C Program, Array inside a case in continuous Switch Menu loop - c

Question

My array inside case 4 in looping Switch Menu doesn't print/display the value of the last array when the user input goes beyond array[4].
I tried to take the case 4 out and make it a single program to check if it doesn't really work on its own but it works fine, but when I put it back into the Switch, same issue again. I thought that maybe the initialization part is the problem. Help
`
#include <stdio.h>
int main ()
{
char first[20],last[20];
float math,eng,sci,avg;
int a,b,c,d,diff,array[diff],e,i,input;
do{
printf("\nMAIN MENU\n");
printf("[1] Basic Input Output\n[2] Conditional Statement\n[3] Looping Construct\n[4] Array\n[5] About\n[6] Exit");
printf("\n\nChoose: ");
scanf("%d",&input);
printf("\n");
switch (input)
{
case 1:
printf("\nEnter your given name:");
scanf("%s",first);
printf("Enter your surname:");
scanf("%s",last);
printf("\nHello %s %s!\n",first,last);
break;
case 2:
printf("\nEnter your grade in Math:");
scanf("%f",&math);
printf("\nEnter your grade in Science:");
scanf("%f",&sci);
printf("\nEnter your grade in English:");
scanf("%f",&eng);
avg=(math+eng+sci)/3;
if(math>eng&&sci)
{
printf("\nHighest grade is in: Math");
}
if(eng>math&&sci)
{
printf("\nHighest grade is in: English");
}
if(sci>eng&&math)
{
printf("\nHighest grade is in: Science");
}
if(math==eng)
{
printf("\n--Math and English tied grades--");
}
if(math==sci)
{
printf("\n--Math and Science tied grades--");
}
if(eng==sci)
{
printf("\n--Science and English tied grades--");
}
printf("\nYour average in 3 subjects:%f\n",avg);
break;
case 3:
printf("Enter beginning number: ");
scanf("%d",&b);
printf("Enter ending number: ");
scanf("%d",&c);
printf("\nCounting from %d to %d\n",b,c);
while(b<=c)
{
printf("%d ",b);
b++;
}
printf("\n");
break;
case 4:
printf("Enter Starting Series of Numbers: ");
scanf("%d",&a);
printf("Enter Ending Series of Numbers: ");
scanf("%d",&d);
diff=(d-a);
array[diff]=d;
printf("Select Array Value to Display: 0 to %d: ",diff);
scanf("%d",&e);
for(i=0;i<=diff;i++)
{
array[i]=a;
if(i==e)
{
printf("%d\n",array[i]);
}
a++;
}
break;
case 5:
printf("Abouts\n");
break;
case 6:
printf("Thank you!");
break;
}
}while(input != 6);
return 0;
}
`

Answer 1

This should be a comment, but I don´t have enough reputation yet.
As Gerhardh said, the problem is that you use diff before being initialized (which causes undefined behaviour)
As you said, if you move the initialization of array inside the case 4: after diff being set the program should work as intended. If you want to keep it before the switch, try using int* and malloc/free inside the case 4: (and add #include <stdlib.h>) Anyway, you should check diff as positive value before using it to set the size of array
About optimizing the code, you can skip the line array[diff]=d; because you are setting the same value inside the for loop. And you can remove the line a++; if you changes array[i]=a; -> array[i]=a+i;