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php的sprintf函数的用法 控制浮点数格式

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Release： 2016-06-13 09:30:41
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控制浮点数打印格式

浮点数的打印和格式控制是sprintf 的一大常用功能，浮点数使用格式符”%f”控制，默认保留小数点后6 位数字，比如：

复制代码 代码如下:


sprintf("%f", 3.1415926); //结果："3.141593"

但，有时我们希望自己控制打印的宽度和小数位数，这时就应该使用：”%m.nf”格式，其中m 表示打印数字的整体宽度，n 表示小数点后的位数。比如：

复制代码 代码如下:


sprintf(" %9.3f", 3.1415926); //右对齐：位数不够用空格补全。结果："    3.142"
sprintf(" %-9.3f", 3.1415926); //左对齐：位数不够用空格补全。结果："3.142    "
sprintf(" %.3f", 3.1415926); //不指定总宽度，结果："3.142"

注意一个问题

复制代码 代码如下:


$num = 100;
sprintf("%.2f", $num );
sprintf("%.2f", (double)$num);


上面的两个结果真的一样吗？虽然看起来是一样的，但是下面的原因，可能会有所启发.
原因是：参数压栈时调用者并不知道跟num相对应的格式控制符是个”%f”。而函数执行时函数本身则并不知道当年被压入栈里的是个整数，于是可怜的保存整数$num的那4个字节就被不由分说地强行作为浮点数格式来解释了，整个乱套了。
Related labels：
php floating point number
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