I am trying to write a program which lets a user specify a tax
category (Single, head of household, married-joint, and
married-seperate) and a taxable income (single-15% of first $17,850
plus 28% of excess, head of household-15% of first $23,900 plus 28% of
excess, married-joint-15% of first 29,750 plus 28% of excess and
married-seperate-15% of first $14,875 plus 28% of excess) and then
calculates the tax. I am to use a loop so that the user can enter
several tax cases. I do not even know where to begin. PLEASE HELP!

Dear mandragora20,

Here's a fully commented C program for you to study.



#include<stdlib.h>
#include<stdio.h>
#include<string.h>
/* the above are required headers for standard libraries */

int main() {
    char entry[1000], code;         /* entry is a character buffer for input */
    double income, threshold, tax;  /* needed for calculations */

    printf("=== TAX CALCULATOR ===\n");     /* title */

    while (1) {   /* infinite loop: true until we break out of it */

        printf("--- tax categories ---\n"); /* display the codes */
        printf("s = single\n");
        printf("h = head of household\n");
        printf("m = married-joint\n");
        printf("n = married-separate\n");
        printf("\n");
        printf("Enter one of the above codes, or anything else to quit >");

        fgets(entry, 1000, stdin);          /* read a line of input */
        if (strlen(entry) != 2)   /* if it's not one character plus... */
            break;                /* ... a newline character, we quit */

        code = entry[0];          /* the code is the first character */

        if (!(code == 's' || code == 'S' || code == 'h' || code == 'H'
           || code == 'm' || code == 'M' || code == 'n' || code == 'N'))
            break;    /* if it's not a legal code, we quit */

        printf("Enter your income in dollars, without a comma >");
        fgets(entry, 1000, stdin); /* read another line */
        income = atof(entry);      /* convert into a floating-point number */

        if (code == 's' || code == 'S')   /* check the code (uppercase, too) */
            threshold = 17850.0;   /* set the threshold */

        else if (code == 'h' || code == 'H') /* otherwise, check another code */
            threshold = 23900.0;   /* appropriate threshold here */

        else if (code == 'm' || code == 'N') /* same thing again */
            threshold = 29750.0;

        else          /* if no other code matches, it must be 'n' or 'N' */
            threshold = 14875.0;

        if (income < threshold)    /* are we under the threshold?   */
            tax = 0.15*income;     /* then just apply the base rate   */

        else                       /* otherwise, apply the excess rate, too  */
            tax = 0.15*threshold + 0.28*(income-threshold);

        printf("-----------> You owe $%.2f in tax.\n\n", tax);

    }                        /* while loop returns to beginning */

    printf("\nGoodbye!\n");  /* if we broke out of the loop, we land here */
    return 0;                /* always return zero at the end of main() */
}



If you have any trouble compiling this program, please let me know.
Any questions that aren't answered by the comments, don't hesitate to
ask.

At your service,

leapinglizard

Thanks for this answer, it has helped me a great deal. I will return
in the future if I need help