ssum;


   printf("Input: ");
   scanf("%20s", expr);


   ssum = evaluate(&a);


   printf("\nOutput: %d", ssum);
}


int evaluate(char expr[])
{
    int i, sum=0;


    for(i=0; i<21; i++)
    {
        sum = sum + expr[i];
    }


    return sum;
}

the program runs, but the output is not coming out correct.
Thanks for all the help in advance.

Hi, it's an easy function you're trying to do let me do a psuedo code for you if that's fine.

Code:

You first give the name to the function and then two places inside the function. One should include the expr, and the other should be about the number of elements.

Integer i, and the sum both are equal to 0
for i is 0, and i is less than the number of elements, add to i
sum is the solution of adding sum to the expr.
return sum

I'm not that good for writing the psuedo code for someone else but this is how it came to my mind blah

You do realize that your input expression is not just numbers. Its numbers and operators (+, -,...) You can't just add expr[i] to a sum.

You store a string in your array.
Your "evaluate()" function expects that array as an argument.
So why are you passing the address of a random integer called 'a' to that function - especially considering 'a' is not used anywhere in your program?

DaveH pretty much summed up the big issue with your code. The "for()" loop in your "evaluate()" function goes through each element of your array. Different things are supposed to happen depending on if an element is a number, a '+', or a '-'.

I could show you my code on how I made it work, but this would go against of this forum which is let the OP work on his code until he just can't anymore.

Use a if condition for checking the + and - operators instead of a for loop

The "for()" loop will still be needed to cycle through the elements of the array.

I am still a little confused. Would it be best to maybe make 2 for loops in the evaluate function, odd indices for the operators and even indices for the numbers. If this is the best case, can anyone give me an idea of how to impliment this because I am extremely lost with this. IM DYING OVER HERE!!!(PLEASE HELP!!!)

Code:

int main()
{
   char expr[21];
   int ssum;
   printf("Input: ");
   scanf("%20s", expr);
   ssum = evaluate(expr);
   printf("Output: %d", &ssum);
   return 0;
}


int evaluate(char expr[])
{
    int i, sum=0;


    for(i=0; i!='\0'; i++)
    {
        if(expr[i]=='+')
        {
            sum = atoi(expr[i-1] + atoi(expr[i+1]));
        }


        if(expr[i]=='-')
        {
            sum = atoi(expr[i-1] - atoi(expr[i+1]));
        }
    }


    return sum;
}

This is what I have now and it is compiling, but still not giving a correct output.

Thanks for all the help everyone! I finally figured it out and am very happy!!!