Add leading Zeros to integer doesn't work (arduino, c)

Question

I want to convert, for example, the number 12 to 0012. So that the number is always 4 characters long. I tried it with this code:

int number;
int result;

void setup() {
    number = 12;
    Serial.begin(9600);
    result = printf("%03d", number);
}

void loop() {
    Serial.println(result);
}

However this code outputs only -1 instead of 0012. What is wrong with this code?

Answer 1

Integers cannot and do not recognize formatting like leading zeros.

The only way to store this type of information would be to use a string (like a char[] instead of an integer, however it looks like you are simply writing out the value so you shouldn't need a variable for it (as printf() will handle that for you).

// %04d% will pad your number to 4 digits
printf("%04d", number);

If you do actually need to store your value, you can use sprintf() and declare your result as seen below :

int number;
char *result = malloc(5);

and then set it within your setup() function as such :

sprintf(result, "%04d", number);

Using both of these changes, your updated code might look like :

int number;
char *result = malloc(5);

void setup() {
    number = 12;
    Serial.begin(9600);
    sprintf(result, "%04d", number);
}

void loop() {
    Serial.println(result);
}

Consider formatting your values when you are outputting them, otherwise you can use the integer values to perform any arithmetic and calculations with.

Answer 2

代码的错误在于您正在打印＆lt; printf＆＃39;的返回值。呼叫。它返回打印的字符数。由于看起来stdio尚未设置，因此返回-1作为错误标志。请仔细阅读手册页。

在这一行：

result = printf("%03d", number);

我们想要使用＆＃39; sprintf＆＃39;而不是＆＃39; printf＆＃39;。

＆＃39; sprintf的＆＃39;需要将其输出字符串放在某处。也许预先分配它，确保它大于你的最长输出字符串加一（用于保存终止空字符）。不要在＆＃39; setup＆＃39;中分配它，而是将它放在任何例程之外（因为你在一个例程中初始化它，并在另一个例程中使用它）。

char buf[6];

r = sprintf(buf, "%04d", number);

// now print the string inside of 'buf'
Serial.println(buf);