问题描述

我从一个未知的来源导入了一组坐标，而我并没有权限去查看这些坐标。

set yMin [lindex [lindex $bbox 0] 1]
puts "yMin: <$yMin>"

一切正常。

yMin: <-6.149999999999999e-02>

括号是用来检查其中是否有空格甚至是隐藏的制表符。 但是 ，如果将 yMin 乘以任意数（比如 0.5），就会出错。

set Y2 [expr {0.5 * $yMin}]

can't use empty string as operand of "*"
while executing "expr {0.5 * $yMin}"
invoked from within "set Y2 [expr {0.5 * $yMin}]"

即使只打印 yMin ，也会出现空操作数错误。

set Y1 [expr $yMin]
puts "Y1: <$Y1>"

empty expression in expression "" (parsing expression "")
invoked from within "expr $yMin"
invoked from within "set Y1 [expr $yMin]"

但是有一个有趣的测试。如果添加花括号，它就会起作用！

set Y1 [expr {$yMin}]
puts "Y1: <$Y1>"

Y1: <-0.06149999999999999>

如何重现问题

感谢 Glenn Jackman（见下面的回复），

% set bbox {{-4.599999999999999e-02 -6.149999999999999e-02} {8.000000000000002e-02 6.149999999999999e-02}}
{-4.599999999999999e-02 -6.149999999999999e-02} {8.000000000000002e-02 6.149999999999999e-02}
% set yMin [lindex [lindex $bbox 0] 1]
-6.149999999999999e-02
% expr {1 + $yMin}
0.9385

实际上，这似乎无法重现问题（这就是我写这篇文章的原因）。但它至少可以是一个模板。

反复试验

以下代码用于检查它是否真的为空。事实证明它不为空。

if {$yMin eq {}} {
    puts "Empty records"
    exit 1 
} else {
    puts "yMin is not empty: <$yMin>
}

yMin is not empty: <-6.149999999999999e-02>

最后，我尝试了 trim 、 ma​​p 和 regsub 来删除所有空格和制表符，但都不起作用。

set trim_yMin [string trim $yMin]
puts "trim_yMin: <$trim_yMin>"

set map_yMin [string map {" " ""} $yMin]
puts "map_yMin: <$map_yMin>"

regsub -all {\s} $yMin {} reg_yMin
puts "reg_yMin: <$reg_yMin>"

if {$trim_yMin eq {} || $map_yMin eq {} || $reg_yMin eq {}} {
    puts "Empty records"
    exit 1
} else {
    puts "trim_yMin is not empty: <$reg_yMin>"
    puts "map_yMin is not empty: <$reg_yMin>"
    puts "reg_yMin is not empty: <$reg_yMin>"
}

trim_yMin: <-6.149999999999999e-02>
map_yMin: <-6.149999999999999e-02>
reg_yMin: <-6.149999999999999e-02>
trim_yMin is not empty: <-6.149999999999999e-02>
map_yMin is not empty: <-6.149999999999999e-02>
reg_yMin is not empty: <-6.149999999999999e-02>

这里我只显示了 regsub 的结果。其他人的结果是一样的。

set reg_Y2 [expr {0.5 * $reg_yMin}]
puts "0.5 * reg_yMin: $reg_Y2"

can't use empty string as operand of "*"
while executing "expr {0.5 * $reg_yMin}"
invoked from within "set reg_Y2 [expr {0.5 * $reg_yMin}]"

你能帮帮我吗？我真的不知道我还能尝试什么。提前致谢。

更新和回复

致 Ergwun ：

puts $bbox;
set yMin [lindex [lindex $bbox 0] 1]
puts "yMin: <$yMin>"

{-4.599999999999999e-02 -6.149999999999999e-02} {8.000000000000002e-02 6.149999999999999e-02}
yMin: <-6.149999999999999e-02>

set Y2 [expr {0.5 * $yMin}]
puts $Y2

can't use empty string as operand of "*"
while executing "expr {0.5 * $yMin}"
invoked from within "set Y2 [expr {0.5 * $yMin}]"

致 benaja ：

puts [tcl::unsupported::representation $yMin]

value is a string with a refcount of 4, object pointer at 0x44ed0910, internal representation 0x450d6120:(nil), string representation "-6.14999999999...."

致 Shawn ，

puts [tcl::unsupported::representation $yMin]
puts [binary encode hex $yMin]
puts [tcl::unsupported::representation $yMin]
puts [string is double -strict $yMin]
puts [tcl::unsupported::representation $yMin]

value is a string with a refcount of 4, object pointer at 0x44ed6030, internal representation 0x450de4b0:(nil), string representation "-6.1499999999..."

2d362e313439393939393939393939393939652d3032

value is a bytearray with a refcount of 4, object pointer at 0x44ed6030, internal representation 0x44ee0dc0:(nil), string representation "-6.1499999999..."

1

value is a double with a refcount of 4, object pointer at 0x44ed6030, internal representation 0xbfaf7ced916872af:(nil), string representation "-6.1499999999..."

set Y2 [expr {0.5 * -6.149999999999999e-02}]
puts "Y2: $Y2"
set new_Y2 [expr {0.5*-6.149999999999999e-02}]
puts "new_Y2: $new_Y2"

Y2: -0.030749999999999996
new_Y2: -0.030749999999999996

致 Schelte Bron ，

错误信息已更改！也许这暗示了什么？

puts [tcl::unsupported::representation $yMin]
set yMin " $yMin"
puts [tcl::unsupported::representation $yMin]
set Y2 [expr {0.5 * $yMin}]
puts $Y2

value is a string with a refcount of 4, object pointer at 0x44ec28a0, internal representation 0x450be540:(nil), string representation "-6.1499999999..."

value is a string with a refcount of 2, object pointer at 0x446bbc70, internal representation 0x44695fd0:(nil), string representation " -6.149999999..."

can't use non-numeric string as operand of "*"
while executing "expr {0.5 * $yMin}"
invoked from within
"set Y2 [expr {0.5 * $yMin}]"

If the calculation still fails after the value has changed to a double after string is double -strict $yMin, then take the result of binary encode hex $yMin, convert it back to a string (using binary decode hex) and use that in the calculation.

puts "Is yMin double? : [string is double -strict $yMin]"
set binary_yMin [binary encode hex $yMin]
set double_binary_yMin [binary decode hex $binary_yMin]
puts "yMin: $yMin"
puts "Binary of double yMin: $binary_yMin"
puts "Double binary double: $double_binary_yMin"
set Y2 [expr {0.5 * $double_binary_yMin}]
puts $Y2

Is yMin double? : 1
yMin: -6.149999999999999e-02
Binary of double yMin: 2d362e313439393939393939393939393939652d3032
Double binary double: -6.149999999999999e-02
can't use empty string as operand of "*"
while executing "expr {0.5 * $double_binary_yMin}"
invoked from within "set Y2 [expr {0.5 * $double_binary_yMin}]"

What do you get with tcl::mathop::* 0.5 $yMin?

puts [tcl::mathop::* 0.5 $yMin]

can't use empty string as operand of "*"
while executing
"tcl::mathop::* 0.5 $yMin"
invoked from within
"puts [tcl::mathop::* 0.5 $yMin]"

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