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stringtclmatch
Kriti Kapoor
Kriti Kapoor

Reputation: 157

Match first letter of a string in Tcl

I want to compare the first letter of a string with a known character. For example, I want to check if the string "example"'s first letter matches with "e" or not. I'm sure there must be a very simple way to do it, but I could not find it.

Upvotes: 1

Views: 8947

Answers (4)

Peter Lewerin
Peter Lewerin

Reputation: 13252

I think it's a good idea to collect the different methods in a single answer.

Assume

set mystring example
set mychar   e

The goal is to test whether the first character in $mystring is equal to $mychar.

My suggestion was (slightly edited):

if {[string match $mychar* $mystring]} {
    ...

This invocation does a glob-style match, comparing $mystring to the character $mychar followed by a sequence of zero or more arbitrary characters. Due to shortcuts in the algorithm, the comparison stops after the first character and is quite efficient.

Donal Fellows:

if {[string index $mystring 0] eq $mychar} {
    ...

This invocation specifically compares a string consisting of the first character in $mystring with the string $mychar. It uses the efficient eq operator rather than the == operator, which is the only one available in older versions of Tcl.

Another way to construct a string consisting of the first character in $mystring is by invoking string range $mystring 0 0.

Mark Kadlec:

if {[string first $mychar $mystring] == 0 }
    ...

This invocation searches the string $mystring for the first occurrence of the character $mychar. If it finds any, it returns with the index where the character was found. This index number is then compared to 0. If they are equal the first character of $mystring was $mychar.

This solution is rather inefficient in the worst case, where $mystring is long and $mychar does not occur in it. The command will then examine the whole string even though only the first character is of interest.

One more string-based solution:

if {[string compare -length 1 $mychar $mystring] == 0} {
    ...

This invocation compares the first n characters of both strings (n being hardcoded to 1 here): if there is a difference the command will return -1 or 1 (depending on alphabetical order), and if they are equal 0 will be returned.

Another solution is to use a regular expression match:

if {[regexp -- ^$mychar.* $mystring]} {
    ...

This solution is similar to the string match solution above, but uses regular expression syntax rather than glob syntax. Don't forget the ^ anchor, otherwise the invocation will return true if $mychar occurs anywhere in $mystring.

Documentation: eq and ==, regexp, string

Upvotes: 2

user3913961
user3913961

Reputation: 1

set mychar "e"
if { [string first $mychar $myString] == 0}{
  ....

Upvotes: -1

Mark Kadlec
Mark Kadlec

Reputation: 8440

if { [string first e $yourString] == 0 }
  ...

Upvotes: 0

Donal Fellows
Donal Fellows

Reputation: 137567

One way is to get the first character with string index:

if {[string index $yourstring 0] eq "e"} {
    ...

Upvotes: 5

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