Where has the first element of my array come from, and what is it?

Question

I am writing a code to produce a C programmed RPN calculator using command line arguments. I am having an issue with the first calculation to be done by the program, as the first element in my array of operators is unknown to me and affects the calculation.

My Command line reads:

$ ./rpn.exe 1 2 3 4 5 + + + +

My array should be {+, +, +, +}

However the output when printed is:

º + + + +

Here is my for loop adding the operators to the array. Operands in my number of Numbers on the cmd line. Is_Op is simply for errors.

for(int c = operands + 1; c < argc; c++)
{
    char b = *argv[c];
    if(Is_Op(b) == 1)
    {
        fprintf(stderr, "%c is not an operator", b);
        return 1;
    }
    else
    {
        operators[c - operands] = b;
    }
}

Here is my array printing function. TotalOps is the total no. of operators. And operators[] is an array of them.

for(int count = 0; count <= TotalOps; count++)
{
    printf("%c ", operators[count]);
}

Likely you've got an off my one error in populating the array of operators. But without code we cannot confirm. — Richard, Mar 19 '19 at 13:06
@OliverThurston while you edited some code into the question, this still looks somewhat incomplete. Are you absolutely sure, that the presented piece of code reveals your problem? — grek40, Mar 19 '19 at 17:15
I could probably write the correct answer with some help from my awesome crystal ball, but you should still complete your code sample — grek40, Mar 19 '19 at 17:20

score 0 · Accepted Answer · answered Mar 21 '19 at 12:10

Have a closer look at

for(int c = operands + 1; c < argc; c++)
{
    char b = *argv[c];
    if(Is_Op(b) == 1)
    {
        fprintf(stderr, "%c is not an operator", b);
        return 1;
    }
    else
    {
        operators[c - operands] = b;
    }
}

since c starts at operands + 1, the first element will be written to operators[c - operands] => operators[1]. So whatever operators[0] initially contains, will stay there.

You can test this by actually initializing the operators on definition:

char operators[TotalOps] = { '#' }; // will initialize the first element to '#', all others to '\0'

It should output # + + + + instead of º + + + +.

So you need to change your code to use the operators array starting with index 0 instead of 1

Where has the first element of my array come from, and what is it?

1 Answers1