how do you increment a variable using a pointer in C? [duplicate] - c

Question

This question already has an answer here:
Variable changed in function not seen by caller?
(1 answer)
Closed 6 years ago.
My code is about implementing the function of Stack and Queue. If you're wondering why don't I use struct in my program that's because our instructor only allows us to use pointers and I'm a little bit confused using pointers. It's not yet finish, as much as possible I try to finish one function at a time instead of skipping to the next.
#include <stdio.h>
int container[5];
int len=0;
As I understand this part of my in function insert, "len" is suppose to change since it's incremented every time the function is use
int insert(int container[],int len)
{
int i,j,content,*incre;
incre=&len;
for(i=*incre;i<*incre+1;i++)
{
printf("Enter number:\n");
scanf("%d",&content);
container[i]=content;
}
(*incre)++;
}
int printcontent(int container[])
{
int i;
printf("Content of container:\n");
for(i=0;i<len+1;i++)
{
printf("%d ",container[i]);
}
printf("\n");
}
This part asks the user whether to add a content in the container or simply remove it. They can only add or remove one at a time.
int stack(int container[])
{
while(1)
{
int choice;
printf("What do you wanna do?\n");
printf("1.Insert\n2.Pop\n3.isEmpty\n4.Exit\n");
scanf("%d", &choice);
I checked the value of len it remains 0 instead of 1 when I first use insert.
if (choice==1)
{
insert(container,len);
printf("Value of len:%d\n",len);
printcontent(container);
}
else if(choice==4)
{
break;
}
}
}
main()
{
while(1)
{
int choice;
printf("What do you want to do?\n");
printf("1.Stack\n2.Queue\n3.Exit\n");
scanf("%d",&choice);
if(choice==1)
{.
stack(container);
}
else if(choice==3)
{
break;
}
}
}
Thanks for all of your help.

Answer 1

What your insert(...) function does is that it takes the parameter int len by value. That means if you call insert(...) with the local variable len as an argument in main(...) or whereever - it actually copies the value of the local variable len to the argument variable len inside the insert(...) function. The variable len which is inside the insert(...) function is then local to the insert function. That does not change even if you're declaring a pointer named incre pointing to this insert(...)-local variable len.
Then the argument variable len is incremented - but due to the fact that it is a local variable of the insert(...) function you will not see the original local variable len (which is in main(...) e.g.) being changed.
If you want to pass the original len by reference instead of by value, you have to declare your function like this ...
int insert(int container[], int* len) { ... } // Using raw pointers
... or like this ...
int insert(int container[], int& len) { ... } // Using references
... and use it like that (depending on the decision above):
insert(container, &len); // With pointers
insert(container, len); // With references