Installment Loan Calculator. More...

#include <math.h>
#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

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Macros
#define	FALSE 0
#define	TRUE 1

Functions
void	ABEND (int e)
	The ABEND function is a signal handler that is called when the program receives certain signals (e.g., SIGINT, SIGQUIT, SIGHUP). It takes an integer argument representing the signal number and prints an error message indicating that an abnormal end (ABEND) has occurred, along with the signal number. After displaying the message, it exits the program with a failure status.
double	accept_i ()
	The accept_i function prompts the user to enter the annual interest rate for the loan. It validates the input to ensure it is numeric and non-negative. If the input is invalid, an error message is displayed, and the user is prompted to enter the value again until a valid input is provided.
double	accept_n ()
	The accept_n function prompts the user to enter the number of payments for the loan. It validates the input to ensure it is numeric and non-negative. If the input is invalid, an error message is displayed, and the user is prompted to enter the value again until a valid input is provided.
double	accept_pmt ()
double	accept_pv ()
	The following functions are used to accept user input for the present value, number of payments, interest rate, and payment amount. They validate the input to ensure it is numeric and meets the required conditions (e.g., non-negative). If the input is invalid, an error message is displayed, and the user is prompted to enter the value again.
void	accept_str (char *s)
	The accept_str function prompts the user with a given string and reads input from the standard input into a buffer. It uses the fprintf function to display the prompt and the read function to capture the user's input. The input is stored in the global buffer in_str, which can be used by other functions for further processing.
double	calculate_i (double pv, double n, double pmt)
	The calculate_i function calculates the annual interest rate for a loan based on the present value, number of payments, and payment amount. It uses an iterative method to find the interest rate that satisfies the loan equation. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.
double	calculate_n (double pv, double i, double pmt)
	The calculate_n function calculates the number of payments required to pay off a loan based on the present value, interest rate, and payment amount. It uses logarithmic functions to compute the result. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.
double	calculate_pmt (double pv, double n, double i)
	The calculate_pmt function calculates the payment amount for a loan based on the present value, number of payments, and interest rate. It uses the formula for the payment amount of an annuity to compute the result. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.
double	calculate_pv (double n, double i, double pmt)
	The calculate_pv function calculates the present value of a loan based on the number of payments, interest rate, and payment amount. It uses the formula for the present value of an annuity to compute the result. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.
void	error_press_any_key (char *s)
	The error_press_any_key function is a utility function that displays an error message to the user and waits for them to press any key before continuing. It takes a string argument that contains the error message to be displayed. After the user presses a key, it prints two newlines for formatting.
char *	format_currency (float a)
	The format_currency function takes a floating-point number as input and formats it as a currency string. It adds commas as thousand separators and ensures that the number is displayed with two decimal places. The function uses a static buffer to store the formatted string and returns a pointer to that buffer. The formatted string is right-aligned and padded with spaces if necessary.
char *	format_interest (float a)
	The format_interest function takes a floating-point number representing an interest rate and formats it as a string with five decimal places. It uses a static buffer to store the formatted string and returns a pointer to that buffer. The formatted string is left-aligned and padded with spaces if necessary.
int	is_numeric (char *s)
	The is_numeric function checks if a given string consists of numeric characters, including digits, decimal points, and commas. It iterates through each character in the string and returns FALSE if it encounters any character that is not a digit, a decimal point, or a comma. If all characters are valid, it returns TRUE.
int	main (int argc, char **argv)
	iloan is a trivial application to demonstrate how a command-line program can be integrated into C-Menu Form with simple file, argument, or pipe i-o.
void	numbers (char d, char s)
	The numbers function takes two character pointers as arguments: a destination pointer (d) and a source pointer (s). It iterates through the characters in the source string (s) and copies only the numeric characters (digits, decimal points, and minus signs) to the destination string (d). The function effectively filters out any non-numeric characters from the source string and constructs a new string containing only the valid numeric characters. Finally, it null-terminates the destination string.

