Control                 package:base                 R Documentation

_C_o_n_t_r_o_l _F_l_o_w

_D_e_s_c_r_i_p_t_i_o_n:

     These are the basic control-flow constructs of the R language. 
     They function in much the same way as control statements in any
     Algol-like language.  They are all reserved words.

_U_s_a_g_e:

     if(cond) expr
     if(cond) cons.expr  else  alt.expr

     for(var in seq) expr
     while(cond) expr
     repeat expr
     break
     next

_A_r_g_u_m_e_n_t_s:

    cond: A length-one logical vector that is not 'NA'. Conditions of
          length greater than one are accepted with a warning, but only
          the first element is used.  Other types are coerced to
          logical if possible, ignoring any class. 

     var: A syntactical name for a variable.

     seq: An expression evaluating to a vector (including a list and an
          expression) or to a pairlist or 'NULL'.  A factor value will
          be coerced to a character vector.

expr, cons.expr, alt.expr: An _expression_ in a formal sense.  This is
          either a simple expression or a so called _compound
          expression_, usually of the form '{ expr1 ; expr2 }'. 

_D_e_t_a_i_l_s:

     'break' breaks out of a 'for', 'while' or 'repeat' loop; control
     is transferred to the first statement outside the inner-most loop.
     'next' halts the processing of the current iteration and advances
     the looping index.  Both 'break' and 'next' apply only to the
     innermost of nested loops.

     Note that it is a common mistake to forget to put braces ('{ ..
     }') around your statements, e.g., after 'if(..)' or 'for(....)'.
     In particular, you should not have a newline between '}' and 
     'else' to avoid a syntax error in entering a 'if ... else'
     construct at the keyboard or via 'source'. For that reason, one
     (somewhat extreme) attitude of defensive programming is to always
     use braces, e.g., for 'if' clauses.

     The 'seq' in a 'for' loop is evaluated at the start of the loop;
     changing it subsequently does not affect the loop. If 'seq' is
     'NULL' the body of the loop is skipped. Otherwise the variable
     'var' is assigned in turn the value of each element of 'seq'. You
     can assign to 'var' within the body of the loop, but this will not
     affect the next iteration. When the loop terminates, 'var' remains
     as a variable containing its latest value.

_V_a_l_u_e:

     'if' returns the value of the expression evaluated, or 'NULL' if
     none was (which may happen if there is no 'else').

     'for', 'while' and 'repeat' return the value of the last
     expression evaluated (or 'NULL' if none was), invisibly. 'for'
     sets 'var' to the last used element of 'seq', or to 'NULL' if it
     was of length zero.

     'break' and 'next' have value 'NULL', although it would be strange
     to look for a return value.

_R_e_f_e_r_e_n_c_e_s:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_. Wadsworth & Brooks/Cole.

_S_e_e _A_l_s_o:

     'Syntax' for the basic R syntax and operators, 'Paren' for
     parentheses and braces; further, 'ifelse', 'switch'.

_E_x_a_m_p_l_e_s:

     for(i in 1:5) print(1:i)
     for(n in c(2,5,10,20,50)) {
        x <- stats::rnorm(n)
        cat(n,":", sum(x^2),"\n")
     }
     f = factor(sample(letters[1:5], 10, replace=TRUE))
     for( i in unique(f) ) print(i)