VTU BPOPS103 · Lab Program 3

Electricity Bill with Slab Rates and Surcharge in C

Q: Why use double for the bill?

The rates are decimal fractions such as 0.80 and 0.90, so a floating-point type keeps the paise values instead of truncating them.

Q: What does %49s guard against in scanf?

It limits the number of characters stored in name so that a long input cannot overflow the 50-byte array.

Q: Why is the surcharge applied after the meter charge?

The rule compares the total bill with Rs 400, and the meter charge is part of that total.

Read a customer's name and units consumed, apply the slab tariff, add the meter charge and the 15% surcharge above Rs 400, and print the bill.

Algorithm

Read the customer name (one word) and the units consumed.
Reject negative units.
Compute the energy charge slab by slab: first 200 units at 0.80, next 100 units at 0.90, and every unit beyond 300 at 1.00.
Add the fixed meter charge of Rs 100 to the energy charge.
If the total is more than Rs 400, add a 15% surcharge on the total.
Print the name, units and the bill amount with two decimal places.

The syllabus says all users are charged 'a minimum of Rs. 100 as meter charge'. This program treats it as a fixed Rs 100 added to every bill, which is how the reference outputs in the college lab manuals are computed. Confirm the reading with your lab instructor if your manual differs.

C program

#include <stdio.h>

#define METER_CHARGE 100.0
#define SURCHARGE_LIMIT 400.0
#define SURCHARGE_RATE 0.15

double energy_charge(int units) {
    double charge;

    if (units <= 200) {
        charge = units * 0.80;
    } else if (units <= 300) {
        charge = 200 * 0.80 + (units - 200) * 0.90;
    } else {
        charge = 200 * 0.80 + 100 * 0.90 + (units - 300) * 1.00;
    }
    return charge;
}

int main(void) {
    char name[50];
    int units;
    double bill;

    printf("Enter customer name:\n");
    scanf("%49s", name);
    printf("Enter units consumed:\n");
    if (scanf("%d", &units) != 1 || units < 0) {
        printf("Error: units must be a non-negative whole number\n");
        return 1;
    }

    bill = METER_CHARGE + energy_charge(units);
    if (bill > SURCHARGE_LIMIT) {
        bill = bill + bill * SURCHARGE_RATE;
    }

    printf("Customer : %s\n", name);
    printf("Units    : %d\n", units);
    printf("Bill     : Rs %.2f\n", bill);
    return 0;
}

Sample input and output

Crosses 300 units and attracts the surcharge

Input

Ramesh
457

Output

Enter customer name:
Enter units consumed:
Customer : Ramesh
Units    : 457
Bill     : Rs 583.05

Edge case: zero units still pays the meter charge

Input

Shyam
0

Output

Enter customer name:
Enter units consumed:
Customer : Shyam
Units    : 0
Bill     : Rs 100.00

How the program works

The tariff is progressive: each slab is priced only on the units that fall inside it. The energy_charge function therefore adds the cost of the first 200 units, the next 100 units and the remainder, instead of pricing all units at one rate.

The surcharge is applied to the whole bill and only when the bill exceeds Rs 400, so the check happens after the meter charge is added. The constants are defined with #define so the rates are easy to change in one place.

Run this code

Edit the program and press Run. It is free and needs no sign-in.

Program input (what the program reads with scanf)

Copy a sample and paste it into the STDIN box in the editor before you press Run.

Crosses 300 units and attracts the surcharge

Ramesh
457

Expected output

Enter customer name:
Enter units consumed:
Customer : Ramesh
Units    : 457
Bill     : Rs 583.05

Edge case: zero units still pays the meter charge

Shyam
0

Expected output

Enter customer name:
Enter units consumed:
Customer : Shyam
Units    : 0
Bill     : Rs 100.00

Viva questions

Why is the energy charge computed in a separate function?

Keeping the tariff in energy_charge separates the slab logic from input and output, so the rule can be tested with any unit value.

Why does 300 units go into the second slab and not the third?

The slab conditions use <= 200 and <= 300, so 300 is the last unit of the second slab and only units above 300 use the third rate.

Why use double for the bill?

The rates are decimal fractions such as 0.80 and 0.90, so a floating-point type keeps the paise values instead of truncating them.

What does %49s guard against in scanf?

It limits the number of characters stored in name so that a long input cannot overflow the 50-byte array.

Why is the surcharge applied after the meter charge?

The rule compares the total bill with Rs 400, and the meter charge is part of that total.

Common mistakes

Pricing every unit at the first-slab rate instead of charging each slab separately.
Using int for the bill, which discards the paise.
Reading the name with scanf("%s") without a width, which can overflow the buffer on long input.
Applying the surcharge before adding the meter charge, which gives a different total near the Rs 400 limit.

Back to all VTU BPOPS103 lab programs.

Algorithm

Read the customer name (one word) and the units consumed.

Reject negative units.

Compute the energy charge slab by slab: first 200 units at 0.80, next 100 units at 0.90, and every unit beyond 300 at 1.00.

Add the fixed meter charge of Rs 100 to the energy charge.

If the total is more than Rs 400, add a 15% surcharge on the total.

Print the name, units and the bill amount with two decimal places.

C program

#include <stdio.h> #define METER_CHARGE 100.0 #define SURCHARGE_LIMIT 400.0 #define SURCHARGE_RATE 0.15 double energy_charge(int units) { double charge; if (units <= 200) { charge = units * 0.80; } else if (units <= 300) { charge = 200 * 0.80 + (units - 200) * 0.90; } else { charge = 200 * 0.80 + 100 * 0.90 + (units - 300) * 1.00; } return charge; } int main(void) { char name[50]; int units; double bill; printf("Enter customer name:\n"); scanf("%49s", name); printf("Enter units consumed:\n"); if (scanf("%d", &units) != 1 || units < 0) { printf("Error: units must be a non-negative whole number\n"); return 1; } bill = METER_CHARGE + energy_charge(units); if (bill > SURCHARGE_LIMIT) { bill = bill + bill * SURCHARGE_RATE; } printf("Customer : %s\n", name); printf("Units : %d\n", units); printf("Bill : Rs %.2f\n", bill); return 0; }

Sample input and output

Crosses 300 units and attracts the surcharge

Input

Ramesh
457

Output

Enter customer name:
Enter units consumed:
Customer : Ramesh
Units    : 457
Bill     : Rs 583.05

Edge case: zero units still pays the meter charge

Input

Shyam
0

Output

Enter customer name:
Enter units consumed:
Customer : Shyam
Units    : 0
Bill     : Rs 100.00

How the program works

Viva questions

Why is the energy charge computed in a separate function?

Keeping the tariff in energy_charge separates the slab logic from input and output, so the rule can be tested with any unit value.

Why does 300 units go into the second slab and not the third?

The slab conditions use <= 200 and <= 300, so 300 is the last unit of the second slab and only units above 300 use the third rate.

Why use double for the bill?

The rates are decimal fractions such as 0.80 and 0.90, so a floating-point type keeps the paise values instead of truncating them.

What does %49s guard against in scanf?

It limits the number of characters stored in name so that a long input cannot overflow the 50-byte array.

Why is the surcharge applied after the meter charge?

The rule compares the total bill with Rs 400, and the meter charge is part of that total.

Common mistakes

Pricing every unit at the first-slab rate instead of charging each slab separately.

Using int for the bill, which discards the paise.

Reading the name with scanf("%s") without a width, which can overflow the buffer on long input.

Applying the surcharge before adding the meter charge, which gives a different total near the Rs 400 limit.