Variables
int	f_i = 0
int	f_n = 0
int	f_pmt = 0
int	f_pv = 0
bool	f_quiet = false
char	in_str [BUFSIZ+1]

Detailed Description

Installment Loan Calculator.

Author: Bill Waller Copyright (c) 2025 MIT License billx.nosp@m.wall.nosp@m.er@gm.nosp@m.ail..nosp@m.com

Date: 2026-02-09

Definition in file iloan.c.

Macro Definition Documentation

◆ FALSE

#define FALSE 0

Definition at line 18 of file iloan.c.

◆ TRUE

#define TRUE 1

Definition at line 19 of file iloan.c.

Function Documentation

◆ ABEND()

void ABEND ( int e )

The ABEND function is a signal handler that is called when the program receives certain signals (e.g., SIGINT, SIGQUIT, SIGHUP). It takes an integer argument representing the signal number and prints an error message indicating that an abnormal end (ABEND) has occurred, along with the signal number. After displaying the message, it exits the program with a failure status.

Definition at line 419 of file iloan.c.

                  {
    printf("ABEND: Error %d:\n", e);
    exit(EXIT_FAILURE);
}

Referenced by main().

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◆ accept_i()

double accept_i ( )

The accept_i function prompts the user to enter the annual interest rate for the loan. It validates the input to ensure it is numeric and non-negative. If the input is invalid, an error message is displayed, and the user is prompted to enter the value again until a valid input is provided.

Definition at line 227 of file iloan.c.

                  {
    double i;
    while (1) {
        accept_str("Rate (annual) - - -> ");
        if (in_str[0] == '\0') {
            i = 0;
            break;
        } else {
            if (is_numeric(in_str)) {
                i = atof(in_str);
                if (i < 0)
                    error_press_any_key("interest Rate can't be less than 0");
                else
                    break;
            } else
                error_press_any_key("Interest Rate must be numeric");
        }
    }
    return (i);
}

References accept_str(), error_press_any_key(), in_str, and is_numeric().

Referenced by main().

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◆ accept_n()

double accept_n ( )

The accept_n function prompts the user to enter the number of payments for the loan. It validates the input to ensure it is numeric and non-negative. If the input is invalid, an error message is displayed, and the user is prompted to enter the value again until a valid input is provided.

Definition at line 210 of file iloan.c.

                  {
    double n;
    while (1) {
        accept_str("Number of Payments > ");
        if (is_numeric(in_str)) {
            n = atof(in_str);
            if (n < 0)
                error_press_any_key("Number of Payments can't be less than 0");
            else
                break;
        } else
            error_press_any_key("Number of Payments must be numeric");
    }
    return (n);
}

References accept_str(), error_press_any_key(), in_str, and is_numeric().

Referenced by main().

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◆ accept_pmt()

double accept_pmt ( )

brief The accept_pmt function prompts the user to enter the payment amount for the loan. It validates the input to ensure it is numeric and non-negative. If the input is invalid, an error message is displayed, and the user is prompted to enter the value again until a valid input is provided.

Definition at line 249 of file iloan.c.

                    {
    double pmt;
    while (1) {
        accept_str("Payment Amount  - -> ");
        if (in_str[0] == '\0') {
            pmt = 0;
            break;
        } else {
            if (is_numeric(in_str)) {
                pmt = atof(in_str);
                break;
            } else
                error_press_any_key("Payment Amount must be numeric");
        }
    }
    return (pmt);
}

References accept_str(), error_press_any_key(), in_str, and is_numeric().

Referenced by main().

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◆ accept_pv()

double accept_pv ( )

The following functions are used to accept user input for the present value, number of payments, interest rate, and payment amount. They validate the input to ensure it is numeric and meets the required conditions (e.g., non-negative). If the input is invalid, an error message is displayed, and the user is prompted to enter the value again.

Definition at line 188 of file iloan.c.

