MATLAB: Recursive MEX function, structures within structure

mexstructures

I'm trying to make a MEX function that takes a structure (STRUCT0) and a number of strings (STRING1, STRING2, …) as input arguments. The function should test whether STRUCT0.STRING1.STRING2 is a valid field. The function "checkForFields" below will verify that STRUCT0 is a structure, that STRING1 is a valid field and that STRUCT0.STRING1 is itself a structure. It should then recursively call itself, to check whether STRING2 is a field of STRUCT0.STRING1. However, that's where it goes wrong. When the function recursively calls itself, STRUCT0.STRING1 is no longer recognised as a structure. How should I define the input arguments for the recursive function call?
Many thanks, Dries
#include "mex.h"
#include "matrix.h"
void checkForFields(mxArray *output[], int nInputArguments, const mxArray *inputArguments[]);
void mexFunction(
        int nlhs, mxArray *plhs[],
        int nrhs, const mxArray *prhs[]) {
      // define a boolean output argument, initialised to false
      plhs[0] = mxCreateLogicalScalar(0);
      // check number of input arguments:
      if(nrhs<2) {
          mexErrMsgIdAndTxt("driesMex:fieldExists:nrhs1",
                  "Function requires a structure and at least one field.");
      }
      // check number of output arguments:
      if(nlhs>1) {
          mexErrMsgIdAndTxt("driesMex:fieldExists:nlhs",
                  "Function supports maximum one output.");
      }
      // if first argument is a struct
      if (mxIsStruct(prhs[0])) {
          // call a subroutine that will recursively call itself
          checkForFields(plhs, nrhs, prhs);
      } else {
          // first argument is not a struct, issue a warning:
          mexWarnMsgIdAndTxt("driesMex:fieldExists:firstArg", "First argument is not a structure.");
      }
  }
void checkForFields(mxArray *output[], int nInputArguments, const mxArray *inputArguments[]){
    // *output[]:       pointer to output mxArray
    // nInputArguments: number of input arguments
    // *inputArguments: pointer to input mxArray,
    //                  first argument must be struct, rest are fields to be tested
      // initialise output to false
      output[0] = mxCreateLogicalScalar(0);
      // if first argument is not a struct
      if (!mxIsStruct(inputArguments[0])) {
          // issue a warning:        
          mexWarnMsgIdAndTxt("driesMex:fieldExists:checkForFields:firstArg", "CheckForFields: First argument is not a structure.");
      }
      // if second argument is a string
      if (mxIsChar(inputArguments[1])) {
          // if second argument is a field
          if (mxGetFieldNumber(inputArguments[0], mxArrayToString(inputArguments[1]))+1) {
              // output argument should be set to true
              output[0] = mxCreateLogicalScalar(1);
              // check if there are more than 2 input arguments
              if (nInputArguments>2) {                
                  // check if second argument is also struct
                  if (mxIsStruct(mxGetField(inputArguments[0], 0, mxArrayToString(inputArguments[1])))){
                      // recursively call this subroutine
                      checkForFields(output, nInputArguments-1, &inputArguments[1]);
                      // output argument may be overwritten if subsequent field  was not a field
                  }
              }
          }
          // else second argument is not a field
          // output argument was already initialised to false
      } else {
          // second argument is not a string, throw error
          mexErrMsgIdAndTxt("driesMex:fieldExists:nrhs2",
                  "Function requires field argument(s) to be a string.");
      }
  }

Best Answer

I would not use a recursion at all.
#include "mex.h"   // Includes matrix.h already
void mexFunction(
      int nlhs, mxArray *plhs[],
      int nrhs, const mxArray *prhs[])
{
  mwSize i;
  char Field[mxMAXNAM];
  S = prhs[0];
  if (!mxIsStruct(S)) {
    mexErrMsgIdAndTxt(...);
  }
  for (i = 1; i < nrhs; i++) {
    // mxGetString replies 1 if the input is not a string
    // or too long:
    if (mxGetString(prhs[i], Field, mxMAXNAM)) {
      // Check for: prhs[i] is not a string, or too long
      mexErrMsgIdAndTxt(...);
    }   
    // mxGetFields replies NULL, if S is not a struct or doe not
    // have the wanted field:
    S = mxGetField(S, 0, Field);
    if (S == NULL) {  // Field not found:
      plhs[0] = mxCreateLogicalScalar(0);
      return;
    }
  } 
  // All subfields have been found: 
  plhs[0] = mxCreateLogicalScalar(1);
  return;
}
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