Loops & Functions: How do I calculate how many years it takes to accumulate a given amount of money to retire?

Question

I am having a bit of trouble with an assignment for my C programming class; I seem to be stuck on how to correctly implement & terminate a loop within the program.

This is my first time asking a question on StackOverflow, so I beg for your patience as I am a programming noob; I've been kind of afraid of posting here so as not to seem like an idiot. :P

Here are the instructions for the assignment:

"Write a program that calculates how many years it takes to accumulate a given amount of money to retire.

The program must prompt for: - starting balance - amount deposited every year - estimated annual interest rate earned (%) - target balance

Program outputs a table that shows information for each year. Calculate each entry in the table exactly as described here:

• year number
• amount deposited that year
• interest earned that year. Calculate as (balance at start of year + deposit) * rate
• balance at end of year. Is balance at start of year + deposit + interest

The loop must stop when the target is exactly reached or exceeded, and then a summary line is displayed. User input, the first few lines of output and example of the summary line are given below:

Enter starting balance ($): 250000.0 
Enter amount deposited every year ($): 20000.0 
Enter estimated annual interest rate (%): 10.0 
Enter target balance ($): 2000000.0 

Year Deposit Interest Balance 
---- ------- -------- ------- 
   0 250000.00 0.00 250000.00 
   1 20000.00 27000.00 297000.00 
   2 20000.00 31700.00 348700.00 
   3 20000.00 36870.00 405570.00 
   4 20000.00 42557.00 468127.00 
   . . . 
   . . . 
In year ??, balance ????? reaches target 2000000.00"

And here is my sad code so far (sorry if the formatting looks weird):

/* CSCI 112; online class */
#include <stdio.h>

/* Week 6: Lab 2 - Program 2 (Retirement) */
void main(void) {
    int year;
    double balance, target, endbalance, deposit, rate, interest;

    printf("Enter starting balance ($): ");
    scanf("%lf", &balance);
    printf("Enter amount deposited every year ($): ");
    scanf("%lf", &deposit);
    printf("Enter estimated annual interest rate (\%%): ");
    scanf("%lf", &rate);
    printf("Enter target balance ($): ");
    scanf("%lf", &target);

    year = 0;
    interest = 0;
    rate = rate / 100.0;
    endbalance = balance + deposit + interest;

    printf("\nYear    Deposit    Interest    Balance");
    printf("\n----    -------    --------    -------");
    do {
        endbalance = endbalance + deposit + interest;
        printf("\n%d    %.2lf    %.2lf    %.2lf", year, deposit, interest, endbalance);
        year += 1;
        interest = (endbalance + deposit) * rate;
    } while (endbalance <= target); 

    printf("\nIn year %d, balance %.2lf reaches target %.2lf", year, balance, target);
}

Output:

Enter starting balance ($): 250000.0
Enter amount deposited every year ($): 20000.0
Enter estimated annual interest rate (%): 10.0
Enter target balance ($): 2000000.0

Year    Deposit    Interest    Balance
----    -------    --------    -------
0    20000.00    0.00    290000.00
1    20000.00    31000.00    341000.00
2    20000.00    36100.00    397100.00
3    20000.00    41710.00    458810.00
4    20000.00    47881.00    526691.00
5    20000.00    54669.10    601360.10
6    20000.00    62136.01    683496.11
7    20000.00    70349.61    773845.72
8    20000.00    79384.57    873230.29
9    20000.00    89323.03    982553.32
10    20000.00    100255.33    1102808.65
11    20000.00    112280.87    1235089.52
12    20000.00    125508.95    1380598.47
13    20000.00    140059.85    1540658.32
14    20000.00    156065.83    1716724.15
15    20000.00    173672.42    1910396.57
16    20000.00    193039.66    2123436.22
In year 17, balance 250000.00 reaches target 2000000.00

I would really appreciate some feedback! :D

Thanks in advance!

Answer 1

while (endbalance != target);    

your loop is running infinitely because the loop terminates only when it's equal to the target, if it exceeds target the loop continues...that's why its not terminating...sometimes endbalance may exceed target without being just equal to target...so, modify your code like this...

while (endbalance <= target);

here is your updated code...

/* CSCI 112; online class */
#include <stdio.h>

/* Week 6: Lab 2 - Program 2 (Retirement) */
void main(void) 
{

    int year;
    double balance, target, endbalance, deposit, rate, interest;

    printf("Enter starting balance ($): ");
    scanf("%lf", &balance);
    printf("Enter amount deposited every year ($): ");
    scanf("%lf", &deposit);
    printf("Enter estimated annual interest rate (\%%): ");
    scanf("%lf", &rate);
    printf("Enter target balance ($): ");
    scanf("%lf", &target);

    year = 0;
    interest = 0;
    rate = rate / 100.0;
    endbalance = balance + deposit + interest;

    printf("\nYear    Deposit    Interest    Balance");
    printf("\n----    -------    --------    -------");
    printf("\n%d    %.2lf    %.2lf    %.2lf", year, deposit, interest, endbalance);
    do {
        endbalance = endbalance + deposit + interest;
        printf("\n%d    %.2lf    %.2lf    %.2lf", year, deposit, interest, endbalance);
        year += 1;
        interest = (endbalance + deposit) * rate;
    } while (endbalance <= target);

    printf("\nIn year %d, balance %.2lf reaches target %.2lf", year, balance, target);
}