                   {
    double pv;
    while (1) {
        accept_str("Present Value - - -> ");
        if (in_str[0] == '\0') {
            pv = 0;
            break;
        } else {
            if (is_numeric(in_str)) {
                pv = atof(in_str);
                if (pv < 0)
                    error_press_any_key("Present Value can't be less than 0");
                else
                    break;
            } else
                error_press_any_key("Present Value must be numeric");
        }
    }
    return (pv);
}

References accept_str(), error_press_any_key(), in_str, and is_numeric().

Referenced by main().

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◆ accept_str()

void accept_str ( char * s )

The accept_str function prompts the user with a given string and reads input from the standard input into a buffer. It uses the fprintf function to display the prompt and the read function to capture the user's input. The input is stored in the global buffer in_str, which can be used by other functions for further processing.

Definition at line 363 of file iloan.c.

                         {
    fprintf(stderr, "%s", s);
    read(0, in_str, BUFSIZ);
}

References in_str.

Referenced by accept_i(), accept_n(), accept_pmt(), and accept_pv().

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◆ calculate_i()

double calculate_i	(	double	pv,
		double	n,
		double	pmt )

The calculate_i function calculates the annual interest rate for a loan based on the present value, number of payments, and payment amount. It uses an iterative method to find the interest rate that satisfies the loan equation. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.

Definition at line 302 of file iloan.c.

                                                    {
    double i1 = 0, i;
    double delta = 0.0000001;
    double xdelta;
    double fdelta;
    double fprimi;
    double ffact;
    double fi;
    double fman;
    double fmann;
    if (pv == 0 || n == 0 || pmt == 0)
        error_press_any_key(
            "3 non-zero values required to calculate Interest Rate");
    ffact = pv / pmt;
    xdelta = 0;
    if (ffact < n) {
        i1 = 0.0125;
        xdelta = 0.9;
    }
    while (xdelta > delta) {
        fman = 1 / (1 + i1);
        fmann = pow(fman, n);
        fi = ffact * i1 + fmann - 1;
        fprimi = ffact - n * fmann * fman;
        fdelta = fi / fprimi;
        i1 = i1 - fdelta;
        if (fdelta < 0)
            xdelta = -fdelta;
        else
            xdelta = fdelta;
    }
    i = i1 * 1200;
    if (!f_quiet)
        printf("interest Rate - - - - - -> %s\n", format_interest(i));
    return (i);
}

References error_press_any_key(), f_quiet, and format_interest().

Referenced by main().

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◆ calculate_n()

double calculate_n	(	double	pv,
		double	i,
		double	pmt )

The calculate_n function calculates the number of payments required to pay off a loan based on the present value, interest rate, and payment amount. It uses logarithmic functions to compute the result. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.

Definition at line 289 of file iloan.c.

                                                    {
    double i1, n;
    if (pv == 0 || i == 0 || pmt == 0)
        error_press_any_key("3 non-zero values required to calculate "
                            "Number of Payments");
    i1 = i / 1200;
    n = -log(1 - pv * i1 / pmt) / log(1 + i1);
    if (!f_quiet)
        printf("Number of Payments  - - -> %s\n", format_currency(n));
    return (n);
}

References error_press_any_key(), f_quiet, and format_currency().

Referenced by main().

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◆ calculate_pmt()

double calculate_pmt	(	double	pv,
		double	n,
		double	i )

The calculate_pmt function calculates the payment amount for a loan based on the present value, number of payments, and interest rate. It uses the formula for the payment amount of an annuity to compute the result. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.

Definition at line 340 of file iloan.c.

                                                    {
    double i1, pmt;
    if (pv == 0 || n == 0 || i == 0)
        error_press_any_key(
            "3 non-zero values required to calculate Payment Amount");
    i1 = i / 1200;
    pmt = pv * i1 / (1 - pow(1 + i1, -n));
    if (!f_quiet)
        printf("Payment Amount  - - - - -> %s\n", format_currency(pmt));
    return (pmt);
}

References error_press_any_key(), f_quiet, and format_currency().

