Thread: C Program to evaluate y=exp(x); Novice, HELP PLEASE!! :)

Hi! I have written a part of the program but don't know if I'm going in the right direction. I was also wondering how to use getchar() because I've never used that before. I'm quite a novice at this so any help would be greatly appreciated. Thanks!


Scan a double variable x and evaluate y=exp(x) by
using the math.h library of functions.

Your objective in this homework is to compare the so calculated
value of y with the approximate value Y obtained by
using 1, 2, 3, 4, and 5 leading terms of the Taylor series:

exp(x) = x^0/0! + x^1/1! + x^2/2! + x^3/3! + x^4/4! + ....

Recall that k!=k*(k-1)*(k-2)* ... *3*2*1 and 0!=1.

Use a do/while loop in conjunction with a switch statement
switch(). Enter a character 1 for one term approximation,
2 for two terms, etc., by using getchar() function. Use
0 to exit the program. Within switch, use default:
if (char != '0') => unrecognized operator.

Let your opening case calculate the fourth term x^4/4!, let the
subsequent case calculate the x^3/3! term, etc. (In this way,
you wan't need to use repeatedly a break statement within your switch).

Evaluate the corresponding Y, print the result in one line,
and the relative error (Y-y)/y in another line.

Execute your program for all five cases (with corresponding
printouts).


............................................
Your output should be like this:


Enter x: 1.25


True value of exp(1.2500) = 3.490343

Enter a character 1-5 (0 to exit):
1
1 term(s) approximation
Approximate exp(1.2500) = 1.000000
Relative error = -71.349520 percent

Enter a character 1-5 (0 to exit):
2
2 term(s) approximation
Approximate exp(1.2500) = 2.250000
Relative error = -35.536421 percent

Enter a character 1-5 (0 to exit):
3
3 term(s) approximation
Approximate exp(1.2500) = 3.031250
Relative error = -13.153233 percent

Enter a character 1-5 (0 to exit):
4
4 term(s) approximation
Approximate exp(1.2500) = 3.356771
Relative error = -3.826905 percent

Enter a character 1-5 (0 to exit):
5
5 term(s) approximation
Approximate exp(1.2500) = 3.458496
Relative error = -0.912428 percent

Enter a character 1-5 (0 to exit):
6
unrecognized operator

Enter a character 1-5 (0 to exit):
0

Code:

#include<stdio.h>
#include<math.h>


/* Scanning a double variable x and evaluating y=exp(x) by
using the math.h library of functions */

int main() {

/* Defining variables */
int k, N;
char ch;
double x, y, Y;

/* Two empty lines and data input + true value of exp(x) at beginning */
printf("\n\nEnter x:\n\n");
scanf("%lf", &x);

y=exp(x);
printf("True value of exp(%lf) = %9.6f\n", x, y);


/* Data input: do/while statement with a switch statement within */
do {
        printf("Enter x:\n");
        scanf("%d", &N);
        y=0;

        switch(N) {
                case 5;
                        y-=




}
}

}

You're fine until your do-while loop.

Code:

do
    get N
    switch N
        case 0:
            // do nothing
        case 1-5:
            call exp_approx(N)
        default:
            print "unrecognized operator"
while N is not 0

You will also need to define the exp_approx() function, and probably a factorial() function.

EDIT: Duh, you need a switch

Thank you!!
Is there any specific way to use getchar() ?
Is that kind of like scanf?

getchar() just returns a single character from the standard input device, not like scanf, which does a myriad of things. Check your docs for specifics or google for examples. Also note that I didn't read your directions very well. You need to let your case statements for 5, 4, 3, 2 and 1 fall through to each other and total the parts of the approximation as you fall through.

Thanks so much! That clarifies a lot.

So I wrote the program and it compiled but it wouldn't run for some reason.
Then I realized that I needed to edit this:

Code:

printf("Enter a character 1-5 (0 to exit):\n");
        scanf("%lf", &ch);
        y=0;

And turn it into this (b/c I'm supposed to use getchar()):

Code:

printf("Enter a character 1-5 (0 to exit):\n");
        getchar("%lf", &ch);
        y=0;

But now I'm getting an error:

Code:

109:20: error: macro "getchar" passed 2 arguments, but takes just 0

What does that even mean??

That means you gave getchar two arguments ("%lf" and &ch), but getchar doesn't take any arguments at all (0). You should look at the manual for getchar to see how to use it.

When I put (0), I got:

error: macro "getchar" passed 1 arguments, but takes just 0

So I left it blank: getchar()

I don't know if that's right but it compiled and I got in the assignment right on time so thank you!!
I'll definitely read up on it.