calculating amount based on uneven ranges in C

Question

I was going through previous paper questions and found a question that requires me to produce a bill based on the number of units(of electricity) each customer have consumed. there's a table given instructing how the calculation is done. I already made the program in a very basic way using if else statements. I want to know if there is a better approach to this than using if else. maybe loops ? I tried using loops but its impractical to me since the ranges are not constant. This is a screenshot of a portion of that question.Question Screenshot

The function I created to calculate is given below.

void findBill(int table[],float ar[],int size)
{
int i; 
float bill;
for(i=0; i<7 ; i++)
{
    if((table[i]>=0)&&(table[i]<=5))
    {   bill=table[i]*3.0;
    }
    else if((table[i]>=6)&&(table[i]<=10))
    {   bill=5*3.0+(table[i]-5)*7.0;
    }
    else if((table[i]>=11)&&(table[i]<=15))
    {   bill=5*3.0+5*7.0+(table[i]-10)*15.0;
    }
    else if((table[i]>=16)&&(table[i]<=20))
    {   bill=5*3.0+5*7.0+5*15.0+(table[i]-15)*30.0;
    }
    else if((table[i]>=21)&&(table[i]<=25))
    {   bill=5*3.0+5*7.0+5*15.0+5*30.0+(table[i]-20)*50.0;
    }
    else if((table[i]>=26)&&(table[i]<=30))
    {   bill=5*3.0+5*7.0+5*15.0+5*30.0+5*50.0+(table[i]-25)*75.0;
    }
    else if((table[i]>=31)&&(table[i]<=40))
    {   bill=5*3.0+5*7.0+5*15.0+5*30.0+5*50.0+5*75.0+(table[i]-30)*90.0;
    }
    else if((table[i]>=41)&&(table[i]<=50))
    {   bill=5*3.0+5*7.0+5*15.0+5*30.0+5*50.0+5*75.0+10*90.0+(table[i]-40)*105.0;
    }
    else if((table[i]>=51)&&(table[i]<=75))
    {   bill=5*3.0+5*7.0+5*15.0+5*30.0+5*50.0+5*75.0+10*90.0+10*105.0+(table[i]-50)*110.0;
    }
    else if(table[i]>75)
    {   bill=5*3.0+5*7.0+5*15.0+5*30.0+5*50.0+5*75.0+10*90.0+10*105.0+25*110.0+(table[i]-75)*120.0;
    }

    ar[i]=bill; 
}

}

Even though this works i feel that this is bad coding, what if there were 100 ranges. Please suggest me another easier way to do this rather than writing simple if else statements.

P.S : I am a beginner, so please be kind enough to suggest answers using stdio.h in C.

Thanks in advance.

Answer 1

Since there is no fixed pattern for the calculation of the costs depending on the number of the units it is difficult to avoid the hardcoded array of the values.

But the implementation you made can be much cleaner i think.

The approach that i think is better is the following

void findBill(int table[], float arr[], int size) {

    int levels[]={75,50,40,30,25,20,15,10,5,0};
    float costs[]={120,110,105,90,75,50,30,15,7,3};
    int level_cnt=sizeof(levels) / sizeof(int);

    for(int i=0;i<size;i++) {
        arr[i]=0;
        for(int c=0;c<level_cnt;c++) {
            if(table[i]>levels[c]) {
                arr[i]+=(table[i]-levels[c])*costs[c];
                table[i]=levels[c];
            }
        }
    }
}