Referenced by main().

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◆ calculate_pv()

double calculate_pv	(	double	n,
		double	i,
		double	pmt )

The calculate_pv function calculates the present value of a loan based on the number of payments, interest rate, and payment amount. It uses the formula for the present value of an annuity to compute the result. If any of the input values are zero, it displays an error message and prompts the user to provide valid inputs.

Definition at line 275 of file iloan.c.

                                                    {
    double i1, pv;
 
    if (n == 0 || i == 0 || pmt == 0)
        error_press_any_key(
            "3 non-zero values required to calculate Present Value");
    i1 = i / 1200;
    pv = pmt * (1 - pow(1 + i1, -n)) / i1;
    if (!f_quiet)
        printf("Present Value - - - - - -> %s\n", format_currency(pv));
    return (pv);
}

References error_press_any_key(), f_quiet, and format_currency().

Referenced by main().

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◆ error_press_any_key()

void error_press_any_key ( char * s )

The error_press_any_key function is a utility function that displays an error message to the user and waits for them to press any key before continuing. It takes a string argument that contains the error message to be displayed. After the user presses a key, it prints two newlines for formatting.

Definition at line 268 of file iloan.c.

                                  {
    printf("%s", s);
    getc(stdin);
    printf("\n\n");
}

Referenced by accept_i(), accept_n(), accept_pmt(), accept_pv(), calculate_i(), calculate_n(), calculate_pmt(), calculate_pv(), and main().

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◆ format_currency()

char * format_currency ( float a )

The format_currency function takes a floating-point number as input and formats it as a currency string. It adds commas as thousand separators and ensures that the number is displayed with two decimal places. The function uses a static buffer to store the formatted string and returns a pointer to that buffer. The formatted string is right-aligned and padded with spaces if necessary.

Definition at line 369 of file iloan.c.

                               {
    int digit_count, left_of_point;
    static char sstr[80];
    static char fstr[80];
    char *src_ptr, *dst_ptr, *sptr, *FPtr;
 
    sprintf(sstr, "%-18.2f", a);
    sptr = sstr;
    src_ptr = sstr;
    FPtr = fstr;
    dst_ptr = fstr;
    while (*src_ptr++ != '\0')
        ;
    src_ptr -= 2;
    left_of_point = FALSE;
    digit_count = 0;
    while (src_ptr >= sptr) {
        if (left_of_point) {
            if (*src_ptr >= '0' && *src_ptr <= '9') {
                if (digit_count == 3) {
                    *dst_ptr++ = ',';
                    digit_count = 1;
                } else
                    digit_count++;
            }
        } else if (*src_ptr == '.')
            left_of_point = TRUE;
        *dst_ptr++ = *src_ptr--;
    }
    src_ptr = dst_ptr - 1;
    dst_ptr = sptr;
    while (src_ptr >= FPtr)
        *dst_ptr++ = *src_ptr--;
    while (*--dst_ptr == ' ')
        ;
    *++dst_ptr = '\0';
    return (sptr);
}

Referenced by calculate_n(), calculate_pmt(), calculate_pv(), and main().

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◆ format_interest()

char * format_interest ( float a )

The format_interest function takes a floating-point number representing an interest rate and formats it as a string with five decimal places. It uses a static buffer to store the formatted string and returns a pointer to that buffer. The formatted string is left-aligned and padded with spaces if necessary.

Definition at line 409 of file iloan.c.

                               {
    static char sstr[80];
    char *s;
 
    sprintf(sstr, "%-3.5f", a);
    s = sstr;
    return (s);
}

Referenced by calculate_i(), and main().

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◆ is_numeric()

int is_numeric ( char * s )

The is_numeric function checks if a given string consists of numeric characters, including digits, decimal points, and commas. It iterates through each character in the string and returns FALSE if it encounters any character that is not a digit, a decimal point, or a comma. If all characters are valid, it returns TRUE.

