c - Unable to properly terminate "while" loop -

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i'm trying out programming in c first time, , applying concrete stuff...

the program i'm creating problem deals while loop. goal of program calculate average miles per gallon set of trucks. want terminate -1 inputted number of gallons consumed, instead have input twice, once number of gallons, , once number of miles. have found input in fact used part of calculation of result. here code:

#include <stdio.h> int main() {      int tanks, miles;     float gallons = 0, average = 0, miles_per_gallon = 0;     tanks = 0;      while (gallons != -1) {              tanks += 1;             miles_per_gallon = (float)miles / gallons;             average = average + miles_per_gallon;             printf("the miles / gallon tank %.3f\n",                    miles_per_gallon);              printf("enter gallons used (-1 end): ");             scanf("%f", &gallons);              printf("enter miles driven: ");             scanf("%d", &miles);     }      average /= tanks;     printf("the overall average miles/gallon %.3f", average);     return 0; }

here sample output:

c:\>gallons enter gallons used (-1 end): 12.3 enter miles driven: 700 miles / gallon tank 56.911 enter gallons used (-1 end): 13.4 enter miles driven: 666 miles / gallon tank 49.701 enter gallons used (-1 end): 17.3 enter miles driven: 644 miles / gallon tank 37.225 enter gallons used (-1 end): 15.5 enter miles driven: 777 miles / gallon tank 50.129 enter gallons used (-1 end): -1 enter miles driven: -1 miles / gallon tank 1.000 overall average miles/gallon 38.993

thanks help.

the problem sequence of statements in code such check loop's exit condition not reached until after second input requested. add check -1 it's entered, , break out loop. alternatively, ask miles entered ahead of gallons.

for (;;) { /* "forwver" loop; break out middle */          tanks += 1;         miles_per_gallon = (float)miles / gallons;         average = average + miles_per_gallon;         printf("the miles / gallon tank %.3f\n",                miles_per_gallon);          printf("enter gallons used (-1 end): ");         scanf("%f", &gallons);          /* break loop: */         if (gallons == -1) return 0;          printf("enter miles driven: ");         scanf("%d", &miles); }

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