strlen(), series evaluation, and for loops

[flamehead144]

basically, this is what i have to do:

In case 1 for the switch
(b) print a blank line, a line with the message:"Part I:" and a blank line.
© Prompt the user to enter a text message from the keyboard
and accept a text from the keyboard by using gets().
(d) Print the original message on the screen.
(e) Find the number of characters in the message by using strlen() and
print the message in the reverse order. For example, "ROSIE IS CUTE!"
becomes "!ETUC SI EISOR".

In case 2 for the switch
(f) print a blank line, a line with the message:"Part II:" and a blank line.
(g) Print a message:
"Series evaluation: x^n/n! from n=0 to n=n_end.\n" and a blank line.
In the above n! (n factorial)=1*2*...*n for n>0 and 0!=1.
(h) Enter x, a real number and n_end, a large positive integer
from the keyboard and print those values using %g and %d.
(i) In a for-loop evaluate the sum. In addition, print the partial sums
using %16.8e for n=10, 20, 30,.... Print the values of n_end, sum,
and x using %4d, %16.8e, and %g. Finally, print the value of
exp(x) using %g for x and %16.8e for exp(x).


I tried to do part one and i think i got most of it but it doesnt work when i do the a.out. any suggestions?

#include <stdio.h>
#include <math.h>
int main(void)
{
int menu, n, n_end, length;
char message[111];
printf("\nHomework 4:\n\n");
printf("There are two parts: part 1 and part 2.\n");
printf("Enter the part number to execute (1 or 2):");
scanf("%d", &menu);

switch(menu){
case 1:
printf("Part I:\n\n");
printf("\nEnter a text message:");

gets(text);
printf("echo: %s\n", text);
length = (int)strlen(text);
printf("There are %d characters.\n", length);
for(k=length-1; k>=0; k--) printf("%c", text[k]);
printf("\n");
printf("\nPart II:\n\n");

}

[mordekai]

What is the variable "text"? Shouldn't it be "message"? I wonder, why the compiler doesn't comlpain...

And i think the scanf() leaves a newline character in the buffer, wich is read by gets(). I think, if you change it to
scanf("%d\n",&menu);
it should work. If this doeasn't work, please give us the error message or tell what the program is doing now. I have got no other idea.

[flamehead144]

ok, this is what it says after i compile.
#include <stdio.h>
#include <math.h>
int main(void)
{
int menu, n, n_end, length;
char message[111];
printf("\nHomework 4:\n\n");
printf("There are two parts: part 1 and part 2.\n");
printf("Enter the part number to execute (1 or 2):");
scanf("%d", &menu);

switch(menu){
case 1:
printf("Part I:\n\n");
printf("\nEnter a text message:");

gets(text);
printf("echo: %s\n", text);
length = (int)strlen(text);
printf("There are %d characters.\n", length);
for(k=length-1; k>=0; k--) printf("%c", text[k]);
printf("\n");
printf("\nPart II:\n\n");
}
}


"hw4.c" 26 lines, 691 characters
iacs5.ucsd.edu% gcc hw4.c
hw4.c: In function `main':
hw4.c:17: error: `text' undeclared (first use in this function)
hw4.c:17: error: (Each undeclared identifier is reported only once
hw4.c:17: error: for each function it appears in.)
hw4.c:21: error: `k' undeclared (first use in this function)


can someone please help me? and can someone help me out with part 2 also? dont really understand what im supposed to do. thanks.

[mordekai]

You haven't declared the variable "text". You have got a char array, but you called it "message", so you should rename it to "text"
You haven't declared the integer "k". You have got to type "int k;" before the for-loop.

For part 2 you'll need a for-loop and sum up the elements of the series. I suggest, you write a function to calculate the factorial. The math.h library contains a function exp().

[biospark444]

hey i got the same questions here. i figured out the first part but i need help w/ the second myself. heres what i have:

#include <stdio.h>
#include <math.h>
int main(void)
{
int part, k, x, n, n_end, length, oneByn_fac, fact;
char text[111];
printf("\nHomework 4:\n\n");
printf("There are two parts: part 1 and part 2.\n\n");
printf("Enter the part number to execute (1 or 2):");
scanf("%d", &part);
switch(part){
case 1:
printf("\nPart I:\n\n");
printf("Enter a text message:");
while(strlen(gets(text))==0);
printf("%s\n", text);
length = (int)strlen(text);
printf("There are %d characters.\n", length);
for(k=length-1; k>=0; k--) printf("%c", text[k]);
printf("\n");
break;
case 2:
printf("\nPart II:\n\n");
printf("Series evaluation: x^n/n! from n=0 t n=n_en
d.\n\n");
printf("Enter a value for x (a real number) and a v
alue for n_end (a large positive interger):\n");
printf("x=");
scanf("%g", &x);
printf("n_end=");
scanf("%d", &n_end);
for(k=1, oneByn_fac=1.; k<=n_end; k++) oneByn_fac /
=(double)k;
fact = pow(x, n_end);
printf(" %g/%d!\n", fact, n_end);
}
}


as you can probably see i haven't got a clue what im doing in part 2, but part one works fine. hope this helps.

can anyone help us figure out part 2? :D

[mordekai]

I really can't see, what you are doing in part two... did you get the idea of a series? Else, read wikipedia about the basic properties of a series.
http://en.wikipedia.org/wiki/Series_(mathematics)

I would suggest, that you write a function to calculate the factorial of a given number. Then you can calculate the partial sum with an easy for-loop. And in the end, you can compare it with the function exp() from math.h