Definition at line 353 of file iloan.c.

                        {
    char c;
 
    while ((c = *s++) != '\0' && c != '\n')
        if ((c < '0' || c > '9') && c != '.' && c != ',')
            return (FALSE);
    return (TRUE);
}

Referenced by accept_i(), accept_n(), accept_pmt(), and accept_pv().

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◆ main()

int main	(	int	argc,
		char **	argv )

iloan is a trivial application to demonstrate how a command-line program can be integrated into C-Menu Form with simple file, argument, or pipe i-o.

Positional arguments: present_value The present value of the loan (the amount borrowed). number_of_payments The total number of payments to be made. interest_rate The annual interest rate (as a percentage). payment_amount The amount of each payment. The program calculates the missing value based on the three provided values. For example, if the present value, number of payments, and interest rate are provided, it will calculate the payment amount. If the present value, interest rate, and payment amount are provided, it will calculate the number of payments, and so on. The program can be used in a non-interactive way by passing the four values as arguments, with the value to be calculated set to 0. For example, to calculate the payment amount for a $10,000 loan with a 5% annual interest rate and 60 monthly payments, you could run:

iloan 10000 60 5 0

This is proof-of-concept code, and is not intended for production use. It is not designed to be robust, and does not handle all edge cases or input errors. It is intended solely to demonstrate how a simple command-line program can be integrated into C-Menu Form. In the future, more sophisticated abstractions, such as async event handlers, serialization, rpc, database, and a standardized plugin interface will be integrated into C-Menu. Feel free to modify and enhance this code as needed, but please do not use it in production without proper testing and validation. It is provided as-is without any warranty or support. Use at your own risk.

Definition at line 74 of file iloan.c.

                                {
    double pv = 0, pmt = 0, i = 0, n = 0;
    char tmp_str[BUFSIZ];
 
    signal(SIGINT, ABEND);
    signal(SIGQUIT, ABEND);
    signal(SIGHUP, ABEND);
 
    if (argc > 1 &&
        ((strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-h") == 0) ||
         argc < 4)) {
        printf("Usage: iloan [present_value number_of_payments "
               "interest_rate payment_amount]\n\n");
        exit(EXIT_SUCCESS);
    }
    if (argc > 4) {
        numbers(tmp_str, argv[1]);
        sscanf(tmp_str, "%lf", &pv);
        sscanf(argv[2], "%lf", &n);
        sscanf(argv[3], "%lf", &i);
        numbers(tmp_str, argv[4]);
        sscanf(tmp_str, "%lf", &pmt);
        if (pv != 0)
            f_pv = 1;
        if (n != 0)
            f_n = 1;
        if (i != 0)
            f_i = 1;
        if (pmt != 0)
            f_pmt = 1;
        if (f_pv + f_n + f_i + f_pmt < 3) {
            error_press_any_key(
                "Error: At least three values must be greater than zero.");
            exit(EXIT_FAILURE);
        }
        f_quiet = true;
    } else {
        if (argc != 1) {
            printf("Usage: iloan [present_value number_of_payments "
                   "interest_rate payment_amount]\n\n");
            exit(EXIT_FAILURE);
        } else {
            if (argc == 1) {
                printf("\nInstallment Loan Calculator\n\n");
                printf("Three of these values must be greater than 0.  The "
                       "field\n");
                printf("with a value of 0 will be calculated.\n\n");
                while (f_pv + f_n + f_i + f_pmt < 3) {
                    if (pv == 0) {
                        pv = accept_pv();
                        if (pv != 0)
                            f_pv = 1;
                    }
                    if (n == 0) {
                        n = accept_n();
                        if (n != 0)
                            f_n = 1;
                    }
                    if (i == 0) {
                        i = accept_i();
                        if (i != 0)
                            f_i = 1;
                    }
                    if (pmt == 0) {
                        pmt = accept_pmt();
                        if (pmt != 0)
                            f_pmt = 1;
                    }
                    if (f_pv + f_n + f_i + f_pmt < 3) {
                        error_press_any_key("Error: At least three values must "
                                            "be greater than zero.");
                    }
                }
                printf("\nYou entered:\n\n");
                if (pv != 0)
                    printf("Present Value - - - - - -> %s\n",
                           format_currency(pv));
                if (n != 0)
                    printf("Number of Payments  - - -> %s\n",
                           format_currency(n));
                if (i != 0)
                    printf("Interest Rate - - - - - -> %s\n",
                           format_interest(i));
                if (pmt != 0)
                    printf("Payment Amount  - - - - -> %s\n",
                           format_currency(pmt));
                printf("\n\nCalculation result:\n\n");
            }
        }
    }
    if (pv == 0)
        pv = calculate_pv(n, i, pmt);
    else if (n == 0)
        n = calculate_n(pv, i, pmt);
    else if (i == 0)
        i = calculate_i(pv, n, pmt);
    else if (pmt == 0)
        pmt = calculate_pmt(pv, n, i);
 
