What’s going on here on this C show?

Question

I’m making a program where the user enters with 10 values, and at the end of this, presents a message saying the total value.

My code :

int rep,valor,soma;

    while(rep < 10) {

    printf("Digite um valor : ");
    scanf("%d",&valor);

    soma += valor;

    rep = rep + 1;  

    }

    printf("Total dos valores : %d ", soma);

What I couldn’t figure out is why it adds one more to the sum total, for example if the input of the 10 values is 100, it adds as 101.

Screenshot :

When I put in the code soma = 0, it goes normal, but in case I don’t, this occurs.

What’s going on then ?

Answer 1

Whenever you create a local variable to a function, it is allocated to a memory area called "the stack". It contains the local variables and some more information that allows the computer to know, when a function finishes being executed, to which point of the previous function it has to return, and so on, until it reaches the main().

One feature of this stack is that if a function is invoked, it uses part of that memory to store its local functions. After this function returns, the memory it used is released, and if later the code that called this function calls another, it uses the same memory space as the first function used to store the local variables of the second. Besides, she doesn’t care about zeroing out that memory region even after allocating space, or before releasing it again.

The result of all this is that the value you find in an uninitialized local variable will depend on which functions previously ran in that program and how they finished executing them. In other words, and by borrowing the terminology from the C standard, when you read the value of an uninitialized local variable, the behavior is undefined. That is, any value can be there. In your case, it happened to be one; it could be -1,357,928, too.

What is the lesson learned? Always initialize your local variables when declaring them. You’ll save yourself trouble sooner or later.