How Can I Format Output with Leading Zeros in C ?

Format Output with Leading Zeros in C

In C , formatting output is crucial to display data in a specific way. One common scenario is to print leading zeros, similar to how printf handles it with d. While using printf is an option, you may prefer the C stream output operator (<<).

One option to achieve leading zeros is by concatenating a formatted string using sprintf with your output operator, as shown below:

std::cout << "ZIP code: " << sprintf("%05d", zipCode) << std::endl;

However, there's a cleaner solution using IO manipulators:

#include <iomanip>

using namespace std;

cout << setw(5) << setfill('0') << zipCode << endl;

// or without 'using namespace std;'
std::cout << std::setw(5) << std::setfill('0') << zipCode << std::endl;</p>
<p>setw(width) sets the field width, while setfill(char) specifies the character used to fill the remaining space. In this case, '0' is used to add leading zeros.</p>
<p>Note that these manipulators affect the global state of the cout object. To avoid unintended effects in later uses, you should undo them by resetting the flags.</p>
<p>If you prefer concise formatting, consider using the fmt library:</p>
<pre class="brush:php;toolbar:false">cout << fmt::format("{:05d}", zipCode);

C 20 and later have built-in support for std::format, and C 23 introduces std::print, providing even more convenient options for formatted output.

For non-negative numbers like ZIP codes, the above code will suffice. However, if you need to handle negative numbers, add std::internal to place the fill character between the sign and the magnitude:

cout << internal << setw(5) << setfill('0') << zipCode << endl;

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