我不明白为什么我的C程序正确运行一次，然后给我不想要的结果

Question

我不明白为什么我的程序运行正常，然后当做...当重复它进入我的其他功能，当我不想要它。完成后，我的程序就会像我想要的那样重复。对于糟糕的解释我很抱歉，但我认为“我的计划”下面的图片比我能解释发生的事情要好。

#include <stdio.h> 
#include <stdlib.h> 

int main() 
{ 
float fahrenheit,celsius; 
char selection[30]; 
char *idontknow; 
long trueselection; 

do{ 
printf("1: Fahrenheit to Celsius."); 
printf("\n2: Celsius to Fahrenheit."); 
printf("\n3: Quit program.\n"); 
fgets(selection,10,stdin); //This line and the line below are in the program 
//so that if the user inputs a character instead of an integer it does not infinite loop. 
trueselection = strtol(selection, &idontknow, 10); 

if(trueselection==1){ 
    printf("Enter temperature in Fahrenheit: "); 
    scanf("%f",&fahrenheit); 
    celsius= (fahrenheit - 32)/1.8; 
    printf("Temperature in Celsius: %.2f\n\n\n",celsius); 
} 
else if(trueselection==2){ 
    printf("Enter temperature in Celsius: "); 
    scanf("%f",&celsius); 
    fahrenheit= (celsius*1.8)+32; 
    printf("Temperature in Fahrenheit: %.2f\n\n\n",fahrenheit); 
} 
else if(trueselection==3){ //This "else if" statement is so the program does not say invalid selection before exiting if 3 is entered. 
return 0; 
} 
else{ 
    printf("Invalid selection !!!\n\n\n"); 
} 
} 
while(trueselection!=3); //This line tells the program to repeat back to    line 3 if any selection but 3 is selected. 
} 

Answer 1

#include <stdio.h> 
#include <stdlib.h> 

int main() 
{ 
    float fahrenheit,celsius; 
    int selection; 
    char *idontknow; 
    long trueselection; 

    do{ 
     printf("1: Fahrenheit to Celsius."); 
     printf("\n2: Celsius to Fahrenheit."); 
     printf("\n3: Quit program.\n"); 
     scanf("%d",&selection); 

     if(trueselection==1){ 
      printf("Enter temperature in Fahrenheit: "); 
      scanf("%f",&fahrenheit); 
      celsius= (fahrenheit - 32)/1.8; 
      printf("Temperature in Celsius: %.2f\n\n\n",celsius); 
     } 
     else if(trueselection==2){ 
      printf("Enter temperature in Celsius: "); 
      scanf("%f",&celsius); 
      fahrenheit= (celsius*1.8)+32; 
      printf("Temperature in Fahrenheit: %.2f\n\n\n",fahrenheit); 
     } 
     else if(trueselection==3){ //This "else if" statement is so the program does not say invalid selection before exiting if 3 is entered. 
      break; 
     } 
     else{ 
      printf("Invalid selection !!!\n\n\n"); 
     } 
    } 
    while(trueselection != 3); //This line tells the program to repeat back to line 3 if any selection but 3 is selected. 
}