Using strcmp in an if statement - c

Question

I am fairly new to C, and I am trying to understand using strings and strcmp to compare two strings in an if statement.
My goal is to be able to run a different function depending on what the user has inputted.
#include <stdio.h>
#include <iostream>
#include <string.h>
#include <stdlib.h>
void gasbill();
void electricitybill();
int main()
{
char input[20];
const char gasCheck[4] = "gas";
const char electricityCheck[13] = "electricity";
printf("Your bills explained!\n\n");
printf("In this application I will go through your gas and electricty bills.\n");
printf("I will explain how each of the billing payments work, \nand the calculations that go on,\n");
printf("to create your bill.\n\n");
printf("Please choose a bill to get started with:\n- gas\n- electricity\n\n");
fgets(input, 20, stdin);
if (strcmp (input, gasCheck)== 0){
printf("\nPreparing to run Gas bill!\n\n");
system("PAUSE");
system("cls");
gasbill();
system("PAUSE");
}
else if (strcmp (input, electricityCheck)== 0){
printf("\nPreparing to run Electricity bill!\n\n");
system("cls");
electricitybill();
system("PAUSE");}
else {
printf("\nError exiting...\n\n");
system("PAUSE");
}
return 0;
}
void gasbill()
{
float balanceBroughtForward, gasThisQuarter, subTotalPerQuarter;
char poundSign = 156;
printf("******Your gas bill, explained!******\n\n\n");
printf("Hello, and welcome to your gas bill, explained. Let's get started.\n");
printf("Please enter the balance brought forward from your previous statement: \n\n%c", poundSign);
scanf("%f", &balanceBroughtForward);
printf("\nHow this works:\n- The money that you did not pay last quarter for your gas bill\nhas been added to this quarterly payment\n\n");
printf("\nNext let's add this to the amount of gas you have spent this quarter. \n(how much gas have you used so far in this billing period?)");
printf(": %c", poundSign);
scanf("%f", &gasThisQuarter);
printf("\n\nNow what? The two values that you have entered\n(balance brought forward
and gas spent this quarter)\nare added together, %c%3.2f + %c%3.2f\n", poundSign,
balanceBroughtForward, poundSign, gasThisQuarter);
subTotalPerQuarter = (balanceBroughtForward + gasThisQuarter);
printf("This is");
}
void electricitybill()
{
printf("Empty");
system("PAUSE");
}
When ever it runs the if statement it always executes the gasBill function and not the electricityBill function.
Thanks in advance.

Answer 1

fgets will return a string which ends with a newline (\n). From its documentation
Reads characters from stream and stores them as a C string into str
until (num-1) characters have been read or either a newline or the
end-of-file is reached, whichever happens first.
You could either test for a trailing newline and strip it off
fgets(input, 20, stdin);
size_t len = strlen(input);
if (input[len-1] == '\n') {
input[len-1] = '\0';
}
or read user input using scanf instead.
scanf("%19s", input);
As an aside
const char gasCheck[4] = "gas";
const char electricityCheck[13] = "electricity";
could be declared slightly more easily and safely as
const char *gasCheck = "gas";
const char *electricityCheck = "electricity";
(This form saves copying the string literals into stack variables. More importantly, it removes a potential source of bugs if you hard-code too small a length for the arrays.)

Answer 2

fgets() read characters from stdin until a newline is seen or an EOF, if newline was seen, it would be stored in the array.Anyway, fgets() appends a null character to the array.
so if you want end your input by newline, my minor alteration is here, change this
const char gasCheck[4] = "gas";
const char electricityCheck[13] = "electricity";
to
const char gasCheck[5] = "gas\n";
const char electricityCheck[14] = "electricity\n";