CODEWITHSUNDEEP
cstringcharpadding
MortalMan
MortalMan

Reputation: 2612

Padding a char with 0s in C

I am trying to pad a certain amount of zeros to the beginning of a char array, which is to be later read as ints. Here is my code:

// charArray is '123' right now
// want charArray to be 00123
// Shift elements in array forward 'difference' times
// difference is amount of 0s to be padded to beginning of string, difference = 2 here
for (i = strlen(charArray) - 1; i >= 0; i--) {
    charArray[i+difference] = charArray[i];
}

This is where I have problems. The way the code below runs, charArray is ??123

for (i = 0; i < difference; i++) {
    y[i] = 0 - '0';
}

If I do this instead:

for (i = 0; i < difference; i++) {
    y[i] = 0;
}

My string gets terminated early. Any ideas?

Upvotes: 0

Views: 4557

Answers (3)

waterjuice
waterjuice

Reputation: 849

So the problem is the character you are setting. In the first case you use y[i] = 0 - '0'; which will give you a character value of -48, which will be a byte of hex 0xD0. This is a character in the extended ascii range which is why you are getting a peculiar character there.

In the second case you used y[i] = 0; Which is a byte value of zero which is used as a zero terminator (hence the early termination of your string).

What you want to use is y[i] = '0';

Upvotes: 0

selbie
selbie

Reputation: 104589

If your originally allocated array is big enough to hold both the original string, it's null terminator, and the padding together, then you can do this:

memmove(charArray+difference, charArray, strlen(charArray));
memset(charArray, '\0', difference);

If you are shifting a string that doesn't have the extra allocate bytes, then some characters are going to get dropped. Hence:

Shift a string by N "difference" bytes and pad the leading space with zeros

int original_array_size = strlen(charArray) + 1;
memmove(charArray+difference, charArray, original_array_size-difference);
memset(charArray, '\0', difference);
charArray[original_array_size - 1] = '\0'; // assumes original_array_size is >= 1

Upvotes: 0

chqrlie
chqrlie

Reputation: 145317

To set the padding area, just use this code:

for (i = 0; i < difference; i++) {
    charArray[i] = '0';
}

or possibly:

memset(charArray, '0', difference);

Upvotes: 1

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