    if (f_quiet) {
        printf("%s\n", format_currency(pv));
        printf("%s\n", format_currency(n));
        printf("%s\n", format_interest(i));
        printf("%s\n", format_currency(pmt));
    }
    signal(SIGINT, SIG_DFL);
    signal(SIGQUIT, SIG_DFL);
    signal(SIGHUP, SIG_DFL);
}

References ABEND(), accept_i(), accept_n(), accept_pmt(), accept_pv(), calculate_i(), calculate_n(), calculate_pmt(), calculate_pv(), error_press_any_key(), f_i, f_n, f_pmt, f_pv, f_quiet, format_currency(), format_interest(), and numbers().

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◆ numbers()

void numbers	(	char *	d,
		char *	s )

The numbers function takes two character pointers as arguments: a destination pointer (d) and a source pointer (s). It iterates through the characters in the source string (s) and copies only the numeric characters (digits, decimal points, and minus signs) to the destination string (d). The function effectively filters out any non-numeric characters from the source string and constructs a new string containing only the valid numeric characters. Finally, it null-terminates the destination string.

Definition at line 425 of file iloan.c.

                               {
    while (*s != '\0') {
        if (*s == '-' || *s == '.' || (*s >= '0' && *s <= '9'))
            *d++ = *s++;
        else
            s++;
    }
    *d = '\0';
}

Referenced by main().

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Variable Documentation

◆ f_i

int f_i = 0

Definition at line 39 of file iloan.c.

Referenced by main().

◆ f_n

int f_n = 0

Definition at line 38 of file iloan.c.

Referenced by main().

◆ f_pmt

int f_pmt = 0

Definition at line 40 of file iloan.c.

Referenced by main().

◆ f_pv

int f_pv = 0

Definition at line 37 of file iloan.c.

Referenced by main().

◆ f_quiet

bool f_quiet = false

Definition at line 41 of file iloan.c.

Referenced by calculate_i(), calculate_n(), calculate_pmt(), calculate_pv(), and main().

◆ in_str

char in_str[BUFSIZ+1]

Examples: /usr/local/src/C-Menu/src/enterstr.c.

Definition at line 20 of file iloan.c.

Referenced by accept_i(), accept_n(), accept_pmt(), accept_pv(), and accept_str().

Macros

Functions

Variables

Detailed Description

Macro Definition Documentation

◆ FALSE

◆ TRUE

Function Documentation

◆ ABEND()

◆ accept_i()

◆ accept_n()

◆ accept_pmt()

◆ accept_pv()

◆ accept_str()

◆ calculate_i()

◆ calculate_n()

◆ calculate_pmt()

◆ calculate_pv()

◆ error_press_any_key()

◆ format_currency()

◆ format_interest()

◆ is_numeric()

◆ main()

◆ numbers()

Variable Documentation

◆ f_i

◆ f_n

◆ f_pmt

◆ f_pv

◆ f_quiet

◆ in_str