Getting error - local variable referenced before being initialized

Question

I have written a simple function which reads in a pointer to an array. It looks at the elements of the array and compares adjacent values for equality. If they are equal it adds their value to "sum"

I am getting a runtime error telling me "local "sum" was referenced before being initialized." I don't understand this because sum is definitely initialized in my function. See code snipet.

int arrRead (char *Pinput){

    int sum, a, b = 0;

    while (*Pinput){
        a = *Pinput;
        ++Pinput;
        b = *Pinput;
        if(a == b)
            sum += a;
    }
    return sum;
}

`sum` is not initialized before you start adding to it. Initialization is when you assign it an initial value. You are not giving it an initial value, so it starts with garbage. — Christian Gibbons, Feb 21 '19 at 14:54
You don’t set `sum` to 0 before you use it. You would need `int sum = 0, a = 0, b = 0;` to zero initialize all three variables. — Jonathan Leffler, Feb 21 '19 at 14:54
duplicate of https://stackoverflow.com/questions/1597405/what-happens-to-a-declared-uninitialized-variable-in-c-does-it-have-a-value ? — jhnc, Feb 21 '19 at 14:58
Your compiler complains about a variable being used without initialization. This is not a runtime error. This is a compile time error/warning. — Gerhardh, Feb 21 '19 at 15:15

bolov · Accepted Answer · 2019-02-21T15:02:37.777

6

int sum, a, b = 0;

here only b is initialized with 0.

sum and a are uninitialized.

You need to write:

int sum = 0, a = 0, b = 0;

Or, each variable on its own line:

int sum = 0;
int a = 0;
int b = 0;

Even if in your function technically only sum needs to be initialized for the algorithm to work, it is a very good practice to initialize all the variables upon declaration.

Another good suggestion by @melpomene is to narrow the scope of the variables. For instance a and b are used only in the while loop:

int sum = 0;

while (*Pinput){
    int a = *Pinput;
    ++Pinput;
    int b = *Pinput;
    if(a == b)
        sum += a;
}
return sum;

edited Feb 21 '19 at 15:02

answered Feb 21 '19 at 14:56

bolov

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Better yet: `int sum = 0, a, b;` Best: `int sum = 0; while (*Pinput) { int a = *Pinput++; int b = *Pinput;` – melpomene Feb 21 '19 at 14:58
1

@melpomene I wouldn't consider anything that uses the comma operator unnecessarily to be in the "better yet" category, but I guess that comes down to personal preference. – Christian Gibbons Feb 21 '19 at 15:01
@ChristianGibbons That's not the comma operator. – melpomene Feb 21 '19 at 15:05
@melpomene Perhaps I misused the terminology. What would the proper term be for the use of the comma in that instance? – Christian Gibbons Feb 21 '19 at 15:09
1

@ChristianGibbons Good question. The C standard doesn't mention a name; the grammar simply specifies that a declarator list consists of one or more declarators separated by `,`. – melpomene Feb 21 '19 at 15:15
@Melpomene thanks for suggestion. But wouldn't limiting the scope of a and b in the manner you suggest, result in them being declared anew for each loop iteration? – TestEngineer11 Feb 21 '19 at 15:21
@TestEngineer11 Not really. "Declaring" is a compile-time thing. It does nothing at runtime. – melpomene Feb 21 '19 at 15:23

score 2 · Answer 2 · answered Feb 21 '19 at 14:56

2

You’re not initializing sum with

int sum, a, b = 0;

Only b is being initialized. You need to add an initializer for sum as well.

int sum=0, a=0, b = 0;

answered Feb 21 '19 at 14:56

dbush

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score 0 · Answer 3 · answered Feb 21 '19 at 14:58

0

The operator += adds the right side value to the left side value, and then stores the sum back into the left side value.

If sum is uninitialized, the operation sum += X invokes undefined behavior; the result depends on the previous value of sum.

answered Feb 21 '19 at 14:58

Govind Parmar

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Getting error - local variable referenced before being initialized

3 Answers3